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LM2597, LM2597HV
SNVS119D – MARCH 1998 – REVISED MAY 2016
LM2597, LM2597HV SIMPLE SWITCHER® Power Converter 150-kHz 0.5-A Step-Down
Voltage Regulator
1 Features
3 Description
•
•
The LM2597xx series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 0.5-A load with excellent line and load
regulation. These devices are available in fixed output
voltages of 3.3 V, 5 V, 12 V, and an adjustable output
version, and are packaged in an 8-pin PDIP and an
8-pin surface-mount package.
1
•
•
•
•
•
•
•
•
•
•
•
•
3.3-V, 5-V, 12-V, and Adjustable Output Versions
Adjustable Version Output Voltage Range: 1.2 V
to 37 V (57 V for HV Version) ±4% Maximum Over
Line and Load Conditions
Specified 0.5-A Output Current
Available in 8-Pin Surface-Mount and 8-Pin PDIP
Packages
Input Voltage Range Up to 60 V
150-kHz Fixed-Frequency Internal Oscillator
Shutdown/Soft-Start
Out of Regulation Error Flag
Error Output Delay
Bias Supply Pin (VBS) for Internal Circuitry
Improves Efficiency at High Input Voltages
Low Power Standby Mode, IQ, Typically 85 μA
High Efficiency
Uses Readily Available Standard Inductors
Thermal Shutdown and Current-Limit Protection
2 Applications
•
•
•
•
Simple High-Efficiency Step-Down (Buck)
Regulator
Efficient Preregulator for Linear Regulators
On-Card Switching Regulators
Positive-to-Negative Converter
This series of switching regulators is similar to the
LM2594 series, with additional supervisory and
performance features added.
Requiring a minimum number of external
components, these regulators are simple to use and
include internal frequency compensation, improved
line and load specifications, fixed-frequency oscillator,
Shutdown/Soft-start, error flag delay, and error flag
output.
The LM2597xx series operates at a switching
frequency of 150-kHz, thus allowing smaller-sized
filter components than what would be required with
lower-frequency switching regulators. Because of its
high efficiency, the copper traces on the printedcircuit board are normally the only heat sinking
required.
Device Information(1)
PART NUMBER
LM2597, LM2597HV
PACKAGE
BODY SIZE (NOM)
SOIC (8)
4.90 mm × 3.91 mm
PDIP (8)
9.81 mm × 6.35 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
Fixed Output Voltage Versions
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM2597, LM2597HV
SNVS119D – MARCH 1998 – REVISED MAY 2016
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Description (continued).........................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
3
4
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
4
4
4
4
5
5
5
5
Absolute Maximum Ratings .....................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics – 3.3 V ..............................
Electrical Characteristics – 5 V .................................
Electrical Characteristics – 12 V ...............................
Electrical Characteristics – Adjustable......................
Electrical Characteristics – All Output Voltage
Versions .....................................................................
7.10 Typical Characteristics ............................................
8
6
8
Detailed Description ............................................ 11
8.1
8.2
8.3
8.4
9
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
11
11
11
16
Application and Implementation ........................ 17
9.1 Application Information............................................ 17
9.2 Typical Applications ................................................ 28
10 Power Supply Recommendations ..................... 35
11 Layout................................................................... 35
11.1 Layout Guidelines ................................................. 35
11.2 Layout Example .................................................... 36
11.3 Thermal Considerations ........................................ 37
12 Device and Documentation Support ................. 39
12.1
12.2
12.3
12.4
12.5
Related Links ........................................................
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
39
39
39
39
39
13 Mechanical, Packaging, and Orderable
Information ........................................................... 39
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision C (April 2013) to Revision D
Page
•
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section. ................................................................................................. 1
•
Removed all references to design software Switchers Made Simple ................................................................................... 1
Changes from Revision B (April 2013) to Revision C
•
2
Page
Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 37
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SNVS119D – MARCH 1998 – REVISED MAY 2016
5 Description (continued)
A standard series of inductors (both through-hole and surface-mount types) are available from several different
manufacturers optimized for use with the LM2597xx series. This feature greatly simplifies the design of switchmode power supplies.
Other features include a specified ±4% tolerance on output voltage under all conditions of input voltage and
output load conditions, and ±15% on the oscillator frequency. External shutdown is included, featuring typically
85-μA standby current. Self protection features include a two stage current limit for the output switch and an
overtemperature shutdown for complete protection under fault conditions.
The LM2597HV is for use in applications requiring and input voltage up to 60 V.
6 Pin Configuration and Functions
D or P Package
8-Pin SOIC or PDIP
Top View
Error_Flag
1
8
Output
Delay
2
7
Bias_Supply
3
6
+VIN
Ground
Feedback
4
5
Shutdown/Soft-Start
Not to scale
Pin Functions (1)
PIN
NO.
NAME
I/O
DESCRIPTION
1
Error Flag
O
Open collector output that provides a low signal (flag transistor ON) when the regulated
output voltage drops more than 5% from the nominal output voltage. On start up, Error Flag
is low until VOUT reaches 95% of the nominal output voltage and a delay time determined by
the Delay pin capacitor. This signal can be used as a reset to a microprocessor on power up.
2
Delay
O
At power-up, this pin can be used to provide a time delay between the time the regulated
output voltage reaches 95% of the nominal output voltage, and the time the error flag output
goes high.
3
Bias Supply
I
This feature allows the regulators internal circuitry to be powered from the regulated output
voltage or an external supply, instead of the input voltage. This results in increased efficiency
under some operating conditions, such as low output current or high input voltage.
4
Feedback
I
Senses the regulated output voltage to complete the feedback loop.
5
Shutdown/SoftStart
I
This dual function pin provides the following features: (a) Allows the switching regulator
circuit to be shut down using logic level signals thus dropping the total input supply current to
approximately 80 μA. (b) Adding a capacitor to this pin provides a soft-start feature which
minimizes start-up current and provides a controlled ramp up of the output voltage.
6
Ground
—
7
+VIN
I
This is the positive input supply for the IC switching regulator. A suitable input bypass
capacitor must be present at this pin to minimize voltage transients and to supply the
switching currents required by the regulator.
8
Output
O
Internal switch. The voltage at this pin switches between (+VIN − VSAT) and approximately
−0.5 V, with a duty cycle of VOUT/VIN. To minimize coupling to sensitive circuitry, the PCB
copper area connected to this pin must be kept to a minimum.
(1)
Circuit ground
If any of the above four features (Shutdown/Soft-start, Error Flag, Delay, or Bias Supply) are not used, the respective pins must be left
open.
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7 Specifications
7.1 Absolute Maximum Ratings
(1) (2)
MIN
Maximum supply voltage, VIN (3)
MAX
UNIT
LM2597
45
V
LM2597HV
60
V
6
V
SD/SS pin input voltage (4)
Delay pin voltage
(4)
1.5
V
Flag pin voltage
–0.3
45
V
Bias supply voltage, VBS
–0.3
30
V
Feedback pin voltage
–0.3
25
V
–1
1
V
Output voltage to ground (steady-state)
Power dissipation
Internally limited
D package
Lead temperature
Vapor phase (60 sec)
215
Infrared (15 sec)
220
P package (soldering, 10 sec)
Maximum junction temperature
Storage temperature, Tstg
(1)
(2)
(3)
(4)
°C
260
–65
150
°C
150
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
VIN = 40V for the LM2597 and 60V for the LM2597HV.
Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
7.2 ESD Ratings
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001
VALUE
UNIT
±2000
V
(1) (2)
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
7.3 Recommended Operating Conditions
Supply voltage
MIN
MAX
4.5
40
LM2597
LM2597HV
Temperature
UNIT
V
4.5
60
V
–40
125
°C
7.4 Thermal Information
LM2597, LM2597HV
THERMAL METRIC (1)
RθJA
(1)
(2)
4
Junction-to-ambient thermal resistance (2)
D (SOIC)
P (PDIP)
8 PINS
8 PINS
95
150
UNIT
°C/W
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
Junction-to-ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads.
Additional copper area will lower thermal resistance further.
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7.5 Electrical Characteristics – 3.3 V
Specifications are for TJ = 25°C, VINmax = 40 V for the LM2597, and 60 V for the LM2597HV (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3) (4)
Output
voltage
4.75 V ≤ VIN ≤ VINmax,
0.1 A ≤ ILOAD ≤ 0.5 A
η
Efficiency
VIN = 12 V, ILOAD = 0.5 A
(2)
(3)
(4)
(2)
MAX (1)
3.3
3.432
TYP
UNIT
(See Figure 46 for test circuit)
VOUT
(1)
MIN (1)
TJ = 25°C
3.168
Over full operating temperature range
3.135
3.465
V
80%
All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely norm.
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2597xx is used as shown in the Figure 46 test circuit, system performance is shown in the test conditions column.
No diode, inductor or capacitor connected to output pin.
7.6 Electrical Characteristics – 5 V
Specifications are for TJ = 25°C, VINmax = 40 V for the LM2597, and 60 V for the LM2597HV (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3) (4)
Output voltage
7 V ≤ VIN ≤ VINmax,
0.1 A ≤ ILOAD ≤ 0.5 A
η
Efficiency
VIN = 12 V, ILOAD = 0.5 A
(2)
(3)
(4)
TYP (2)
4.8
5
MAX (1)
UNIT
(See Figure 46 for test circuit)
VOUT
(1)
MIN (1)
TJ = 25°C
Over full operating temperature range
4.75
5.2
5.25
V
82%
All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely norm.
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2597xx is used as shown in the Figure 46 test circuit, system performance is shown in the test conditions column.
No diode, inductor or capacitor connected to output pin.
7.7 Electrical Characteristics – 12 V
Specifications are for TJ = 25°C, VINmax = 40 V for the LM2597, and 60 V for the LM2597HV (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3) (4)
Output voltage
15 V ≤ VIN ≤ VINmax,
0.1 A ≤ ILOAD ≤ 0.5 A
η
Efficiency
VIN = 25 V, ILOAD = 0.5 A
(2)
(3)
(4)
TYP (2)
MAX (1)
11.52
12
12.48
UNIT
(See Figure 46 for test circuit)
VOUT
(1)
MIN (1)
TJ = 25°C
Over full operating temperature range
11.4
12.6
V
88%
All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely norm.
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2597xx is used as shown in the Figure 46 test circuit, system performance is shown in the test conditions column.
No diode, inductor or capacitor connected to output pin.
7.8 Electrical Characteristics – Adjustable
Specifications are for TJ = 25°C, VINmax = 40 V for the LM2597, and 60 V for the LM2597HV (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3) (4)
MIN (1)
η
(1)
(2)
(3)
(4)
Feedback
voltage
Efficiency
MAX (1)
UNIT
(See Figure 46 for test circuit)
4.5 V ≤ VIN ≤ VINmax, 0.1 A ≤ ILOAD ≤ 0.5 A
VFB
TYP (2)
TJ = 25°C
VOUT programmed for 3 V,
circuit of Figure 46
Over full operating temperature range
VIN = 12 V, VOUT = 3 V, ILOAD = 0.5 A
1.230
1.193
1.267
1.18
1.280
V
80%
All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely norm.
External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance.
When the LM2597xx is used as shown in the Figure 46 test circuit, system performance is shown in the test conditions column.
No diode, inductor or capacitor connected to output pin.
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7.9 Electrical Characteristics – All Output Voltage Versions
Specifications are for TJ = 25°C, ILOAD = 100 mA, VIN = 12 V for the 3.3-V, 5-V, and Adjustable version, and VIN = 24 V for the
12-V version (unless otherwise noted).
PARAMETER
MIN (1)
TEST CONDITIONS
TYP (2)
MAX (1)
10
50
UNIT
DEVICE PARAMETERS
Ib
Feedback bias current
fO
Oscillator frequency (3)
VSAT
Saturation voltage
Adjustable version only, VFB
= 1.235 V
TJ = 25°C
Over full operating
temperature range
100
TJ = 25°C
127
Over full operating temperature range
110
TJ = 25°C
IOUT = 0.5 A (4) (5)
0.9
Current limit
IL
Output leakage current
IQ
Operating quiescent
current
V
100%
0%
Peak current (4) (5)
TJ = 25°C
0.65
Over full operating
temperature range
0.58
0.8
Output = −1 V
SD/SS pin open, VBS pin open
(6)
TJ = 25°C
Standby quiescent
current
A
50
μA
2
15
mA
5
10
mA
85
200
Over full operating
temperature range
250
TJ = 25°C
LM2597HV
1.3
1.4
Output = 0 V (4) (6) (7)
LM2597, SD/SS pin = 0 V (6)
ISTBY
kHz
1.1
1.2
Min duty cycle (OFF) (6)
ICL
173
173
Over full operating
temperature range
Max duty cycle (ON) (5)
DC
150
nA
140
Over full operating
temperature range
μA
250
300
μA
SHUTDOWN/SOFT-START CONTROL (see Figure 46 for test circuit)
1.3
VSD
Shutdown threshold
voltage
Low, shutdown mode, over full operating temperature range
High, soft-start mode
VOUT = 20% of nominal output voltage
2
VOUT = 100% of nominal output voltage
3
VSS
Soft-start voltage
ISD
Shutdown current
VSHUTDOWN = 0.5 V
ISS
Soft-start current
VSoft-start = 2.5 V
(1)
(2)
(3)
(4)
(5)
(6)
(7)
6
0.6
V
2
V
5
10
μA
1.6
5
μA
All room temperature limits are 100% production tested. All limits at temperature extremes are specified via correlation using standard
Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely norm.
The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the
severity of current overload.
No diode, inductor or capacitor connected to output pin.
Feedback pin removed from output and connected to 0 V to force the output transistor switch ON.
Feedback pin removed from output and connected to 12 V for the 3.3-V, 5-V, and the adjustable version, and 15 V for the 12-V version,
to force the output transistor switch OFF.
VIN = 40 V for the LM2597 and 60 V for the LM2597HV.
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Electrical Characteristics – All Output Voltage Versions (continued)
Specifications are for TJ = 25°C, ILOAD = 100 mA, VIN = 12 V for the 3.3-V, 5-V, and Adjustable version, and VIN = 24 V for the
12-V version (unless otherwise noted).
PARAMETER
TEST CONDITIONS
MIN (1)
TYP (2)
MAX (1)
92%
96%
98%
UNIT
FLAG/DELAY CONTROL (see Figure 46 for test circuit)
Regulator dropout
detector
threshold voltage
Low (Flag ON)
ISINK = 3 mA
VFSAT
IFL
Flag output saturation
voltage
0.3
TJ = 25°C
VDELAY = 0.5 V
Flag output leakage
current
VFLAG = 40 V
Delay pin threshold
voltage
Low (flag ON)
0.7
Over full operating
temperature range
1
VDELAY = 0.5 V
Delay pin saturation
Low (Flag ON)
μA
1.25
V
1.21
TJ = 25°C
V
0.3
V
High (flag OFF) and VOUT regulated
Delay pin source
current
V
1.29
V
3
6
μA
55
350
Over full operating
temperature range
400
mV
BIAS SUPPLY
IBS
Bias supply pin current
IQ
Operating quiescent
current
VBS = 2 V (6)
120
400
μA
VBS = 4.4 V (6)
4
10
mA
VBS = 4.4 V , Vin pin current (6)
1
2
mA
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7.10 Typical Characteristics
8
Figure 1. Normalized Output Voltage
Figure 2. Line Regulation
Figure 3. Efficiency
Figure 4. Switch Saturation Voltage
Figure 5. Switch Current Limit
Figure 6. Dropout Voltage
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Typical Characteristics (continued)
Figure 7. Quiescent Current
Figure 8. Standby Quiescent Current
Figure 9. Minimum Operating Supply Voltage
Figure 10. Feedback Pin Bias Current
Figure 11. Flag Saturation Voltage
Figure 12. Switching Frequency
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Typical Characteristics (continued)
10
Figure 13. Soft-Start
Figure 14. Shutdown/Soft-Start Current
Figure 15. Delay Pin Current
Figure 16. VIN and VBS Current vs VBS and Temperature
Figure 17. Soft-Start Response
Figure 18. Shutdown/Soft-Start Threshold Voltage
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8 Detailed Description
8.1 Overview
The LM2597 SIMPLE SWITCHER® regulator is an easy-to-use, nonsynchronous step-down DC-DC converter
with a wide input voltage range from 45 V to up to 60 V for the HV version. The regulatorm is capable of
delivering up to 0.5-A DC load current with excellent line and load regulation. These devices are available in
fixed output voltages of 3.3-V, 5-V, 12-V, and an adjustable output version. The family requires few external
components, and the pin arrangement was designed for simple, optimum PCB layout.
8.2 Functional Block Diagram
8.3 Feature Description
8.3.1 Bias Supply Feature
The bias supply (VBS) pin allows the internal circuitry of the LM2597 to be powered from a power source, other
than VIN, typically the output voltage. This feature can increase efficiency and lower junction temperatures under
some operating conditions. The greatest increase in efficiency occur with light load currents, high input voltage
and low output voltage (4 V to 12 V). See efficiency curves shown in Figure 19 and Figure 20. The curves with
solid lines are with the VBS pin connected to the regulated output voltage, while the curves with dashed lines are
with the VBS pin open.
The bias supply pin requires a minimum of approximately 3.5 V at room temperature (4 V at −40°C), and can be
as high as 30 V, but there is little advantage of using the bias supply feature with voltages greater than 15 V or
20 V. The current required for the VIN pin is typically 4 mA.
To use the bias supply feature with output voltages between 4 V and 15 V, wire the bias pin to the regulated
output. Because the VBS pin requires a minimum of 4 V to operate, the 3.3-V part cannot be used this way. When
the VBS pin is left open, the internal regulator circuitry is powered from the input voltage.
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Feature Description (continued)
Figure 19. Effects of Bias Supply Feature
on 5-V Regulator Efficiency
Figure 20. Effects of Bias Supply Feature
on 12-V Regulator Efficiency
8.3.2 SHUTDOWN and Soft-Start
The circuit shown in Figure 23 is a standard buck regulator with 24 VIN, 12 VOUT, 100-mA load, and using a
0.068-μF soft-start capacitor. The photo in Figure 21 and Figure 22 show the effects of soft-start on the output
voltage, the input current, with, and without a soft-start capacitor. Figure 21 also shows the error flag output
going high when the output voltage reaches 95% of the nominal output voltage. The reduced input current
required at start-up is very evident when comparing the two photos. The soft-start feature reduces the start-up
current from 700 mA down to 160 mA, and delays and slows down the output voltage rise time.
Figure 21. Output Voltage, Input Current,
and Error Flag Signal at Start-Up With Soft-start
Figure 22. Output Voltage and Input Current
at Start-Up Without Soft-start
This reduction in start-up current is useful in situations where the input power source is limited in the amount of
current it can deliver. In some applications, soft start can be used to replace undervoltage lockout or delayed
start-up functions.
If a very slow output voltage ramp is desired, the soft-start capacitor can be made much larger. Many seconds or
even minutes are possible.
If only the shutdown feature is required, the soft-start capacitor can be eliminated.
12
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Feature Description (continued)
Figure 23. Typical Circuit Using Shutdown/Soft-Start and Error Flag Features
8.3.3 Inverting Regulator
The circuit in Figure 24 converts a positive input voltage to a negative output voltage with a common ground. The
circuit operates by bootstrapping the regulators ground pin to the negative output voltage, then grounding the
feedback pin, the regulator senses the inverted output voltage and regulates it.
Figure 24. Inverting –5-V Regulator With Shutdown and Soft-Start
This example uses the LM2597-5 to generate a –5-V output, but other output voltages are possible by selecting
other output voltage versions, including the adjustable version. Because this regulator topology can produce an
output voltage that is either greater than or less than the input voltage, the maximum output current greatly
depends on both the input and output voltage. The curve shown in Figure 25 provides a guide as to the amount
of output load current possible for the different input and output voltage conditions.
The maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and
this must be limited to a maximum of 40 V. In this example, when converting 20 V to −5 V, the regulator would
see 25 V between the input pin and ground pin. The LM2597 has a maximum input voltage rating of 40 V (60 V
for the LM2597HV).
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Feature Description (continued)
Figure 25. Maximum Load Current for Inverting Regulator Circuit
An additional diode is required in this regulator configuration. Diode D1 is used to isolate input voltage ripple or
noise from coupling through the CIN capacitor to the output, under light or no load conditions. Also, this diode
isolation changes the topology to closely resemble a buck configuration thus providing good closed loop stability.
A Schottky diode is recommended for low input voltages, (because of its lower voltage drop) but for higher input
voltages, a 1N4001 diode could be used.
Because of differences in the operation of the inverting regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 100-μH, 1-A inductor is the best choice. Capacitor
selection can also be narrowed down to just a few values. Using the values shown in Figure 24 will provide good
results in the majority of inverting designs.
This type of inverting regulator can require relatively large amounts of input current when starting up, even with
light loads. Input currents as high as the LM2597 current limit (approximately 0.8 A) are required for 1 ms or
more, until the output reaches its nominal output voltage. The actual time depends on the output voltage and the
size of the output capacitor. Input power sources that are current limited or sources that can not deliver these
currents without getting loaded down, may not work correctly. Because of the relatively high start-up currents
required by the inverting topology, the soft-start feature shown in Figure 24 is recommended.
Also shown in Figure 24 are several shutdown methods for the inverting configuration. With the inverting
configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but
is now at the negative output voltage. The shutdown methods shown accept ground referenced shutdown
signals.
8.3.4 Undervoltage Lockout
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.
Figure 26 shows an undervoltage lockout circuit for a buck configuration, while Figure 27 and Figure 28 are for
the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 26 uses a Zener
diode to establish the threshold voltage when the switcher begins operating. When the input voltage is less than
the Zener voltage, resistors R1 and R2 hold the Shutdown/Soft-start pin low, keeping the regulator in the
shutdown mode. As the input voltage exceeds the Zener voltage, the Zener conducts, pulling the Shutdown/Softstart pin high, allowing the regulator to begin switching. The threshold voltage for the undervoltage lockout
feature is approximately 1.5 V greater than the Zener voltage.
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Feature Description (continued)
Figure 26. Undervoltage Lockout for a Buck Regulator
Figure 27 and Figure 28 apply the same feature to an inverting circuit. Figure 27 features a constant threshold
voltage for turnon and turnoff (Zener voltage plus approximately 1 V). If hysteresis is required, the circuit in
Figure 28 has a turnon voltage which is different than the turnoff voltage. The amount of hysteresis is
approximately equal to the value of the output voltage. Since the SD/SS pin has an internal 7-V Zener clamp, R2
is required to limit the current into this pin to approximately 1 mA when Q1 is on.
Figure 27. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
Figure 28. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
8.3.5 Negative Voltage Charge Pump
Occasionally a low current negative voltage is required for biasing parts of a circuit. A simple method of
generating a negative voltage using a charge pump technique and the switching waveform present at the OUT
pin, is shown in Figure 29. This unregulated negative voltage is approximately equal to the positive input voltage
(minus a few volts), and can supply up to a 100 mA of output current. There is a requirement however, that there
be a minimum load of several hundred mA on the regulated positive output for the charge pump to work
correctly. Also, resistor R1 is required to limit the charging current of C1 to some value less than the LM2597
current limit (typically 800 mA).
This method of generating a negative output voltage without an additional inductor can be used with other
members of the SIMPLE SWITCHER family, using either the buck or boost topology.
Figure 29. Charge Pump for Generating a
Low Current, Negative Output Voltage
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8.4 Device Functional Modes
8.4.1 Discontinuous Mode Operation
The selection guide chooses inductor values suitable for continuous mode operation, but for low current
applications or high input voltages, a discontinuous mode design may be a better choice. Discontinuous mode
would use an inductor that would be physically smaller, and would need only one half to one third the inductance
value required for a continuous mode design. The peak switch and inductor currents are higher in a
discontinuous design, but at these low load currents (200 mA and below), the maximum switch current will still be
less than the switch current limit.
Discontinuous operation can have voltage waveforms that are considerably different than a continuous design.
The output pin (switch) waveform can have some damped sinusoidal ringing present. This ringing is normal for
discontinuous operation, and is not caused by feedback loop instabilities. In discontinuous operation, there is a
period of time where neither the switch nor the diode are conducting, and the inductor current has dropped to
zero. During this time, a small amount of energy can circulate between the inductor and the switch and diode
parasitic capacitance causing this characteristic ringing. Normally this ringing is not a problem, unless the
amplitude becomes great enough to exceed the input voltage, and even then, there is very little energy present
to cause damage.
Different inductor types or core materials produce different amounts of this characteristic ringing. Ferrite core
inductors have very little core loss and therefore produce the most ringing. The higher core loss of powdered iron
inductors produce less ringing. If desired, a series RC could be placed in parallel with the inductor to dampen the
ringing.
Figure 30. Post Ripple Filter Waveform
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9 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Soft-Start Capacitor, CSS
A capacitor on this pin provides the regulator with a soft-start feature (slow start-up). When the DC input voltage
is first applied to the regulator, or when the Shutdown/Soft-start pin is allowed to go high, a constant current
(approximately 5 μA begins charging this capacitor). As the capacitor voltage rises, the regulator goes through
four operating regions (see the bottom curve in Figure 31).
1. Regulator in Shutdown – When the SD/SS pin voltage is between 0 V and 1.3 V, the regulator is in
shutdown, the output voltage is zero, and the IC quiescent current is approximately 85 μA.
2. Regulator ON, but the output voltage is zero – With the SD/SS pin voltage between approximately 1.3 V and
1.8 V, the internal regulatory circuitry is operating, the quiescent current rises to approximately 5 mA, but the
output voltage is still zero. Also, as the 1.3-V threshold is exceeded, the soft-start capacitor charging current
decreases from 5 μA down to approximately 1.6 μA. This decreases the slope of capacitor voltage ramp.
3. Soft-start Region – When the SD/SS pin voltage is between 1.8 V and 2.8 V (at 25°C), the regulator is in a
soft-start condition. The switch (Pin 8) duty cycle initially starts out very low, with narrow pulses and gradually
get wider as the capacitor SD/SS pin ramps up towards 2.8 V. As the duty cycle increases, the output
voltage also increases at a controlled ramp up. See the center curve in Figure 31. The input supply current
requirement also starts out at a low level for the narrow pulses and ramp up in a controlled manner. This is a
very useful feature in some switcher topologies that require large start-up currents (such as the inverting
configuration) which can load down the input power supply.
NOTE
The lower curve shown in Figure 31 shows the soft-start region from 0% to 100%. This is
not the duty cycle percentage, but the output voltage percentage. Also, the soft-start
voltage range has a negative temperature coefficient associated with it. See the soft-start
curve in Electrical Characteristics.
4. Normal operation – Above 2.8 V, the circuit operates as a standard Pulse Width Modulated switching
regulator. The capacitor will continue to charge up until it reaches the internal clamp voltage of approximately
7 V. If this pin is driven from a voltage source, the current must limit to about 1 mA.
If the part is operated with an input voltage at or below the internal soft-start clamp voltage of approximately
7 V, the voltage on the SD/SS pin tracks the input voltage and can be disturbed by a step in the voltage. To
maintain proper function under these conditions, TI strongly recommends clamping the SD/SS pin externally
between the 3-V maximum soft-start threshold and the 4.5-V minimum input voltage. Figure 32 is an example
of an external 3.7 V (approximately) clamp that prevents a line-step related glitch but does not interfere with
the soft-start behavior of the device.
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Application Information (continued)
Figure 31. Soft-Start, Delay, Error, and Output
VIN
LM2597
5
Q1
SD/SS
CSS
Z1
3V
Figure 32. External 3.7-V Soft-Start Clamp
9.1.2 Delay Capacitor, CDELAY
Provides delay for the error flag output. See the upper curve in Figure 31, and also refer to timing diagrams in
Figure 33. A capacitor on this pin provides a time delay between the time the regulated output voltage (when it is
increasing in value) reaches 95% of the nominal output voltage, and the time the error flag output goes high. A 3μA constant current from the delay pin charges the delay capacitor resulting in a voltage ramp. When this voltage
reaches a threshold of approximately 1.3 V, the open collector error flag output (or power OK) goes high. This
signal can be used to indicate that the regulated output has reached the correct voltage and has stabilized.
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Application Information (continued)
If, for any reason, the regulated output voltage drops by 5% or more, the error output flag (Pin 1) immediately
goes low (internal transistor turns on). The delay capacitor provides very little delay if the regulated output is
dropping out of regulation. The delay time for an output that is decreasing is approximately a 1000 times less
than the delay for the rising output. For a 0.1-μF delay capacitor, the delay time would be approximately 50 ms
when the output is rising and passes through the 95% threshold, but the delay for the output dropping would only
be approximately 50 μs.
Figure 33. Timing Diagram for 5-V Output
9.1.2.1 RPULLUP
The error flag output, (or power OK) is the collector of a NPN transistor, with the emitter internally grounded. To
use the error flag, a pullup resistor to a positive voltage is required. The error flag transistor is rated up to a
maximum of 45 V and can sink approximately 3 mA. If the error flag is not used, it can be left open.
9.1.3 Input Capacitor, CIN
A low ESR aluminum or tantalum bypass capacitor is required between the input pin and ground pin. The
capacitor must be located near the regulator using short leads. This capacitor prevents large voltage transients
from appearing at the input, and provides the instantaneous current required each time the switch turns on.
The important parameters for the Input capacitor are the voltage rating and the RMS current rating. Because of
the relatively high RMS currents flowing in a input capacitor of a buck regulator, this capacitor must be chosen
for its RMS current rating rather than its capacitance or voltage ratings, although the capacitance value and
voltage rating are directly related to the RMS current rating.
The RMS current rating of a capacitor could be viewed as a power rating of the capacitor. The RMS current
flowing through the capacitors internal ESR produces power which causes the internal temperature of the
capacitor to rise. The RMS current rating of a capacitor is determined by the amount of current required to raise
the internal temperature approximately 10°C above an ambient temperature of 105°C. The ability of the capacitor
to dissipate this heat to the surrounding air will determine the amount of current the capacitor can safely sustain.
Capacitors that are physically large and have a large surface area will typically have higher RMS current ratings.
For a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the surrounding air, and therefore will have a higher RMS
current rating.
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Application Information (continued)
Figure 34. RMS Current Ratings for
Low ESR Electrolytic Capacitors (Typical)
Figure 35. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
The consequences of operating an electrolytic capacitor above the RMS current rating is a shortened operating
life. The higher temperature speeds up the evaporation of the electrolyte of the capacitor, resulting in eventual
failure.
Selecting an input capacitor requires consulting the manufacturers data sheet for maximum allowable RMS ripple
current. For a maximum ambient temperature of 40°C, a general guideline would be to select a capacitor with a
ripple current rating of approximately 50% of the DC load current. For ambient temperatures up to 70°C, a
current rating of 75% of the DC load current would be a good choice for a conservative design. The capacitor
voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher
voltage capacitor is required to satisfy the RMS current requirements.
Figure 34 shows the relationship between an electrolytic capacitor value, its voltage rating, and the RMS current
it is rated for. These curves were obtained from the Nichicon PL series of low-ESR, high-reliability electrolytic
capacitors designed for switching regulator applications. Other capacitor manufacturers offer similar types of
capacitors, but always check the capacitor data sheet.
Standard electrolytic capacitors typically have much higher ESR numbers, lower RMS current ratings and
typically have a shorter operating lifetime.
Because of their small size and excellent performance, surface-mount solid tantalum capacitors are often used
for input bypassing, but several precautions must be observed. A small percentage of solid tantalum capacitors
can short if the inrush current rating is exceeded. This can happen at turnon when the input voltage is suddenly
applied, and of course, higher input voltages produce higher inrush currents. Several capacitor manufacturers do
a 100% surge current testing on their products to minimize this potential problem. If high turnon currents are
expected, it may be necessary to limit this current by adding either some resistance or inductance before the
tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the RMS ripple
current rating must be sized to the load current.
9.1.4 Output Capacitor, COUT
An output capacitor is required to filter the output and provide regulator loop stability. Use a low impedance or
low ESR Electrolytic or solid tantalum capacitors designed for switching regulator applications. When selecting
an output capacitor, the important capacitor parameters are; the 100-kHz Equivalent Series Resistance (ESR),
the RMS ripple current rating, voltage rating, and capacitance value. For the output capacitor, the ESR value is
the most important parameter.
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Application Information (continued)
The output capacitor requires an ESR value that has an upper and lower limit. For low output ripple voltage, a
low ESR value is required. This value is determined by the maximum allowable output ripple voltage, typically 1%
to 2% of the output voltage. But if the ESR of the selected capacitor is extremely low, there is a possibility of an
unstable feedback loop, resulting in an oscillation at the output. Using the capacitors listed in the tables, or
similar types, will provide design solutions under all conditions.
If very low output ripple voltage (less than 15 mV) is required, see Output Voltage Ripple and Transients for a
post ripple filter.
An aluminum electrolytic capacitor's ESR value is related to the capacitance value and its voltage rating. In most
cases, Higher voltage electrolytic capacitors have lower ESR values (see Figure 35). Often, capacitors with much
higher voltage ratings are required to provide the low ESR values required for low output ripple voltage.
The output capacitor for many different switcher designs are often satisfied with only three or four different
capacitor values and several different voltage ratings. See Table 4 and Table 6 for typical capacitor values,
voltage ratings, and manufacturers capacitor types.
Electrolytic capacitors are not recommended for temperatures below −25°C. The ESR rises dramatically at cold
temperatures and typically rises 3X at −25°C and as much as 10X at −40°C (see Figure 36).
Solid tantalum capacitors have a much better ESR specifications for cold temperatures and are recommended
for temperatures below −25°C.
9.1.5 Catch Diode
Buck regulators require a diode to provide a return path for the inductor current when the switch turns off. This
must be a fast diode, located close to the LM2594 using short leads and short printed-circuit traces.
Because of their very fast switching speed and low forward voltage drop, Schottky diodes provide the best
performance, especially in low output voltage applications (5 V and lower). Ultra-fast recovery, or high-efficiency
rectifiers are also a good choice, but some types with an abrupt turnoff characteristic may cause instability or
EMI problems. Ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such
as the 1N4001 series are much too slow and must not be used.
Figure 36. Capacitor ESR Change vs Temperature
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Application Information (continued)
Table 1. Diode Selection Table
1-A DIODES
VR
SURFACE-MOUNT
SCHOTTKY
ULTRA FAST RECOVERY
ULTRA FAST RECOVERY
1N5817
20 V
SR102
All of these diodes are rated to
at least 60 V.
30 V
THROUGH-HOLE
SCHOTTKY
MBRS130
All of these diodes are rated to
at least 60 V.
1N5818
SR103
11DQ03
MBRS140
40 V
50 V
or more
10BQ040
MURS120
10BF10
1N5819
SR104
10MQ040
11DQ04
MBRS160
SR105
10BQ050
MBR150
10MQ060
11DQ05
MBRS1100
MBR160
10MQ090
SB160
SGL41-60
SS16
HER101
MUR120
11DF1
11DQ10
9.1.6 Inductor Selection
All switching regulators have two basic modes of operation: continuous and discontinuous. The difference
between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a
period of time in the normal switching cycle. Each mode has distinctively different operating characteristics,
which can affect the regulators performance and requirements. Most switcher designs will operate in the
discontinuous mode when the load current is low.
The LM2597 (or any of the SIMPLE SWITCHER family) can be used for both continuous or discontinuous modes
of operation.
In many cases the preferred mode of operation is the continuous mode. This mode offers greater output power,
lower peak switch, inductor, and diode currents, and can have lower output ripple voltage. However, the
continuous mode requires larger inductor values to keep the inductor current flowing continuously, especially at
low output load currents or high input voltages.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed. This guide
assumes that the regulator is operating in the continuous mode, and selects an inductor that will allow a peak-topeak inductor ripple current to be a certain percentage of the maximum design load current. This peak-to-peak
inductor ripple current percentage is not fixed, but is allowed to change as different design load currents are
selected (see Figure 37).
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Figure 37. (ΔIIND) Peak-to-Peak Inductor Ripple Current
(as a Percentage of the Load Current) vs Load Current
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size
can be kept relatively low.
When operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth
type of waveform (depending on the input voltage), with the average value of this current waveform equal to the
DC output load current.
Inductors are available in different styles such as pot core, toroid, E-core, bobbin core, and so forth, as well as
different core materials, such as ferrites and powdered iron. The least expensive, the bobbin, rod or stick core,
consists of wire wrapped on a ferrite bobbin. This type of construction makes for a inexpensive inductor;
however, because the magnetic flux is not completely contained within the core, it generates more ElectroMagnetic Interference (EMl). This magnetic flux can induce voltages into nearby printed-circuit traces, thus
causing problems with both the switching regulator operation and nearby sensitive circuitry, and can give
incorrect scope readings because of induced voltages in the scope probe (see Open Core Inductors).
The inductors listed in the selection chart include ferrite E-core construction for Schott, ferrite bobbin core for
Renco and Coilcraft, and powdered iron toroid for Pulse Engineering.
Exceeding the maximum current rating of an inductor may cause the inductor to overheat because of the copper
wire losses, or the core may saturate. If the inductor begins to saturate, the inductance decreases rapidly and the
inductor begins to look mainly resistive (the DC resistance of the winding). This can cause the switch current to
rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the DC output load current.
This can also result in overheating of the inductor and the LM2597. Different inductor types have different
saturation characteristics, so consider this when selecting an inductor.
The inductor manufacturers data sheets include current and energy limits to avoid inductor saturation.
For continuous mode operation, see the inductor selection graphs in Figure 38 through Figure 41
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Figure 38. LM2597xx 3.3-V Version
Figure 39. LM2597xx 5-V Version
Figure 40. LM2597xx 12-V Version
Figure 41. LM2597xx Adjustable Voltage Version
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Table 2. Inductor Manufacturers' Part Numbers
INDUCTANCE
(μH)
CURRENT
(A)
L1
220
L2
150
L3
SCHOTT
RENCO
PULSE ENGINEERING
COILCRAFT
THROUGHHOLE
SURFACEMOUNT
THROUGHHOLE
SURFACEMOUNT
THROUGHHOLE
SURFACEMOUNT
SURFACEMOUNT
0.18
67143910
67144280
RL-5470-3
RL1500-220
PE-53801
PE-53801-S
DO1608-224
0.21
67143920
67144290
RL-5470-4
RL1500-150
PE-53802
PE-53802-S
DO1608-154
100
0.26
67143930
67144300
RL-5470-5
RL1500-100
PE-53803
PE-53803-S
DO1608-104
L4
68
0.32
67143940
67144310
RL-1284-68
RL1500-68
PE-53804
PE-53804-S
DO1608-68
L5
47
0.37
67148310
67148420
RL-1284-47
RL1500-47
PE-53805
PE-53805-S
DO1608-473
L6
33
0.44
67148320
67148430
RL-1284-33
RL1500-33
PE-53806
PE-53806-S
DO1608-333
L7
22
0.6
67148330
67148440
RL-1284-22
RL1500-22
PE-53807
PE-53807-S
DO1608-223
L8
330
0.26
67143950
67144320
RL-5470-2
RL1500-330
PE-53808
PE-53808-S
DO3308-334
L9
220
0.32
67143960
67144330
RL-5470-3
RL1500-220
PE-53809
PE-53809-S
DO3308-224
L10
150
0.39
67143970
67144340
RL-5470-4
RL1500-150
PE-53810
PE-53810-S
DO3308-154
L11
100
0.48
67143980
67144350
RL-5470-5
RL1500-100
PE-53811
PE-53811-S
DO3308-104
L12
68
0.58
67143990
67144360
RL-5470-6
RL1500-68
PE-53812
PE-53812-S
DO1608-683
L13
47
0.7
67144000
67144380
RL-5470-7
RL1500-47
PE-53813
PE-53813-S
DO3308-473
L14
33
0.83
67148340
67148450
RL-1284-33
RL1500-33
PE-53814
PE-53814-S
DO1608-333
L15
22
0.99
67148350
67148460
RL-1284-22
RL1500-22
PE-53815
PE-53815-S
DO1608-223
L16
15
1.24
67148360
67148470
RL-1284-15
RL1500-15
PE-53816
PE-53816-S
DO1608-153
L17
330
0.42
67144030
67144410
RL-5471-1
RL1500-330
PE-53817
PE-53817-S
DO3316-334
L18
220
0.55
67144040
67144420
RL-5471-2
RL1500-220
PE-53818
PE-53818-S
DO3316-224
L19
150
0.66
67144050
67144430
RL-5471-3
RL1500-150
PE-53819
PE-53819-S
DO3316-154
L20
100
0.82
67144060
67144440
RL-5471-4
RL1500-100
PE-53820
PE-53820-S
DO3316-104
L21
68
0.99
67144070
67144450
RL-5471-5
RL1500-68
PE-53821
PE-53821-S
DDO3316-683
L26
330
0.8
67144100
67144480
RL-5471-1
—
PE-53826
PE-53826-S
—
L27
220
1
67144110
67144490
RL-5471-2
—
PE-53827
PE-53827-S
—
9.1.7 Output Voltage Ripple and Transients
The output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple
voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth
waveform.
The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output
capacitor. A typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. To
obtain low ripple voltage, the ESR of the output capacitor must be low; however, caution must be exercised when
using extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If
very low output ripple voltage is required (less than 15 mV), TI recommends a post ripple filter (see Figure 46).
The inductance required is typically between 1 μH and 5 μH, with low DC resistance, to maintain good load
regulation. A low ESR output filter capacitor is also required to assure good dynamic load response and ripple
reduction. The ESR of this capacitor may be as low as desired, because it is out of the regulator feedback loop.
Figure 30 shows a typical output ripple voltage, with and without a post ripple filter.
When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground
connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto
the regulator board, preferably at the output capacitor. This provides a very short scope ground, thus eliminating
the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much
cleaner and more accurate picture of the ripple voltage waveform.
The voltage spikes are caused by the fast switching action of the output switch, the diode, the parasitic
inductance of the output filter capacitor, and its associated wiring. To minimize these voltage spikes, the output
capacitor must be designed for switching regulator applications, and the lead lengths must be kept very short.
Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute
to the amplitude of these spikes.
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Figure 42. Peak-to-Peak Inductor
Ripple Current vs Load Current
When a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a
triangular to a sawtooth type of waveform (depending on the input voltage). For a given input and output voltage,
the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current increases or
decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this
current waveform is equal to the DC load current.
If the load current drops to a low enough level, the bottom of the sawtooth current waveform reaches zero, and
the switcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher
designs (regardless how large the inductor value is) will be forced to run discontinuous if the output is lightly
loaded. This is a perfectly acceptable mode of operation.
In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current (ΔIIND) can be
useful for determining a number of other circuit parameters. Parameters such as, peak inductor or peak switch
current, minimum load current before the circuit becomes discontinuous, output ripple voltage and output
capacitor ESR can all be calculated from the peak-to-peak ΔIIND. When the inductor nomographs shown in
Figure 38 through Figure 41 are used to select an inductor value, the peak-to-peak inductor ripple current can
immediately be determined. Figure 42 shows the range of (ΔIIND) that can be expected for different load currents.
Figure 42 also shows how the peak-to-peak inductor ripple current (ΔIIND) changes as the designer goes from the
lower border to the upper border (for a given load current) within an inductance region. The upper border
represents a higher input voltage, while the lower border represents a lower input voltage (see Inductor
Selection).
These curves are only correct for continuous mode operation, and only if the inductor selection guides are used
to select the inductor value
Consider the following example:
VOUT = 5 V, maximum load current of 300 mA
VIN = 15 V (nominal), varying between 11 V and 20 V
The selection guide in Figure 39 shows that the vertical line for a 0.3-A load current and the horizontal line for the
15-V input voltage intersect approximately midway between the upper and lower borders of the 150-μH
inductance region. A 150-μH inductor allows a peak-to-peak inductor current (ΔIIND) to flow a percentage of the
maximum load current. Referring to Figure 42, follow the 0.3-A line approximately midway into the inductance
region, and read the peak-to-peak inductor ripple current (ΔIIND) on the left hand axis (approximately 150 mAp-p).
As the input voltage increases to 20 V, it approaches the upper border of the inductance region, and the inductor
ripple current increases. Figure 42 shows that for a load current of 0.3 A, the peak-to-peak inductor ripple current
(ΔIIND) is 150 mA with 15 VIN, and can range from 175 mA at the upper border (20 VIN) to 120 mA at the lower
border (11 VIN).
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Once the ΔIIND value is known, the following formulas can be used to calculate additional information about the
switching regulator circuit.
1. Peak Inductor or peak switch current
2. Minimum load current before the circuit becomes discontinuous
3. Output Ripple Voltage
–
= (ΔIIND) × (ESR of COUT)
–
= 0.150 A × 0.240 Ω = 36 mVp-p
– or
4. ESR of COUT
9.1.8 Open Core Inductors
Another possible source of increased output ripple voltage or unstable operation is from an open-core inductor.
Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to
the other end. These magnetic lines of flux will induce a voltage into any wire or PCB copper trace that comes
within the magnetic field of the inductor. The strength of the magnetic field, the orientation and location of the PC
copper trace to the magnetic field, and the distance between the copper trace and the inductor determine the
amount of voltage generated in the copper trace. Another way of looking at this inductive coupling is to consider
the PCB copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. Many
millivolts can be generated in a copper trace located near an open-core inductor, which can cause stability
problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is used, it is possible that the location of the inductor
with respect to other PC traces may be the problem. To determine if this is the problem, temporarily raise the
inductor away from the board by several inches and then check circuit operation. If the circuit now operates
correctly, then the magnetic flux from the open-core inductor is causing the problem. Substituting a closed-core
inductor such as a torroid or E-core will correct the problem, or re-arranging the PC layout may be necessary.
Magnetic flux cutting the IC device ground trace, feedback trace, or the positive or negative traces of the output
capacitor must be minimized.
Sometimes, placing a trace directly beneath a bobbin inductor will provide good results, provided it is exactly in
the center of the inductor (because the induced voltages cancel themselves out). However, problems could arise
if the trace is off center. If flux problems are present, even the direction of the inductor winding can make a
difference in some circuits.
This discussion on open-core inductors is not to frighten users, but to alert them on what kind of problems to
watch out for. Open-core bobbin or stick inductors are an inexpensive, simple way of making a compact, efficient
inductor, and they are used by the millions in many different applications.
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9.2 Typical Applications
9.2.1 Series Buck Regulator (Fixed Output)
Component values shown are for VIN = 15 V, VOUT = 5 V, ILOAD = 500 mA
CIN – 47-μF, 50-V, Aluminum Electrolytic Nichico L Series
COUT – 120-μF, 25-V Aluminum Electrolytic, Nichicon PL Series
D1 – 1–A, 30–V Schottky Rectif 1N5818
L1 – 100-μH, L20
Typical Values
CSS – 0.1 μF
CDELAY – 0.1 μF
RPull Up – 4.7k
*Use bias supply pin for 5-V and 12-V versions
Figure 43. Fixed Output Voltage Versions
9.2.1.1 Design Requirements
Table 3 lists the design parameters for this application example.
Table 3. Design Parameters
PARAMETER
EXAMPLE VALUE
Regulated output voltage (3.3 V, 5 V or 12 V), VOUT
5V
Maximum DC input voltage, VIN(max)
12 V
Maximum load current, ILOAD(max)
0.4 A
9.2.1.2 Detailed Design Procedure
9.2.1.2.1 Inductor Selection (L1)
1. Select the correct inductor value selection guide from Figure 38, Figure 39, or Figure 40 (output voltages of
3.3 V, 5 V, or 12 V respectively). For all other voltages, see the design procedure for the adjustable version.
Use the inductor selection guide for the 5-V version shown in Figure 39.
2. From the inductor value selection guide, identify the inductance region intersected by the maximum input
voltage line and the maximum load current line. Each region is identified by an inductance value and an
inductor code (LXX).
From the inductor value selection guide shown in Figure 39, the inductance region intersected by the 12-V
horizontal line and the 0.4-A vertical line is 100 μH, and the inductor code is L20.
3. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2.
The inductance value required is 100 μH. See row L20 of Table 2 and choose an inductor part number from
any of the four manufacturers shown. (In most instance, both through-hole and surface-mount inductors are
available.)
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9.2.1.2.2 Output Capacitor Selection (COUT)
1. In the majority of applications, low ESR (Equivalent Series Resistance) electrolytic capacitors between 82 μF
and 220 μF and low ESR solid tantalum capacitors between 15 μF and 100 μF provide the best results. This
capacitor must be located close to the IC using short capacitor leads and short copper traces. Do not use
capacitors larger than 220 μF.
For additional information, see Output Capacitor, COUT.
2. To simplify the capacitor selection procedure, see Table 4 for quick design component selection. This table
contains different input voltages, output voltages, and load currents, and lists various inductors and output
capacitors that will provide the best design solutions.
From Table 4, locate the 5-V output voltage section. In the load current column, choose the load current line
that is closest to the current required for the application; for this example, use the 0.5-A line. In the maximum
input voltage column, select the line that covers the input voltage required for the application; in this
example, use the 15-V line. The rest of this line shows the recommended inductors and capacitors that will
provide the best overall performance.
The capacitor list contains both through-hole electrolytic and surface-mount tantalum capacitors from four
different capacitor manufacturers. TI recommends using both the manufacturers and the manufacturer's
series that are listed in Table 4.
In this example aluminum electrolytic capacitors from several different manufacturers are available with the
range of ESR numbers required:
120-µF, 25-V Panasonic HFQ Series
120-µF, 25-V Nichicon PL Series
3. The capacitor voltage rating for electrolytic capacitors must be at least 1.5 times greater than the output
voltage, and often require much higher voltage ratings to satisfy the low ESR requirements for low output
ripple voltage.
For a 5-V output, a capacitor voltage rating at least 7.5 V or more is required. But, in this example, even a
low ESR, switching grade, 120-μF, 10-V aluminum electrolytic capacitor would exhibit approximately 400 mΩ
of ESR (see Figure 35). This amount of ESR would result in relatively high output ripple voltage. To reduce
the ripple to 1% of the output voltage, or less, a capacitor with a higher voltage rating (lower ESR) must be
selected. A 16-V or 25-V capacitor will reduce the ripple voltage by approximately half.
9.2.1.2.3 Catch Diode Selection (D1)
1. The catch diode current rating must be at least 1.3 times greater than the maximum load current. Also, if the
power supply design must withstand a continuous output short, the diode must have a current rating equal to
the maximum current limit of the LM2597. The most stressful condition for this diode is an overload or
shorted output condition.
See Table 1. In this example, a 1-A, 20-V, 1N5817 Schottky diode provides the best performance, and will
not be overstressed even for a shorted output.
2. The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
3. This diode must be fast (short reverse recovery time) and must be located close to the LM2597 using short
leads and short printed-circuit traces. Because of their fast switching speed and low forward voltage drop,
Schottky diodes provide the best performance and efficiency, and must be the first choice, especially in low
output voltage applications. Ultra-fast recovery, or high-efficiency rectifiers also provide good results. Ultrafast recovery diodes typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001
must not be used because they are too slow.
9.2.1.2.4 Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is required between the input pin and ground to prevent large
voltage transients from appearing at the input. In addition, the RMS current rating of the input capacitor must be
selected to be at least ½ the DC load current. The capacitor manufacturers data sheet must be checked to
assure that this current rating is not exceeded. Figure 34 shows typical RMS current ratings for several different
aluminum electrolytic capacitor values.
This capacitor must be located close to the IC using short leads and the voltage rating must be approximately 1.5
times the maximum input voltage.
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If solid tantalum input capacitors are used, TI recommends the capacitors are surge-current tested by the
manufacturer.
Exercise caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the
VIN pin.
The important parameters for the Input capacitor are the input voltage rating and the RMS current rating. With a
nominal input voltage of 12 V, an aluminum electrolytic capacitor with a voltage rating greater than 18 V (1.5 ×
VIN) is necessary. The next higher capacitor voltage rating is 25 V.
The RMS current rating requirement for the input capacitor in a buck regulator is approximately ½ the DC load
current. In this example, with a 400-mA load, a capacitor with a RMS current rating of at least 200 mA is
required. Figure 34 shows curves that can be used to select an appropriate input capacitor. From these curves,
locate the 25-V line and note which capacitor values have RMS current ratings greater than 200 mA. Either a 47μF or 68-μF, 25-V capacitor could be used.
For a through-hole design, a 68-μF or 25-V electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or
equivalent) would be adequate. Other types or other manufacturers' capacitors can be used provided the RMS
ripple current ratings are adequate.
For surface-mount designs, solid tantalum capacitors are recommended. The TPS series available from AVX,
and the 593D series from Sprague are both surge current tested.
For additional information, see Input Capacitor, CIN.
Table 4. LM2597xx Fixed Voltage Quick Design Component Selection Table
CONDITIONS
VOLTAGE CURRENT
OUTPUT
LOAD
(V)
(A)
0.5
3.3
0.2
0.5
5
0.2
0.5
12
0.2
30
OUTPUT CAPACITOR
INDUCTOR
VOLTAGE
MAX INPUT
(V)
THROUGH-HOLE
INDUCTANCE INDUCTOR
(μH)
(#)
SURFACE-MOUNT
PANASONIC
HFQ SERIES
(μF/V)
NICHICON
PL SERIES
(μF/V)
AVX TPS
SERIES
(μF/V)
SPRAGUE
595D SERIES
(μF/V)
5
33
L14
220/16
220/16
100/16
100/6.3
7
47
L13
120/25
120/25
100/16
100/6.3
10
68
L21
120/25
120/25
100/16
100/6.3
40
100
L20
120/35
120/35
100/16
100/6.3
6
68
L4
120/25
120/25
100/16
100/6.3
10
150
L10
120/16
120/16
100/16
100/6.3
40
220
L9
120/16
120/16
100/16
100/6.3
8
47
L13
180/16
180/16
100/16
33/25
10
68
L21
180/16
180/16
100/16
33/25
15
100
L20
120/25
120/25
100/16
33/25
40
150
L19
120/25
120/25
100/16
33/25
9
150
L10
82/16
82/16
100/16
33/25
20
220
L9
120/16
120/16
100/16
33/25
40
330
L8
120/16
120/16
100/16
33/25
15
68
L21
82/25
82/25
100/16
15/25
18
150
L19
82/25
82/25
100/16
15/25
30
220
L27
82/25
82/25
100/16
15/25
40
330
L26
82/25
82/25
100/16
15/25
15
100
L11
82/25
82/25
100/16
15/25
20
220
L9
82/25
82/25
100/16
15/25
40
330
L17
82/25
82/25
100/16
15/25
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9.2.1.3 Application Curves
Discontinuous mode switching waveforms
VIN = 20 V, VOUT = 5 V, ILOAD = 200 mA
L = 33 μH, COUT = 220 μF, COUT ESR = 60 mΩ
A: Output pin voltage, 10 V/div.
B: Inductor current, 0.2 A/div.
C: Output ripple voltage, 20 mV/div.
Figure 44. Horizontal Time Base: 2 μs/div
Load transient response for discontinuous mode
VIN = 20 V, VOUT = 5 V, ILOAD = 100 mA to 200 mA
L = 33 μH, COUT = 220 μF, COUT ESR = 60 mΩ
A: Output voltage, 50 mV/div. (AC)
B: 100-mA to 200-mA load pulse
Figure 45. Horizontal Time Base: 200 μs/div
9.2.2 Series Buck Regulator (Adjustable Output)
Select R1 to be approximately 1 kΩ, use a 1% resistor for best
stability.
Component values shown are for VIN = 20 V,
VOUT = 10 V, ILOAD = 500 mA.
CIN – 68-μF, 35-V Aluminum Electrolytic, Nichicon PL Series
COUT – 120-μF, 25-V Aluminum Electrolytic, Nichicon PL Series
D1 – –1-A, 0-V Schottky Rectifier, 1N5818
L1 – 150-μH, L19
R1 – 1 kΩ, 1%
R2 – 7.15k, 1%
CFF – 3.3 nF
Typical Values
CSS – 0.1 μF
CDELAY – 0.1 μF
RPULL UP – 4.7k
*For output voltages between 4 V and 20 V
Figure 46. Adjustable Output Voltage Versions
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9.2.2.1 Design Requirements
Table 5 lists the design parameters for this application example.
Table 5. Design Parameters
PARAMETER
EXAMPLE VALUE
Regulated output voltage, VOUT
20 V
Maximum input voltage, VIN(max)
28 V
Maximum load current, ILOAD(max)
0.5 A
Switching frequency, F
Fixed at a nominal 150 kHz
9.2.2.2 Detailed Design Procedure
9.2.2.2.1 Programming Output Voltage
Select R1 and R2, as shown in Figure 46.
Use Equation 1 to select the appropriate resistor values.
(1)
Select R1 with Equation 2 to be 1 kΩ, 1%. Solve for R2.
(2)
Select a value for R1 with Equation 3 between 240 Ω and 1.5 kΩ. The lower resistor values minimize noise
pickup in the sensitive feedback pin. (For the lowest temperature coefficient and the best stability with time, use
1% metal film resistors.)
(3)
R2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.
R2 = 15.4 kΩ.
9.2.2.2.2 Inductor Selection (L1)
1. Calculate the inductor Volt microsecond constant E • T (V • μs) with Equation 4.
where
•
•
VSAT = internal switch saturation voltage = 0.9 V
VD = diode forward voltage drop = 0.5 V
(4)
Calculate the inductor Volt • microsecond constant (E • T) with Equation 5.
(5)
2. Use the E • T value from the previous formula and match it with the E • T number on the vertical axis of the
Inductor Value Selection Guide shown in Figure 41.
E • T = 35.2 (V • μs)
3. On the horizontal axis, select the maximum load current.
ILOAD(max) = 0.5 A
4. Identify the inductance region intersected by the E • T value and the Maximum Load Current value. Each
region is identified by an inductance value and an inductor code (LXX).
From the inductor value selection guide shown in Figure 41, the inductance region intersected by the 35 (V •
μs) horizontal line and the 0.5A vertical line is 150 μH, and the inductor code is L19.
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5. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2.
From Table 2, locate line L19, and select an inductor part number from the list of manufacturers' part
numbers.
9.2.2.2.3 Output Capacitor Selection (COUT)
1. In the majority of applications, low ESR electrolytic or solid tantalum capacitors between 82 μF and 220 μF
provide the best results. This capacitor must be located close to the IC using short capacitor leads and short
copper traces. Do not use capacitors larger than 220 μF. For additional information, see Output Capacitor,
COUT.
2. To simplify the capacitor selection procedure, see Table 6 for a quick design guide. This table contains
different output voltages, and lists various output capacitors that will provide the best design solutions.
From Table 6, locate the output voltage column. From that column, locate the output voltage closest to the
output voltage in your application. In this example, select the 24-V line. In Output Capacitor, COUT, select a
capacitor from the list of through hole electrolytic or surface mount tantalum types from four different
capacitor manufacturers. TI recommends that both the manufacturers and the manufacturers series that are
listed in Table 6 be used.
In this example, through-hole aluminum electrolytic capacitors from several different manufacturers are
available.
82-µF, 50-V Panasonic HFQ Series
120-µF, 50-V Nichicon PL Series
3. The capacitor voltage rating must be at least 1.5 times greater than the output voltage, and often much
higher voltage ratings are required to satisfy the low ESR requirements required for low-output ripple voltage.
For a 20-V output, a capacitor rating of at least 30 V or more is required. In this example, either a 35-V or
50-V capacitor would work. A 50-V rating was chosen because it has a lower ESR which provides a lower
output ripple voltage.
Other manufacturers or other types of capacitors may also be used, provided the capacitor specifications
(especially the 100-kHz ESR) closely match the types listed in Table 6. Refer to the capacitor manufacturers'
data sheet for this information.
9.2.2.2.4 Feedforward Capacitor (CFF)
For output voltages greater than approximately 10 V, an additional capacitor is required. The compensation
capacitor is typically between 50 pF and 10 nF, and is wired in parallel with the output voltage setting resistor,
R2. It provides additional stability for high output voltages, low input or output voltages, or very low ESR output
capacitors, such as solid tantalum capacitors calculated with Equation 6 (see Figure 46).
(6)
This capacitor type can be ceramic, plastic, silver mica, and so forth (because of the unstable characteristics of
ceramic capacitors made with Z5U material are not recommended.)
Table 6 contains feedforward capacitor values for various output voltages. In this example, a 1-nF capacitor is
required.
9.2.2.2.5 Catch Diode Selection (D1)
1. The catch diode current rating must be at least 1.3 times greater than the maximum load current. Also, if the
power supply design must withstand a continuous output short, the diode must have a current rating equal to
the maximum current limit of the LM2597. The most stressful condition for this diode is an overload or
shorted output condition.
Schottky diodes provide the best performance, and in this example a 1-A, 40-V, 1N5819 Schottky diode is a
good choice. The 1-A diode rating is more than adequate and will not be overstressed even for a shorted
output (see Table 1).
2. The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
3. This diode must be fast (short reverse recovery time) and must be placed close to the LM2597 using short
leads and short-printed circuit traces. Because of their fast switching speed and low forward voltage drop,
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Schottky diodes provide the best performance and efficiency, and must be the first choice, especially in low
output voltage applications. Ultra-fast recovery or high-efficiency rectifiers are also good choices, but some
types with an abrupt turnoff characteristic may cause instability or EMl problems. Ultra-fast recovery diodes
typically have reverse recovery times of 50 ns or less. Rectifiers such as the 1N4001 series must not be
used because they are too slow.
9.2.2.2.6 Input Capacitor (CIN)
A low-ESR aluminum or tantalum bypass capacitor is required between the input pin and ground to prevent large
voltage transients from appearing at the input. In addition, the RMS current rating of the input capacitor must be
selected to be at least ½ the DC load current. The capacitor manufacturers data sheet must be checked to
assure that this current rating is not exceeded. Figure 34 shows typical RMS current ratings for several different
aluminum electrolytic capacitor values.
This capacitor must be located close to the IC using short leads and the voltage rating must be approximately 1.5
times the maximum input voltage.
If solid tantalum input capacitors are used, TI recommend the capacitors are surge-current tested by the
manufacturer.
CAUTION
Use caution when using ceramic capacitors for input bypassing, because it may cause
severe ringing at the VIN pin.
The important parameters for the input capacitor are the input voltage rating and the RMS current rating. With a
nominal input voltage of 28 V, an aluminum electrolytic aluminum electrolytic capacitor with a voltage rating
greater than 42 V (1.5 × VIN) is required. Because the next higher capacitor voltage rating is 50 V, a 50-V
capacitor must be used. The capacitor voltage rating of (1.5 × VIN) is a conservative guideline, and can be
modified somewhat if desired.
The RMS current rating requirement for the input capacitor of a buck regulator is approximately ½ the DC load
current. In this example, with a 400-mA load, a capacitor with a RMS current rating of at least 200 mA is
required.
Figure 34 shows curves that can be used to select an appropriate input capacitor. From the curves, locate the
50-V line and note which capacitor values have RMS current ratings greater than 200 mA. A 47-μF, 50-V, lowESR electrolytic capacitor is required.
For a through hole design, a 47-μF, 50-V electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or
equivalent) would be adequate. Other types or other manufacturers' capacitors can be used provided the RMS
ripple current ratings are adequate.
For surface-mount designs, solid tantalum capacitors are recommended. The TPS series available from AVX,
and the 593D series from Sprague are both surge current tested.
For additional information, see Input Capacitor, CIN.
Table 6. Output Capacitor and Feedforward Capacitor Selection Table
THROUGH-HOLE OUTPUT CAPACITOR
OUTPUT
VOLTAGE
(V)
34
PANASONIC
NICHICON PL
HFQ SERIES
(μF/V)
SERIES
(μF/V)
1.2
220/25
4
180/25
6
9
SURFACE-MOUNT OUTPUT CAPACITOR
FEEDFORWARD
AVX TPS
SPRAGUE
FEEDFORWARD
CAPACITOR
SERIES
(μF/V)
595D SERIES
(μF/V)
CAPACITOR
220/25
0
220/10
220/10
0
180/25
4.7 nF
100/10
120/10
4.7 nF
82/25
82/25
4.7 nF
100/10
120/10
4.7 nF
82/25
82/25
3.3 nF
100/16
100/16
3.3 nF
12
82/25
82/25
2.2 nF
100/16
100/16
2.2 nF
15
82/25
82/25
1.5 nF
68/20
100/20
1.5 nF
24
82/50
120/50
1 nF
10/35
15/35
220 pF
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Table 6. Output Capacitor and Feedforward Capacitor Selection Table (continued)
THROUGH-HOLE OUTPUT CAPACITOR
OUTPUT
VOLTAGE
(V)
SURFACE-MOUNT OUTPUT CAPACITOR
PANASONIC
NICHICON PL
FEEDFORWARD
AVX TPS
SPRAGUE
FEEDFORWARD
HFQ SERIES
(μF/V)
SERIES
(μF/V)
CAPACITOR
SERIES
(μF/V)
595D SERIES
(μF/V)
CAPACITOR
82/50
120/50
820 pF
10/35
15/35
220 pF
28
9.2.2.3 Application Curves
Continuous mode switching waveforms
VIN = 20 V, VOUT = 5 V, ILOAD = 400 mA
L = 100 μH, COUT = 120 μF, COUT ESR = 140 mΩ
A: Output pin voltage, 10 V/div.
B: Inductor current, 0.2 A/div.
C: Output ripple voltage, 20 mV/div.
Figure 47. Horizontal Time Base: 2 μs/div
Load transient response for continuous mode
VIN = 20 V, VOUT = 5 V, ILOAD = 200 mA to 500 mA
L = 100 μH, COUT = 120 μF, COUT ESR = 140 mΩ
A: Output voltage, 50 mV/div. (AC)
B: 200-mA to 500-mA load pulse
Figure 48. Horizontal Time Base: 50 μs/div
10 Power Supply Recommendations
The LM2597 is designed to operate from an input voltage supply up to 45 V and 60 V (HV version). This input
supply must be well regulated and able to withstand maximum input current and maintain a stable voltage.
11 Layout
11.1 Layout Guidelines
As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring
inductance can generate voltage transients which can cause problems. For minimal inductance and ground
loops, the wires indicated by heavy lines must be wide printed-circuit traces and must be kept as short as
possible. For best results, external components must be placed as close to the switcher lC as possible using
ground plane construction or single point grounding.
If open core inductors are used, take care of the location and positioning of this type of inductor. Allowing the
inductor flux to intersect sensitive feedback, lC groundpath and COUT wiring can cause problems.
When using the adjustable version, take special care regarding the location of the feedback resistors and the
associated wiring. Physically place both resistors near the IC, and route the wiring away from the inductor,
especially an open core type of inductor.
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LM2597, LM2597HV
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11.2 Layout Example
CIN – 10-µF, 35-V solid tantalum, AVX, TPS Series (surface-mount, D size)
COUT – 100-µF, 10-V solid tantalum, AVX, TPS Series (surface-mount, D size)
D1 – 1-A, 40-V surface-mount, Schottky rectifier
L1 – Surface-mount inductor, Coilcraft DO33
CSS – Soft-start capacitor (surface-mount, ceramic chip capacitor)
CD – Delay capacitor (surface-mount, ceramic chip capacitor)
R3 – Error flag pullup resistor (surface-mount, chip resistor)
Figure 49. Typical Surface-Mount PCB Layout, Fixed Output (2X Size)
CIN – 10-μF, 35-V solid tantalum, AVX, TPS Series (surface-mount, D size)
COUT – 68-μF, 20-V solid tantalum, AVX, TPS Series (surface-mount, D size)
D1 – 1-A, 40-V Surface-mount, Schottky rectifier
L1 – Surface-mount inductor, Coilcraft DO33
CSS – Soft-start capacitor (surface-mount, ceramic chip capacitor)
CD – Delay capacitor (surface-mount, ceramic chip capacitor)
CFF – Feedforward capacitor (surface-mount, ceramic chip capacitor)
R1 – Output voltage program resistor (surface-mount, chip resistor)
R2 – Output voltage program resistor (surface-mount, chip resistor)
R3 – Error flag pullup resistor (surface-mount, chip resistor)
Figure 50. Typical Surface-Mount PCB Layout, Adjustable Output (2X Size)
36
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LM2597, LM2597HV
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SNVS119D – MARCH 1998 – REVISED MAY 2016
11.3 Thermal Considerations
The LM2597xx is available in two packages: an 8-pin through-hole PDIP (P) and an 8-pin surface-mount SOIC
(D).Both packages are molded plastic with a copper lead frame. When the package is soldered to the PCB, the
copper and the board are the heat sink for the LM2597 and the other heat producing components.
For best thermal performance, wide copper traces must be used. Pins must be soldered to generous amounts of
printed-circuit board (PCB) copper, (one exception to this is the output (switch) pin, which must not have large
areas of copper). Large areas of copper provide the best transfer of heat (lower thermal resistance) to the
surrounding air, and even double-sided or multilayer boards provide a better heat path to the surrounding air.
Unless power levels are small, sockets are not recommended because of the added thermal resistance it adds
and the resultant higher junction temperatures.
Package thermal resistance and junction temperature rise numbers are all approximate, and there are many
factors that will affect the junction temperature. Some of these factors include board size, shape, thickness,
position, location, and even board temperature. Other factors are trace width, printed-circuit copper area, copper
thickness, single- or double-sided multilayer board, and the amount of solder on the board. The effectiveness of
the PCB to dissipate heat also depends on the size, quantity, and spacing of other components on the board.
Furthermore, some of these components such as the catch diode will add heat to the PCB and the heat can vary
as the input voltage changes. For the inductor, depending on the physical size, type of core material, and the DC
resistance, it could either act as a heat sink taking heat away from the board, or it could add heat to the board.
Circuit Data for Temperature Rise Curve
Capacitors
Through hole electrolytic
Inductor
Through hole, Schott, 100 μH
Diode
Through hole, 1-A, 40-V, Schottky
PCB
4 square inches single sided 2 oz. copper (0.0028″)
Figure 51. Junction Temperature Rise, 8-Pin PDIP
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Thermal Considerations (continued)
Circuit Data for Temperature Rise Curve (Surface Mount)
Capacitors
Through hole electrolytic
Inductor
Through hole, Schott, 100 μH
Diode
Through hole, 1-A, 40-V, Schottky
PCB
4 square inches single sided 2 oz. copper (0.0028″)
Figure 52. Junction Temperature Rise, 8-Pin SOIC
Figure 51 and Figure 52 show the LM2597 junction temperature rise above ambient temperature with a 500-mA
load for various input and output voltages. The Bias Supply pin was not used (left open) for these curves.
Connecting the Bias Supply pin to the output voltage would reduce the junction temperature by approximately
5°C to 15°C, depending on the input and output voltages, and the load current. This data was taken with the
circuit operating as a buck switcher with all components mounted on a PCB to simulate the junction temperature
under actual operating conditions. This curve is typical, and can be used for a quick check on the maximum
junction temperature for various conditions, but keep in mind that there are many factors that can affect the
junction temperature.
38
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LM2597, LM2597HV
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SNVS119D – MARCH 1998 – REVISED MAY 2016
12 Device and Documentation Support
12.1 Related Links
The table below lists quick access links. Categories include technical documents, support and community
resources, tools and software, and quick access to sample or buy.
Table 7. Related Links
PARTS
PRODUCT FOLDER
SAMPLE & BUY
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
LM2597
Click here
Click here
Click here
Click here
Click here
LM2597HV
Click here
Click here
Click here
Click here
Click here
12.2 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
12.3 Trademarks
E2E is a trademark of Texas Instruments.
SIMPLE SWITCHER is a registered trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
12.4 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
12.5 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
13 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
Copyright © 1998–2016, Texas Instruments Incorporated
Product Folder Links: LM2597 LM2597HV
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PACKAGE OPTION ADDENDUM
www.ti.com
13-Oct-2022
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
Samples
(4/5)
(6)
LM2597HVM-12/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
Call TI | SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-12
Samples
LM2597HVM-3.3/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-3.3
Samples
LM2597HVM-5.0/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-5.0
Samples
LM2597HVM-ADJ
NRND
SOIC
D
8
95
Non-RoHS
& Green
Call TI
Level-1-235C-UNLIM
-40 to 125
2597H
M-ADJ
LM2597HVM-ADJ/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-ADJ
Samples
LM2597HVMX-12/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-12
Samples
LM2597HVMX-3.3/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-3.3
Samples
LM2597HVMX-5.0/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-5.0
Samples
LM2597HVMX-ADJ/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
2597H
M-ADJ
Samples
LM2597HVN-12/NOPB
ACTIVE
PDIP
P
8
40
RoHS & Green
NIPDAU
Level-1-NA-UNLIM
-40 to 125
LM2597HV
N-12 P+
Samples
LM2597HVN-3.3/NOPB
ACTIVE
PDIP
P
8
40
RoHS & Green
Call TI | NIPDAU
Level-1-NA-UNLIM
-40 to 125
LM2597HV
N-3.3 P+
Samples
LM2597HVN-5.0/NOPB
ACTIVE
PDIP
P
8
40
RoHS & Green
Call TI | NIPDAU
Level-1-NA-UNLIM
-40 to 125
LM2597HV
N-5.0 P+
Samples
LM2597HVN-ADJ/NOPB
ACTIVE
PDIP
P
8
40
RoHS & Green
Call TI | NIPDAU
Level-1-NA-UNLIM
-40 to 125
LM2597HV
N-ADJ P+
Samples
LM2597M-12/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
2597
M-12
Samples
LM2597M-3.3/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
2597
M-3.3
Samples
LM2597M-5.0
NRND
SOIC
D
8
95
Non-RoHS
& Green
Call TI
Level-1-235C-UNLIM
2597
M-5.0
LM2597M-5.0/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
2597
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
Orderable Device
13-Oct-2022
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
Samples
(4/5)
(6)
M-5.0
LM2597M-ADJ/NOPB
ACTIVE
SOIC
D
8
95
RoHS & Green
SN
Level-1-260C-UNLIM
LM2597MX-12/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
SN
LM2597MX-3.3/NOPB
ACTIVE
SOIC
D
8
2500
RoHS & Green
LM2597MX-5.0/NOPB
ACTIVE
SOIC
D
8
2500
LM2597MX-ADJ/NOPB
ACTIVE
SOIC
D
8
LM2597N-12/NOPB
ACTIVE
PDIP
P
LM2597N-3.3/NOPB
ACTIVE
PDIP
LM2597N-5.0/NOPB
ACTIVE
LM2597N-ADJ/NOPB
ACTIVE
-40 to 125
2597
M-ADJ
Samples
Level-1-260C-UNLIM
2597
M-12
Samples
SN
Level-1-260C-UNLIM
2597
M-3.3
Samples
RoHS & Green
SN
Level-1-260C-UNLIM
2597
M-5.0
Samples
2500
RoHS & Green
SN
Level-1-260C-UNLIM
2597
M-ADJ
Samples
8
40
RoHS & Green
NIPDAU
Level-1-NA-UNLIM
LM2597N
-12 P+
Samples
P
8
40
RoHS & Green
NIPDAU
Level-1-NA-UNLIM
LM2597N
-3.3 P+
Samples
PDIP
P
8
40
RoHS & Green
NIPDAU
Level-1-NA-UNLIM
LM2597N
-5.0 P+
Samples
PDIP
P
8
40
RoHS & Green
NIPDAU
Level-1-NA-UNLIM
LM2597N
-ADJ P+
Samples
-40 to 125
-40 to 125
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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