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LM2598
SNVS125D – MARCH 1998 – REVISED MAY 2016
LM2598 SIMPLE SWITCHER® Power Converter 150-kHz 1-A Step-Down Voltage Regulator,
With Features
1 Features
3 Description
•
•
The LM2598 series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 1-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.
1
•
•
•
•
•
•
•
•
•
•
•
•
3.3-V, 5-V, 12-V, and Adjustable Output Versions
Adjustable Version Output Voltage Range, 1.2-V
to 37-V ±4% Max Over Line and Load Conditions
1-A Output Current
Available in 7-Pin TO-220 and DDPAK (Surface
Mount) Package
Input Voltage Range Up to 40 V
Excellent Line and Load Regulation Specifications
150 kHz Fixed Frequency Internal Oscillator
Shutdown/Soft-start
Out of Regulation Error Flag
Error Output Delay
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
The LM2598 is a member of the LM259x family.
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 LM2598 series operates at a switching frequency
of 150 kHz, thus allowing smaller sized filter
components than what would be required with lowerfrequency switching regulators. Available in a
standard 7-lead TO-220 package with several
different lead bend options, and a 7-lead DDPAK
surface mount package. Typically, for output voltages
less than 12 V, and ambient temperatures less than
50°C, no heat sink is required.
Device Information(1)
PART NUMBER
LM2598
PACKAGE
BODY SIZE (NOM)
TO-220 (7)
14.986 mm × 10.16 mm
TO-263 (7)
10.10 mm × 8.89 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.
LM2598
SNVS125D – MARCH 1998 – REVISED MAY 2016
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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
4
4
4
4
5
5
5
Absolute Maximum Ratings .....................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics – 3.3-V Version.................
Electrical Characteristics – 5-V Version....................
Electrical Characteristics – 12-V Version..................
Electrical Characteristics – Adjustable Voltage
Version .......................................................................
7.9 Electrical Characteristics – All Output Voltage
Versions .....................................................................
7.10 Typical Characteristics ............................................
8
6
6
8
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 Application .................................................. 28
10 Power Supply Recommendations ..................... 37
11 Layout................................................................... 37
11.1 Layout Guidelines ................................................. 37
11.2 Layout Examples................................................... 37
11.3 Thermal Considerations ........................................ 38
12 Device and Documentation Support ................. 40
12.1
12.2
12.3
12.4
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
40
40
40
40
13 Mechanical, Packaging, and Orderable
Information ........................................................... 40
Detailed Description ............................................ 11
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 .................................................................................. 38
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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 LM2598. This feature greatly simplifies the design of switch-mode
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.
6 Pin Configuration and Functions
NDZ Package
7-Pin TO-220
Top View
KTW Package
7-Pin TO-263
Top View
Pin Functions
PIN
I/O
DESCRIPTION
NO.
NAME
1
Output
O
Internal switch. The voltage at this pin switches between approximately (+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.
2
+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.
3
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. (1)
4
Ground
—
Circuit ground.
5
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. (1)
6
Feedback
I
Senses the regulated output voltage to complete the feedback loop.
7
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. (1)
(1)
If any of the above three features (Shutdown/Soft-start, Error Flag, or Delay) are not used, the respective pins must be left open.
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7 Specifications
7.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1) (2)
MAX
UNIT
Maximum supply voltage, VIN
MIN
45
V
SD/SS pin input voltage (3)
6
V
1.5
V
Delay pin voltage (3)
Flag pin voltage
–0.3
45
V
Feedback pin voltage
–0.3
25
V
–1
V
Output voltage to ground (steady state)
Power dissipation
Lead temperature
Internally limited
KTW package
Vapor phase (60 s)
215
Infrared (10 s)
245
NDZ package (soldering, 10 s)
Maximum junction temperature
Storage temperature, Tstg
(1)
(2)
(3)
°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.
Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
7.2 ESD Ratings
V(ESD)
(1)
(2)
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1) (2)
Electrostatic discharge
VALUE
UNIT
±2000
V
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
MIN
MAX
Supply voltage
4.5
40
UNIT
V
Temperature
–25
125
°C
7.4 Thermal Information
LM2598
THERMAL METRIC (1)
Junction-to-ambient thermal resistance (2) (3)
RθJA
RθJC(top)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
4
KTW (TO-263)
NDZ (TO-220)
7 PINS
7 PINS
See (4)
—
50
See (5)
50
—
See (6)
30
—
See (7)
20
—
2
2
Junction-to-case (top) thermal resistance
UNIT
°C/W
°C/W
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
The package thermal impedance is calculated in accordance to JESD 51-7.
Thermal Resistances were simulated on a 4 -layer, JEDEC board.
Junction to ambient thermal resistance (no external heat sink) for the package mounted TO-220 package mounted vertically, with the
leads soldered to a printed circuit board with (1 oz.) copper area of approximately 1 in2.
Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 0.5 in2 of (1
oz.) copper area.
Junction to ambient thermal resistance with the TO-263 package tab soldered to a single sided printed circuit board with 2.5 in2 of (1
oz.) copper area.
Junction to ambient thermal resistance with the TO-263 package tab soldered to a double sided printed circuit board with 3 in2 of (1 oz.)
copper area on the LM2598S side of the board, and approximately 16 in2 of copper on the other side of the PCB.
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7.5 Electrical Characteristics – 3.3-V Version
Specifications are for TJ = 25°C, unless otherwise specified.
PARAMETER
SYSTEM PARAMETERS
(3)
Output voltage
4.75 V ≤ VIN ≤ 40 V,
0.1 A ≤ ILOAD ≤ 1 A
η
Efficiency
VIN = 12 V, ILOAD = 1 A
(2)
(3)
TYP (2)
TJ = 25°C
3.168
3.3
Over full operating
temperature range
3.135
MAX
(1)
UNIT
(see Figure 42 and Figure 45 for test circuits)
VOUT
(1)
MIN (1)
TEST CONDITIONS
3.432
3.465
V
78%
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 LM2598 is used as shown in the Figure 42 and Figure 45, system performance is as shown in system parameters of Electrical
Characteristics.
7.6 Electrical Characteristics – 5-V Version
Specifications are for TJ = 25°C, unless otherwise specified.
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3)
TJ = 25°C
Output voltage
7 V ≤ VIN ≤ 40 V,
0.1 A ≤ ILOAD ≤ 1 A
η
Efficiency
VIN = 12 V, ILOAD = 1 A
(2)
(3)
TYP (2)
MAX (1)
4.8
5
5.2
UNIT
(see Figure 42 and Figure 45 for test circuits)
VOUT
(1)
MIN (1)
Over full operating
temperature range
4.75
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 LM2598 is used as shown in the Figure 42 and Figure 45, system performance is as shown in system parameters of Electrical
Characteristics.
7.7 Electrical Characteristics – 12-V Version
Specifications are for TJ = 25°C, unless otherwise specified.
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3)
TJ = 25°C
Output voltage
15 V ≤ VIN ≤ 40 V,
0.1 A ≤ ILOAD ≤ 1 A
η
Efficiency
VIN = 25 V, ILOAD = 1 A
(2)
(3)
TYP (2)
MAX (1)
11.52
12
12.48
UNIT
(see Figure 42 and Figure 45 for test circuits)
VOUT
(1)
MIN (1)
Over full operating
temperature range
11.4
12.6
V
90%
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 LM2598 is used as shown in the Figure 42 and Figure 45, system performance is as shown in system parameters of Electrical
Characteristics.
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7.8 Electrical Characteristics – Adjustable Voltage Version
Specifications are for TJ = 25°C, unless otherwise specified.
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3)
MIN (1)
Feedback voltage
Efficiency
η
(1)
(2)
(3)
MAX (1)
UNIT
(see Figure 42 and Figure 45 for test circuits)
4.5 V ≤ VIN ≤ 40 V, 0.1 A ≤ ILOAD ≤ 1 A
VFB
TYP (2)
VOUT programmed for 3 V, TJ = 25°C
circuit of Figure 42 and
Over full operating
Figure 45
temperature range
1.23
1.193
1.267
1.18
1.28
VIN = 12 V, VOUT = 3 V, ILOAD = 1 A
V
78%
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 LM2598 is used as shown in the Figure 42 and Figure 45, system performance is as shown in system parameters of Electrical
Characteristics.
7.9 Electrical Characteristics – All Output Voltage Versions
Specifications are for TJ = 25°C unless otherwise noted. Unless otherwise specified, VIN = 12 V for the 3.3-V, 5-V, and
Adjustable version and VIN = 24 V for the 12-V version. ILOAD = 500 mA
PARAMETER
TEST CONDITIONS
MIN (1)
TYP (2)
MAX (1)
10
50
UNIT
DEVICE PARAMETERS
TJ = 25°C
Ib
Feedback bias
current
Adjustable version only,
VFB = 1.3 V
fO
Oscillator frequency
See (3)
VSAT
Saturation voltage
IOUT = 1 A
Max duty cycle (ON)
See (5)
100%
DC
Minimum duty cycle
(OFF)
See (6)
0%
ICL
Current limit
Peak current (4) (5)
IL
Output leakage
current
Output = 0 V, see (4) (6) (7)
50
μA
Output = –1 V
2
15
mA
IQ
Operating quiescent
current
SD/SS pin open (6)
5
10
mA
Current standby
quiescent
85
200
ISTBY
SD/SS pin = 0 V (7)
Over full operating
temperature range
100
TJ = 25°C
127
Over full operating
temperature range
110
TJ = 25°C
(4) (5)
(2)
(3)
(4)
(5)
(6)
(7)
6
Over full operating
temperature range
TJ = 25°C
Over full operating
temperature range
173
173
1
Over full operating
temperature range
TJ = 25°C
(1)
150
1.5
1.15
kHz
1.2
1.3
1.2
nA
V
2.4
2.6
250
A
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.
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Electrical Characteristics – All Output Voltage Versions (continued)
Specifications are for TJ = 25°C unless otherwise noted. Unless otherwise specified, VIN = 12 V for the 3.3-V, 5-V, and
Adjustable version and VIN = 24 V for the 12-V version. ILOAD = 500 mA
PARAMETER
TEST CONDITIONS
MIN (1)
TYP (2)
MAX (1)
UNIT
SHUTDOWN AND SOFT-START CONTROL (see Figure 42 and Figure 45 for test circuits)
TJ = 25°C
VSD
Shutdown threshold
voltage
1.3
Low, (Shutdown Mode), over full operating temperature
range
High, (Soft-start Mode), over full operating temperature
range
0.6
2
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
V
V
5
10
μA
1.6
5
μA
96%
98%
FLAG AND DELAY CONTROL (see Figure 42 and Figure 45 for test circuits)
Regulator dropout
detector threshold
voltage
Low (Flag ON)
92%
ISINK = 3 mA
VFSAT
IFL
Voltage flag output
saturation
0.3
TJ = 25°C
VDELAY = 0.5 V
Flag output leakage
current
VFLAG = 40 V
Voltage delay pin
threshold
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
Over full operating
temperature range
1.29
3
6
55
350
400
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V
0.3
High (Flag OFF) and VOUT Regulated
Delay pin source
current
V
V
μA
mV
7
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7.10 Typical Characteristics
Circuit of Figure 45
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)
Circuit of Figure 45
Figure 7. Operating Quiescent Current
Figure 8. Shutdown 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)
Circuit of Figure 45
Figure 13. Soft-start
Figure 14. Shutdown/Soft-start Current
Figure 15. Delay Pin Current
Figure 16. Soft-start Response
Figure 17. Shutdown and Soft-start Threshold Voltage
10
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8 Detailed Description
8.1 Overview
The LM2598 SIMPLE SWITCHER® regulator is an easy-to-use, nonsynchronous, step-down DC-DC converter
with a wide input voltage range up to 40 V. The regulator is capable of delivering up to 1-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 SHUTDOWN and Soft-Start
The circuit shown in Figure 20 is a standard buck regulator with 24-VIN, 12-VOUT, 280-mA load, and using a
0.068-μF soft-start capacitor. The photo in Figure 18 and Figure 19 show the effects of Soft-start on the output
voltage, the input current, with, and without a soft-start capacitor. Figure 18 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 1 A down to 240 mA, and delays and slows down the output voltage rise time.
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.
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Feature Description (continued)
Figure 18. Output Voltage, Input Current, and Error Flag
Signal at Start-Up With Soft-start
Figure 19. Output Voltage and Input Current at Start-Up
Without Soft-start
Figure 20. Typical Circuit Using Shutdown/Soft-start and Error Flag Features
12
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Feature Description (continued)
Figure 21. Inverting –5-V Regulator With Shutdown and Soft-start
8.3.2 Inverting Regulator
The circuit in Figure 21 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.
This example uses the LM2598-5 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 22 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 LM2598 has a maximum input voltage rating of 40 V.
Figure 22. Maximum Load Current for Inverting
Regulator Circuit
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Feature Description (continued)
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 1N5400 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 68-μH, 1.5-A inductor is the best choice. Capacitor
selection can also be narrowed down to just a few values. Using the values shown in Figure 21 provides 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 LM2598 current limit (approximately 1.5 A) are required for 2 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 21 is recommended.
Also shown in Figure 21 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.
8.3.3 Undervoltage Lockout
Some applications require the regulator to remain off until the input voltage reaches a predetermined voltage.
Figure 23 shows an undervoltage lockout feature applied to a buck regulator, while Figure 24 and Figure 25 are
for the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). Figure 23 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 or 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.
Figure 23. Undervoltage Lockout for a Buck Regulator
Figure 24 and Figure 25 apply the same feature to an inverting circuit. Figure 24 features a constant threshold
voltage for turnon and turnoff (Zener voltage plus approximately 1 V). Because 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. If hysteresis is
required, the circuit in Figure 25 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.
14
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Feature Description (continued)
Figure 24. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
Figure 25. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
8.3.4 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 26. This unregulated negative voltage is approximately equal to the positive input voltage
(minus a few volts), and can supply up to a 200 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 LM2598
current limit (typically 1.5 A).
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 26. 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 require only one half to one third the
inductance value required for a continuous mode design. The peak switch and inductor currents is higher in a
discontinuous design, but at these low load currents (200 mA and below), the maximum switch current is still 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 (see Figure 46) 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 or 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 27. 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 28).
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 regulator 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 Softstart condition. The switch (Pin 1) 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 28. 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 28 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.
4. Normal operation: Above 2.8 V, the circuit operates as a standard pulse width modulated switching regulator.
The capacitor continues 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 be limited 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, it is strongly recommended that the SD/SS pin be clamped externally
between the 3-V maximum soft-start threshold and the 4.5-V minimum input voltage. Figure 30 is an example of
an external approximately 3.7-V clamp that prevents a line-step related glitch but does not interfere with the softstart behavior of the device.
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Application Information (continued)
Figure 28. Soft-start, Delay, Error, Output
Figure 29. Timing Diagram for 5-V Output
18
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Application Information (continued)
VIN
LM2598
5
Q1
SD/SS
CSS
Z1
3V
Figure 30. 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 28, and also refer to timing diagrams in
Figure 29. 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.
If, for any reason, the regulated output voltage drops by 5% or more, the error output flag (Pin 3) 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.
The error flag output, RPull Up (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 Feedforward Capacitor (CFF)
NOTE
Adjustable output voltage version only
Figure 45 shows a feedfoward capacitor across R2 which is used when the output voltage is greater than 10 V or
then COUT has a very low ESR. This capacitor adds lead compensation to the feedback loop and increases the
phase margin for better loop stability.
If the output ripple is large (> 5% of the nominal output voltage), this ripple can be coupled to the feedback pin
through the feedforward capacitor and cause the error comparator to trigger the error flag. In this situation,
adding a resistor, RFF, in series with the feedforward capacitor, approximately 3 times R1, attenuates the ripple
voltage at the feedback pin.
9.1.4 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 buck regulator's input capacitor, 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.
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Application Information (continued)
The RMS current rating of a capacitor could be viewed as a capacitor's power rating. 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 determines the amount of current the capacitor can safely sustain. Capacitors that
are physically large and have a large surface area typically has higher RMS current ratings. For a given capacitor
value, a higher voltage electrolytic capacitor is physically larger than a lower voltage capacitor, and thus be able
to dissipate more heat to the surrounding air, and therefore has a higher RMS current rating.
Figure 31. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
Figure 32. 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 capacitor's electrolyte, 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.
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Application Information (continued)
Figure 31 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.
9.1.5 Output Capacitor (COUT)
An output capacitor is required to filter the output and provide regulator loop stability. Low impedance or low ESR
Electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. 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.
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 selected capacitor's ESR 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, provides 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 32). Often, capacitors with much
higher voltage ratings may be required to provide the low ESR values required for low output ripple voltage.
The output capacitor for many different switcher designs often can be satisfied with only three or four different
capacitor values and several different voltage ratings. See Figure 38 and Table 1 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 curve shown in Figure 33.
Solid tantalum capacitors have a much better ESR specifications for cold temperatures and are recommended
for temperatures below –25°C.
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Application Information (continued)
Table 1. Output Capacitor and Feedforward Capacitor Selection Table
OUTPUT
VOLTAGE
(V)
THROUGH-HOLE ELECTROLYTIC
PANASONIC
HFQ SERIES
(μF/V)
NICHICON PL
SERIES
(μF/V)
1.2
330/50
4
220/25
6
SURFACE-MOUNT TANTALUM
FEEDFORWARD
CAPACITOR
AVX TPS
SERIES
(μF/V)
SPRAGUE
595D SERIES
(μF/V)
330/50
0
330/6.3
330/6.3
0
220/25
4.7 nF
220/10
220/10
4.7 nF
220/25
220/25
3.3 nF
220/10
220/10
3.3 nF
FEEDFORWARD
CAPACITOR
9
180/25
180/25
1.5 nF
100/16
180/16
1.5 nF
12
120/25
120/25
1.5 nF
68/20
120/20
1.5 nF
15
120/25
120/25
1.5 nF
68/20
100/20
1.5 nF
24
82/35
82/35
1 nF
33/25
33/35
220 pF
28
82/50
82/50
1 nF
10/35
33/35
220 pF
9.1.6 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 and must be located close to the LM2598 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 1N5400 series are much too slow and must not be used.
Figure 33. Capacitor ESR Change vs Temperature
9.1.7 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 operate in the
discontinuous mode when the load current is low.
The LM2598 (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.
22
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To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Table 1
through Figure 37). This guide assumes that the regulator is operating in the continuous mode, and selects an
inductor that allows a peak-to-peak 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 34.)
Figure 34. (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 wound on a ferrite bobbin. This type of construction makes for an inexpensive inductor; however,
because the magnetic flux is not completely contained within the core, it generates more Electro-Magnetic
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. Also seeOpen Core Inductors.
When multiple switching regulators are located on the same PCB, open core magnetics can cause interference
between two or more of the regulator circuits, especially at high currents. A torroid or E-core inductor (closed
magnetic structure) must be used in these situations.
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 an inductor's maximum current rating 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 or the LM2598. Different inductor types have different
saturation characteristics, and this must be kept in mind when selecting an inductor.
The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.
For continuous mode operation, see the inductor selection graphs in Figure 35 through Figure 38.
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Figure 35. LM2598-3.3
Figure 36. LM2598-5.0
Figure 37. LM2598-12
Figure 38. LM2598-ADJ
Table 2. Inductor Manufacturers Part Numbers
INDUCTANCE
(μH)
CURRENT
(A)
L4
68
L5
47
L6
L9
L10
SCHOTTKY
RENCO
PULSE ENGINEERING
COILCRAFT
THROUGH
HOLE
SURFACE
MOUNT
THROUGH
HOLE
SURFACE
MOUNT
THROUGH
HOLE
SURFACE
MOUNT
0.32
67143940
67144310
RL-1284-68-43
RL1500-68
PE-53804
PE-53804-S
DO1608-68
0.37
67148310
67148420
RL-1284-47-43
RL1500-47
PE-53805
PE-53805-S
DO1608-473
33
0.44
67148320
67148430
RL-1284-33-43
RL1500-33
PE-53806
PE-53806-S
DO1608-333
220
0.32
67143960
67144330
RL-5470-3
RL1500-220
PE-53809
PE-53809-S
DO3308-224
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
DO3308-683
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-1284-33-43
RL1500-33
PE-53814
PE-53814-S
DO3308-333
L15
22
0.99
67148350
67148460
RL-1284-22-43
RL1500-22
PE-53815
PE-53815-S
DO3308-223
L16
15
1.24
67148360
67148470
RL-1284-15-43
RL1500-15
PE-53816
PE-53816-S
DO3308-153
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
DO3316-683
24
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Table 2. Inductor Manufacturers Part Numbers (continued)
INDUCTANCE
(μH)
CURRENT
(A)
L22
47
L23
33
L24
SCHOTTKY
RENCO
PULSE ENGINEERING
COILCRAFT
THROUGH
HOLE
SURFACE
MOUNT
THROUGH
HOLE
SURFACE
MOUNT
THROUGH
HOLE
SURFACE
MOUNT
SURFACE
MOUNT
1.17
67144080
67144460
RL-5471-6
—
PE-53822
PE-53822-S
DO3316-473
1.4
67144090
67144470
RL-5471-7
—
PE-53823
PE-53823-S
DO3316-333
22
1.7
67148370
67144480
RL-1283-22-43
—
PE-53824
PE-53824-S
DO3316-223
L26
330
0.8
67144100
67144480
RL-5471-1
—
PE-53826
PE-53826-S
DO5022P-334
L27
220
1
67144110
67144490
RL-5471-2
—
PE-53827
PE-53827-S
DO5022P-224
L28
150
1.2
67144120
67144500
RL-5471-3
—
PE-53828
PE-53828-S
DO5022P-154
L29
100
1.47
67144130
67144510
RL-5471-4
—
PE-53829
PE-53829-S
DO5022P-104
L30
68
1.78
67144140
67144520
RL-5471-5
—
PE-53830
PE-53830-S
DO5022P-683
L35
47
2.15
67144170
—
RL-5473-1
—
PE-53935
PE-53935-S
—
9.1.8 Output Voltage Ripple and Transients
The output voltage of a switching power supply operating in the continuous mode contains 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 20 mV), TI recommends a post ripple filter (see Figure 45).
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 27 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.
Figure 39. Peak-to-Peak Inductor
Ripple Current vs Load Current
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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 smoothly changes from a continuous to a discontinuous mode of operation. Most switcher designs
(regardless how large the inductor value is) is 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 35 through Figure 38 are used to select an inductor value, the peak-to-peak inductor ripple current can
immediately be determined. Figure 39 shows the range of (ΔIIND) that can be expected for different load currents.
Figure 39 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
Guides).
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 800 mA
VIN = 12 V, nominal, varying between 10 V and 14 V.
The selection guide in Figure 36 shows that the vertical line for a 0.8-A load current and the horizontal line for the
12-V input voltage intersect approximately midway between the upper and lower borders of the 68-μH inductance
region. A 68-μH inductor allows a peak-to-peak inductor current (ΔIIND) to a percentage of the maximum load
current. Referring to Figure 39, follow the 0.8-A line approximately midway into the inductance region, and read
the peak-to-peak inductor ripple current (ΔIIND) on the left hand axis (approximately 300-mA p-p).
As the input voltage increases to 14 V, it approaches the upper border of the inductance region, and the inductor
ripple current increases. Figure 39 shows that for a load current of 0.8 A, the peak-to-peak inductor ripple current
(ΔIIND) is 300 mA with 12-V in, and can range from 340 mA at the upper border (14-V in) to 225 mA at the lower
border (10-V in).
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) = 0.3 A × 0.16 Ω = 48 mVp-p
4. ESR of COUT
26
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9.1.9 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 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 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 provides 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 when using them. 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.
Circuit Data for Temperature Rise Curve TO-220 Package (T)
Capacitors
Through hole electrolytic
Inductor
Through hole, Schott, 68 μH
Diode
Through hole, 3-A, 40-V, Schottky
Printed-circuit board
3 square inches single sided 2 oz. copper (0.0028″)
Figure 40. Junction Temperature Rise, TO-220
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Circuit Data for Temperature Rise Curve DDPAK Package (S)
Capacitors
Surface mount tantalum, molded D size
Inductor
Surface mount, Schott, 68 μH
Diode
Surface mount, 3-A, 40-V, Schottky
Printed-circuit board
3 square inches single sided 2 oz. copper (0.0028″)
Figure 41. Junction Temperature Rise, DDPAK
9.2 Typical Application
9.2.1 LM2598 Fixed Output Series Buck Regulator
Component Values shown are for VIN = 15 V, VOUT = 5 V, ILOAD = 1 A.
120-μF, 50-V, Aluminum Electrolytic Nichicon PL Series
120-μF, 35-V Aluminum Electrolytic, Nichicon PL Series
3-A, 40-V Schottky Rectifier, 1N5822
68-μH, L30
Typical Values
*CSS: — 0.1 μF
CDELAY: — 0.1 μF
RPull Up: — 4.7k
Figure 42. Fixed Output Voltage Version
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Typical Application (continued)
9.2.1.1 Design Requirements
Table 3 lists the design parameters of this application example.
Table 3. Design Parameters
PARAMETERS
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)
1A
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 35, Figure 36, or Figure 37 (Output voltages of
3.3 V, 5 V, or 12 V respectively.) Use the inductor selection guide for the 5-V version shown in Figure 36.
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 36, the inductance region
intersected by the 12-V horizontal line and the 1-A vertical line is 68 μH, and the inductor code is L30.
3. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2. The inductance
value required is 68 μH. See row L30 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.)
9.2.1.2.2 Output Capacitor Selection (COUT)
1. In the majority of applications, low ESR (Equivalent Series Resistance) electrolytic capacitors between 47 μF
and 330 μF and low ESR solid tantalum capacitors between 56 μF and 270 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 330 μF.
For additional information, see section on output capacitors in Output Capacitor (COUT) section.
2. To simplify the capacitor selection procedure, see Figure 38 for quick design component selection. This table
contains different input voltages, output voltages, and load currents, and lists various inductors and output
capacitors that provide the best design solutions.
From Figure 38, 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 1-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
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 Figure 38.
In this example aluminum electrolytic capacitors from several different manufacturers are available with the
range of ESR numbers required.
– 220-μF, 25-V Panasonic HFQ Series
– 220 μF, 25-V Nichicon PL Series
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Table 4. LM2598 Fixed Voltage Quick Design Component Selection Table
OUTPUT CAPACITOR
CONDITIONS
OUTPUT
LOAD
VOLTAGE CURRENT
(V)
(A)
1
3.3
0.5
1
5
0.5
1
12
0.5
INDUCTOR
MAX INPUT
VOLTAGE
(V)
THROUGH-HOLE
ELECTROLYTIC
INDUCTANCE INDUCTOR
(μH)
(#)
SURFACE-MOUNT TANTALUM
PANASONIC
HFQ SERIES
(μF/V)
NICHICON
PL SERIES
(μF/V)
AVX TPS
SERIES
(μF/V)
SPRAGUE
595D SERIES
(μF/V)
5
22
L24
330/16
330/16
220/10
330/10
7
33
L23
270/25
270/25
220/10
270/10
10
47
L31
220/25
220/35
220/10
220/10
40
68
L30
180/35
220/35
220/10
180/10
6
47
L13
220/25
220/16
220/16
220/10
10
68
L21
150/35
150/25
100/16
150/16
40
100
L20
150/35
82/35
100/16
100/20
8
33
L28
330/16
330/16
220/10
270/10
10
47
L31
220/25
220/25
220/10
220/10
15
68
L30
180/35
180/35
220/10
150/16
40
100
L29
180/35
120/35
100/16
120/16
9
68
L21
180/16
180/16
220/10
150/16
20
150
L19
120/25
120/25
100/16
100/20
40
150
L19
100/25
100/25
68/20
68/25
15
47
L31
220/25
220/25
68/20
120/20
18
68
L30
180/35
120/25
68/20
120/20
30
150
L36
82/25
82/25
68/20
100/20
40
220
L35
82/25
82/25
68/20
68/25
15
68
L21
180/25
180/25
68/20
120/20
20
150
L19
82/25
82/25
68/20
100/20
40
330
L26
56/25
56/25
68/20
68/25
3. The capacitor voltage rating for electrolytic capacitors 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 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, 220-μF, 10-V aluminum electrolytic capacitor would exhibit approximately 225 mΩ
of ESR (see the curve in Figure 32 for the ESR vs voltage rating). 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 reduces 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 LM2598. The most stressful condition for this diode is an overload or
shorted output condition. See Table 5. In this example, a 3-A, 20-V, 1N5820 Schottky diode provides the
best performance, and does not overstressed even for a shorted output.
30
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Table 5. Diode Selection Table
1-A DIODES
VR
SURFACE MOUNT
SCHOTTKY
ULTRA FAST
RECOVERY
SK12
30 V
SCHOTTKY
ULTRA FAST
RECOVERY
SURFACE MOUNT
SCHOTTKY
ULTRA FAST
RECOVERY
1N5817
All of these
diodes are rated
to at least 50 V.
20 V
3-A DIODES
THROUGH HOLE
SR102
SK13
1N5818
MBRS130
SR103
SK32
All of these
diodes are rated
to at least 50 V.
ULTRA FAST
RECOVERY
SR302
MBR320
All of these
diodes are rated
to at least 50 V.
1N5821
SK33
MBR330
11DQ03
50 V
or
more
SCHOTTKY
IN5820
All of these
diodes are rated
to at least 50 V.
31DQ03
SK14
40 V
THROUGH HOLE
1N5822
MBRS140
1N5819
SK34
SR304
10BQ040
SR104
MBRS340
MBR340
10MQ040
MURS120
11DQ04
30WQ04
MURS320
31DQ04
MUR320
MBRS160
10BF10
SR105
MUR120
SK35
30WF10
SR305
30WF10
10BQ050
MBR150
MBRS360
MBR350
10MQ060
11DQ05
30WQ05
31DQ05
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 LM2598 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 1N5400
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 31 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 recommends they be surge current tested by the manufacturer.
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 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 1-A load, a capacitor with a RMS current rating of at least 500 mA is required.
Figure 31 shows curves that can be used to select an appropriate input capacitor. From the curves, locate the
25-V line and note which capacitor values have RMS current ratings greater than 500 mA. Either a 180-μF or
220-μF, 25-V capacitor could be used.
For a through-hole design, a 220-μF, 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.
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9.2.1.3 Application Curves
Load Transient Response for Continuous Mode
VIN = 20 V, VOUT = 5 V, ILOAD = 250 mA to 750 mA,
L = 68 μH, COUT = 120 μF, COUT ESR = 100 mΩ
A: Output Voltage, 100 mV/div. (AC)
B: 250-mA to 750-mA Load Pulse
Continuous Mode Switching Waveforms
VIN = 20 V, VOUT = 5 V, ILOAD = 1 A, L = 68 μH,
COUT = 120 μF, COUT ESR = 100 mΩ
A: Output Pin Voltage, 10 V/div.
B: Inductor Current 0.5 A/div.
C: Output Ripple Voltage, 50 mV/div.
Figure 43. Horizontal Time Base: 2 μs/div
32
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Figure 44. Horizontal Time Base: 100 μs/div
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9.2.2 LM2598 Adjustable Output Series Buck Regulator
where VREF = 1.23 V
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 = 1 A.
CIN — 120 μF, 35-V, Aluminum Electrolytic Nichicon PL Series
COUT — 120 μF, 35-V Aluminum Electrolytic, Nichicon PL Series
D1 —3-A, 40-V Schottky Rectifier, 1N5822
L1 —100 μH, L29
R1 —1 kΩ, 1%
R2 —7.1 kΩ, 1%
CFF — 3.3 nF, See Feedforward Capacitor (CFF)
RFF — 3 kΩ, See Feedforward Capacitor (CFF)
Typical Values
CSS—0.1 μF
CDELAY—0.1 μF
RPULL UP—4.7 kΩ
Figure 45. Adjustable Output Voltage Version
9.2.2.1 Design Requirements
Table 6 lists the design parameters for this application example.
Table 6. Design Parameters
PARAMETERS
EXAMPLE VALUE
Regulated output voltage (3.3 V, 5 V or 12 V), VOUT
20 V
Maximum DC input voltage, VIN(max)
28 V
Maximum load current, ILOAD(max)
1A
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 45.
Use Equation 1 to select the appropriate resistor values.
(1)
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Select a value for R1 with Equation 2 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.)
(2)
Select R1 with Equation 3 to be 1 kΩ, 1%. Solve for R2.
(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 = 1 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
see the inductor selection graphs in Figure 35 through Figure 38.
E • T = 34.8 (V • μs)
3. On the horizontal axis, select the maximum load current.
ILOAD(max) = 1 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 selection graphs in Figure 35 through Figure 38, the inductance region intersected by the
35 (V • μs) horizontal line and the 1-A vertical line is 100 μH, and the inductor code is L29.
5. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 2.
From the table in Table 2, locate line L29, 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 1 for a quick design guide. This table contains
different output voltages, and lists various output capacitors that provide the best design solutions.
From Table 1, 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. Under the Output Capacitor (COUT)
section, 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 1 be used.
In this example, through hole aluminum electrolytic capacitors from several different manufacturers are
available:
34
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– 82-μF, 35-V Panasonic HFQ Series
– 82-μF, 35-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 35-V rating was chosen although a 50-V rating could also be used if a lower
output ripple voltage is required.
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 1. 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 (use Equation 6; see
Figure 45). 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.
(6)
This capacitor type can be ceramic, plastic, silver mica, etc. (Because of the unstable characteristics of ceramic
capacitors made with Z5U material, they are not recommended.)
The table shown in Table 1 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 LM2598. The most stressful condition for this diode is an overload or
shorted output condition.
See Table 5. Schottky diodes provide the best performance, and in this example a 3-A, 40-V, 1N5822
Schottky diode is a good choice. The 3-A diode rating is more than adequate and does not 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 placed close to the LM2598 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 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 31 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, it is recomended that they be surge current tested by the
manufacturer.
Use caution when using a high dielectric constant ceramic capacitor for input bypassing, because it may cause
severe ringing at the VIN pin.
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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 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 1-A load, a capacitor with a RMS current rating of at least 500 mA is required.
Figure 31 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 500 mA. Either a 100-μF or
120-μF, 50-V capacitor could be used.
For a through-hole design, a 120-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 can be used, but caution must be exercised with regard to
the capacitor surge current rating (see Input Capacitor (CIN)). The TPS series available from AVX, and the 593D
series from Sprague are both surge current tested.
9.2.2.3 Application Curves
Load Transient Response for Discontinuous Mode
VIN = 20 V, VOUT = 5 V, ILOAD = 250 mA to 750 mA,
L = 22 μH, COUT = 220 μF, COUT ESR = 50 mΩ
A: Output Voltage, 100 mV/div. (AC)
B: 250-mA to 750-mA Load Pulse
Discontinuous Mode Switching Waveforms
VIN = 20 V,VOUT = 5 V, ILOAD = 600 mA, L = 22 μH,
COUT = 220 μF,COUT ESR = 50 mΩ
A: Output Pin Voltage, 10 V/div.
B: Inductor Current 0.5 A/div.
C: Output Ripple Voltage, 50 mV/div.
Figure 46. Horizontal Time Base: 2 μs/div
36
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Figure 47. Horizontal Time Base: 200 μs/div
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10 Power Supply Recommendations
The LM2598 is designed to operate from an input voltage supply up to 40 V. 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 special care regarding 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, special care must be taken as to 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 (see Open Core Inductors for more information).
11.2 Layout Examples
CIN—150-μF, 50-V Aluminum Electrolytic, Panasonic HFQ series
COUT—120-μF, 25-V Aluminum Electrolytic, Panasonic HFQ series
D1 — 3-A, 40-V Schottky Rectifier, 1N5822
L1 — 68-μH, L30, Renco, Through hole
RPULL-UP — 10 kΩ
CDELAY — 0.1 μF
CSD/SS — 0.1 μF
Figure 48. Typical Through-Hole PCB Layout, Fixed Output (1x Size), Double-Sided, Through-Hole Plated
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Layout Examples (continued)
CIN — 150-μF, 50-V, Aluminum Electrolytic, Panasonic HFQ series
COUT — 120-μF, 25-V Aluminum Electrolytic, Panasonic HFQ series
D1 — 3-A, 40-V Schottky Rectifier, 1N5822
L1 — 68-μH, L30, Renco, Through hole
R1 — 1 kΩ, 1%
R2—Use formula in Design Procedure
CFF—See Feedforward Capacitor (CFF).
RFF—See Feedforward Capacitor (CFF).
RPULL-UP—10 kΩ
CDELAY — 0.1-μF
CSD/SS — 0.1 μF
Figure 49. Typical Through-Hole PCB Layout, Adjustable Output (1x Size), Double-Sided, Through-Hole
Plated
11.3 Thermal Considerations
The LM2598 is available in two packages: a 7-pin TO-220 (T) and a 7-pin surface mount DDPAK (S).
The TO-220 package can be used without a heat sink for ambient temperatures up to approximately 50°C
(depending on the output voltage and load current). Figure 40 shows the LM2598T junction temperature rises
above ambient temperature for different input and output voltages. The data for these curves was taken with the
LM2598T (TO-220 package) operating as a switching regulator in an ambient temperature of 25°C (still air).
These temperature rise numbers are all approximate and there are many factors that can affect these
temperatures. Higher ambient temperatures require some heat sinking, either to the PCB or a small external heat
sink.
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Thermal Considerations (continued)
The DDPAK surface mount package tab is designed to be soldered to the copper on a printed-circuit board
(PCB). The copper and the board are the heat sink for this package and the other heat producing components,
such as the catch diode and inductor. The PCB copper area that the package is soldered to must be at least 0.4
in2, and ideally must have 2 or more square inches of 2 oz. (0.0028) in) copper. Additional copper area improves
the thermal characteristics, but with copper areas greater than approximately 3 in2, only small improvements in
heat dissipation are realized. If further thermal improvements are required, TI recommends double-sided or
multilayer PCB with large copper areas.
Figure 41 shows the LM2598S (DDPAK package) junction temperature rise above ambient temperature with a 1A load for various input and output voltages. This data was taken with the circuit operating as a buck switching
regulator with all components mounted on a PCB to simulate the junction temperature under actual operating
conditions. This curve can be used for a quick check for the approximate junction temperature for various
conditions, but be aware that there are many factors that can affect the junction temperature.
For the best thermal performance, wide copper traces and generous amounts of PCB copper must be used in
the board layout. (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
moving air lowers the thermal resistance even further.
Package thermal resistance and junction temperature rise numbers are all approximate, and there are many
factors that affect these numbers. Some of these factors include board size, shape, thickness, position, location,
and even board temperature. Other factors are trace width, total 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, as well as
whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode
adds 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.
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12 Device and Documentation Support
12.1 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.
12.2 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.
12.3 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.
12.4 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.
40
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
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)
(4/5)
(6)
LM2598S-12/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-12 P+
LM2598S-3.3/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-3.3 P+
LM2598S-5.0/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-5.0 P+
LM2598S-ADJ/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-ADJ P+
LM2598SX-12/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-12 P+
LM2598SX-3.3/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-3.3 P+
LM2598SX-5.0/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-5.0 P+
LM2598SX-ADJ/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2598S
-ADJ P+
LM2598T-12/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2598T
-12 P+
LM2598T-3.3/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2598T
-3.3 P+
LM2598T-5.0/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2598T
-5.0 P+
LM2598T-ADJ/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2598T
-ADJ P+
(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".
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
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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