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LM2599
SNVS123D – APRIL 1998 – REVISED MAY 2016
LM2599 SIMPLE SWITCHER® Power Converter 150-kHz 3-A Step-Down Voltage Regulator,
With Features
1 Features
3 Description
•
•
The LM2599 series of regulators are monolithic
integrated circuits that provide all the active functions
for a step-down (buck) switching regulator, capable of
driving a 3-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% Maximum Over Line and Load
Conditions
3-A Output Current
Available in 7-Pin TO-220 and TO-263 (SurfaceMount) Package
Input Voltage Range Up to 40 V
150-kHz Fixed-Frequency Internal Oscillator
Shutdown and Soft-Start
Out-of-Regulation Error Flag
Error Output Delay
Low Power Standby Mode, IQ, Typically 80 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, 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 and Soft-start, error flag delay, and error
flag output.
Device Information(1)
PART NUMBER
LM2599
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.
LM2599
SNVS123D – APRIL 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
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
5
6
7
8.1
8.2
8.3
8.4
9
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
10
10
10
15
Application and Implementation ........................ 16
9.1 Application Information............................................ 16
9.2 Typical Applications ................................................ 25
10 Power Supply Recommendations ..................... 34
11 Layout................................................................... 34
11.1 Layout Guidelines ................................................. 34
11.2 Layout Examples................................................... 34
11.3 Thermal Considerations ........................................ 35
12 Device and Documentation Support ................. 38
12.1
12.2
12.3
12.4
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
38
38
38
38
13 Mechanical, Packaging, and Orderable
Information ........................................................... 38
Detailed Description ............................................ 10
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 .................................................................................. 35
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5 Description (continued)
The LM2599 series operates at a switching frequency of 150 kHz, thus allowing smaller sized filter components
than what would be needed with lower frequency switching regulators. Available in a standard 7-pin TO-220
package with several different lead bend options, and a 7-pin TO-263 surface-mount package.
A standard series of inductors (both through-hole and surface-mount types) are available from several different
manufacturers optimized for use with the LM2599 series. This feature greatly simplifies the design of switchmode power supplies.
Other features include a ±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 80-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 (1)
PIN
NO.
NAME
I/O
DESCRIPTION
1
+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 needed by the regulator.
2
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.
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 powerup.
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.
6
Feedback
I
Senses the regulated output voltage to complete the feedback loop.
7
Shutdown/Soft-start
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)
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
(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)
260
Maximum junction temperature
−65
Storage temperature, Tstg
(1)
(2)
(3)
°C
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)
Electrostatic
discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001
(1)
VALUE
UNIT
±2000
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
7.3 Recommended Operating Conditions
MIN
MAX
UNIT
Supply voltage
4.5
40
V
Temperature
–40
125
°C
7.4 Thermal Information
LM2599
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 PCB 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 PCB 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 PCB 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 PCB with 3 in2 of 1-oz copper area on
the LM2599S 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 noted).
PARAMETER
SYSTEM PARAMETERS
(3)
Output voltage
4.75 V ≤ VIN ≤ 40 V,
0.2 A ≤ ILOAD ≤ 3 A
η
Efficiency
VIN = 12 V, ILOAD = 3 A
(2)
(3)
TYP (2)
TJ = 25°C
3.168
3.3
–40°C ≤ TJ ≤ 125°C
3.135
MAX
(1)
UNIT
(see Figure 43 for test circuit)
VOUT
(1)
MIN (1)
TEST CONDITIONS
3.432
3.465
V
73%
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 LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.6 Electrical Characteristics – 5-V Version
Specifications are for TJ = 25°C (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3)
TJ = 25°C
Output voltage
7 V ≤ VIN ≤ 40 V,
0.2 A ≤ ILOAD ≤ 3 A
η
Efficiency
VIN = 12 V, ILOAD = 3 A
(2)
(3)
TYP (2)
4.8
5
MAX (1)
UNIT
(see Figure 43 for test circuit)
VOUT
(1)
MIN (1)
–40°C ≤ TJ ≤ 125°C
4.75
5.2
5.25
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 LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.7 Electrical Characteristics – 12-V Version
Specifications are for TJ = 25°C (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3)
TJ = 25°C
Output voltage
15 V ≤ VIN ≤ 40 V,
0.2 A ≤ ILOAD ≤ 3 A
η
Efficiency
VIN = 25 V, ILOAD = 3 A
(2)
(3)
TYP (2)
MAX (1)
11.52
12
12.48
UNIT
(see Figure 43 for test circuit)
VOUT
(1)
MIN (1)
–40°C ≤ TJ ≤ 125°C
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 LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
7.8 Electrical Characteristics – Adjustable Voltage Version
Specifications are for TJ = 25°C (unless otherwise noted).
PARAMETER
SYSTEM PARAMETERS
TEST CONDITIONS
(3)
MIN (1)
η
(1)
(2)
(3)
Feedback voltage
Efficiency
MAX (1)
UNIT
(see Figure 43 for test circuit)
4.5 V ≤ VIN ≤ 40 V, 0.2 A ≤ ILOAD ≤ 3 A
VFB
TYP (2)
VOUT programmed for 3 V,
circuit of Figure 43
TJ = 25°C
–40°C ≤ TJ ≤ 125°C
VIN = 12 V, VOUT = 3 V, ILOAD = 3 A
1.23
1.193
1.267
1.18
1.28
V
73%
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 LM2599 is used as shown in Figure 43, system performance is shown in the test conditions column.
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7.9 Electrical Characteristics – All Output Voltage Versions
Specifications are for TJ = 25°C, ILOAD = 500 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)
UNIT
DEVICE PARAMETERS
Ib
Feedback bias current
fO
Oscillator frequency (3)
VSAT
Saturation voltage
Adjustable voltage
version only, VFB = 1.3
V
TJ = 25°C
10
–40°C ≤ TJ ≤ 125°C
TJ = 25°C
127
–40°C ≤ TJ ≤ 125°C
110
TJ = 25°C
IOUT = 3 A (4) (5)
Current limit
173
173
1.16
1.4
1.5
nA
kHz
V
100%
Min duty cycle (OFF) (6)
ICL
150
–40°C ≤ TJ ≤ 125°C
Max duty cycle (ON) (5)
DC
50
100
0%
Peak current (4)
(5)
TJ = 25°C
3.6
–40°C ≤ TJ ≤ 125°C
3.4
Output = 0 V, VIN = 40 V (4)
4.5
6.9
A
7.5
(6)
50
μA
Output = −1 V
2
30
mA
Operating quiescent current
SD/SS pin open (6)
5
10
mA
Current standby quiescent
SD/SS pin = 0 V, VIN =
40 V
80
200
μA
250
μA
0.6
V
IL
Output leakage current
IQ
ISTBY
TJ = 25°C
–40°C ≤ TJ ≤ 125°C
SHUTDOWN/SOFT-START CONTROL – See Figure 43
1.3
VSD
Shutdown threshold voltage
Low (Shutdown mode), –40°C ≤ TJ ≤ 125°C
High (Soft-start mode), –40°C ≤ TJ ≤ 125°C
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
5
10
μA
1.6
5
μA
96%
98%
FLAG/DELAY CONTROL – See Figure 43
Regulator dropout detector
threshold voltage
Low (flag ON)
92%
ISINK = 3 mA
VFSAT
IFL
Voltage flag output saturation
VDELAY = 0.5 V
Flag output leakage current
VFLAG = 40 V
Voltage delay pin threshold
Low (flag ON)
0.3
TJ = 25°C
–40°C ≤ TJ ≤ 125°C
1
μA
1.25
V
1.21
Delay pin saturation
(1)
(2)
(3)
(4)
(5)
(6)
6
1.29
VDELAY = 0.5 V
Low (flag ON)
TJ = 25°C
–40°C ≤ TJ ≤ 125°C
V
0.3
High (flag OFF) and VOUT regulated
Delay pin source current
V
0.7
3
6
55
350
400
V
μA
mV
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 versions, and 15 V for the 12-V
version, to force the output transistor switch OFF.
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7.10 Typical Characteristics
See Figure 43 for test circuit
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)
See Figure 43 for test circuit
8
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)
See Figure 43 for test circuit
Figure 13. Soft-Start
Figure 14. Shutdown/Soft-Start Current
Figure 15. Daisy Pin Current
Figure 16. Soft-Start Response
Figure 17. Shutdown/Soft-Start Threshold Voltage
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8 Detailed Description
8.1 Overview
The LM2599 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 3-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/Soft-Start
The circuit shown in Figure 20 is a standard buck regulator with 20-VIN, 12-VOUT, 1-A load using a 0.068-μF softstart capacitor. 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. The reduced input current required at startup is very evident when
comparing the two photos. The soft-start feature reduces the start-up current from 2.6 A down to 650 mA, and
delays and slows down the output voltage rise time.
10
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Feature Description (continued)
Figure 18. Output Voltage and Input Current
at Start-Up With Soft-Start
Figure 19. 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 needed, the soft-start capacitor can be eliminated.
Figure 20. Typical Circuit Using Shutdown/Soft-Start and Error Flag Features
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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 ground pin of the regulator to the negative output voltage, then grounding
the feedback pin, the regulator senses the inverted output voltage and regulates it.
This example uses the LM2599-5 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 LM2599 has a maximum input voltage rating of 40 V.
Figure 22. Maximum Load Current for Inverting Regulator Circuit
12
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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.
TI recommends a Schottky diode for low input voltages, (because of its lower voltage drop) but for higher input
voltages, a IN5400 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 33-μH, 3.5-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 LM2599 current limit (approximately 4.5 A) are needed 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 startup 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 contains a undervoltage lockout circuit for a buck configuration, 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/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 turn on and turn off (Zener voltage plus approximately one volt). If hysteresis is needed, the circuit in
Figure 25 has a turn ON voltage which is different than the turn OFF voltage. The amount of hysteresis is
approximately equal to the value of the output voltage. Because the SD/SS pin has an internal 7-V Zener clamp,
R2 is needed to limit the current into this pin to approximately 1 mA when Q1 is on.
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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 needed 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 600 mA of output current. There is a requirement however, that there
be a minimum load of 1.2 A 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 LM2599 current limit (typically
4.5 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
14
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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. The design would use
an inductor that would be physically smaller, and would need only one half to one third the inductance value
needed for a continuous mode design. The peak switch and inductor currents is higher in a discontinuous design,
but at these low load currents (1 A 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 44). 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 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)
The 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
soft-start condition. The switch (Pin 2) 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. See the soft-start
section in Electrical Characteristics – All Output Voltage Versions.
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, 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 30 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 28. Soft-Start, Delay, Error, Output
Figure 29. Timing Diagram for 5-V Output
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Application Information (continued)
VIN
LM2599
5
Q1
SD/SS
CSS
Z1
3V
Figure 30. External 3.7-V Soft-Start Clamp
9.1.2 Delay Capacitor (CDELAY)
This capacitor provides delay for the error flag output (see Figure 28 and 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 3-μA 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.
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 needed. 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) for Adjustable Output Voltage Version Only
A feedforward capacitor, CFF, is used when the output voltage is greater than 10 V or when 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 placed near the regulator using short leads. This capacitor prevents large voltage transients
from appearing at the input, and provides the instantaneous current needed 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 the 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.
18
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Application Information (continued)
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 determines the amount of current the capacitor can safely sustain.
Capacitors that are physically large and have a large surface area typically has a 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 needed to satisfy the RMS current requirements.
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.
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Application Information (continued)
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 needed. 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, 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 needed 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 the quick design component selection tables in
Table 3 and Table 6 for typical capacitor values, voltage ratings, and manufacturer 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 33).
Solid tantalum capacitors have a much better ESR specification for cold temperatures and are recommended for
temperatures below −25°C.
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 placed close to the LM2599 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 IN5400 series are much too slow and must not be used.
Figure 33. Capacitor ESR Change vs Temperature
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Application Information (continued)
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 operates in the
discontinuous mode when the load current is low.
The LM2599 (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 and/or high input voltages.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 35
through Figure 38). 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, but
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 (see Open Core Inductors).
When multiple switching regulators are placed 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.
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Application Information (continued)
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 and/or the LM2599. Different inductor types have different
saturation characteristics, so keep this 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.
Figure 35. 3.3-V Version
Figure 36. 5-V Version
Figure 37. 12-V Version
Figure 38. Adjustable Voltage Version
Table 1. Inductor Manufacturers Part Numbers
22
INDUCTANCE
(μH)
CURRENT
(A)
L15
22
L21
68
L22
SCHOTTKY
RENCO
THROUGHHOLE
SURFACEMOUNT
0.99
67148350
0.99
67144070
47
1.17
L23
33
L24
PULSE ENGINEERING
COILCRAFT
THROUGH-HOLE
SURFACEMOUNT
THROUGHHOLE
SURFACEMOUNT
SURFACEMOUNT
67148460
RL-1284-22-43
RL1500-22
PE-53815
PE-53815-S
DO3308-223
67144450
RL-5471-5
RL1500-68
PE-53821
PE-53821-S
DO3316-683
67144080
67144460
RL-5471-6
—
PE-53822
PE-53822-S
DO3316-473
1.40
67144090
67144470
RL-5471-7
—
PE-53823
PE-53823-S
DO3316-333
22
1.70
67148370
67148480
RL-1283-22-43
—
PE-53824
PE-53825-S
DO3316-223
L25
15
2.1
67148380
67148490
RL-1283-15-43
—
PE-53825
PE-53824-S
DO3316-153
L26
330
0.80
67144100
67144480
RL-5471-1
—
PE-53826
PE-53826-S
DOS022P-334
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Application Information (continued)
Table 1. Inductor Manufacturers Part Numbers (continued)
INDUCTANCE
(μH)
CURRENT
(A)
L27
220
L28
150
L29
SCHOTTKY
RENCO
THROUGHHOLE
SURFACEMOUNT
1.00
67144110
1.20
67144120
100
1.47
L30
68
L31
PULSE ENGINEERING
COILCRAFT
THROUGH-HOLE
SURFACEMOUNT
THROUGHHOLE
SURFACEMOUNT
SURFACEMOUNT
67144490
RL-5471-2
—
PE-53827
PE-53827-S
DOS022P-224
67144500
RL-5471-3
—
PE-53828
PE-53828-S
DOS022P-154
67144130
67144510
RL-5471-4
—
PE-53829
PE-53829-S
DOS022P-104
1.78
67144140
67144520
RL-5471-5
—
PE-53830
PE-53830-S
DOS022P-683
47
2.2
67144150
67144530
RL-5471-6
—
PE-53831
PE-53831-S
DOS022P-473
L32
33
2.5
67144160
67144540
RL-5471-7
—
PE-53932
PE-53932-S
DOS022P-333
L33
22
3.1
67148390
67148500
RL-1283-22-43
—
PE-53933
PE-53933-S
DOS022P-223
L34
15
3.4
67148400
67148790
RL-1283-15-43
—
PE-53934
PE-53934-S
DOS022P-153
L35
220
1.70
67144170
—
RL-5473-1
—
PE-53935
PE-53935-S
—
L36
150
2.1
67144180
—
RL-5473-4
—
PE-54036
PE-54036-S
—
L37
100
2.5
67144190
—
RL-5472-1
—
PE-54037
PE-54037-S
—
L38
68
3.1
67144200
—
RL-5472-2
—
PE-54038
PE-54038-S
—
L39
47
3.5
67144210
—
RL-5472-3
—
PE-54039
PE-54039-S
—
L40
33
3.5
67144220
67148290
RL-5472-4
—
PE-54040
PE-54040-S
—
L41
22
3.5
67144230
67148300
RL-5472-5
—
PE-54041
PE-54041-S
—
L42
150
2.7
67148410
—
RL-5473-4
—
PE-54042
PE-54042-S
—
L43
100
3.4
67144240
—
RL-5473-2
—
PE-54043
—
—
L44
68
3.4
67144250
—
RL-5473-3
—
PE-54044
—
—
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, exercise caution when using
extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If very
low output ripple voltage is needed (less than 20 mV), TI recommends a post ripple filter (see Figure 40). 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, and 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 39. 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 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 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 2.5 A
VIN = 12 V, nominal, varying between 10 V and 16 V.
The selection guide in Figure 36 shows that the vertical line for a 2.5-A load current and the horizontal line for the
12-V input voltage intersect approximately midway between the upper and lower borders of the 33-μH inductance
region. A 33-μH inductor allows a peak-to-peak inductor current (ΔIIND) to flow at a percentage of the maximum
load current. In Figure 39, follow the 2.5-A line approximately midway into the inductance region, and read the
peak-to-peak inductor ripple current (ΔIIND) on the left hand axis (approximately 620 mAp-p).
As the input voltage increases to 16 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 2.5 A, the peak-to-peak inductor ripple current
(ΔIIND) is 620 mA with 12 VIN, and can range from 740 mA at the upper border (16 VIN) to 500 mA at the lower
border (10 VIN).
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
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3. Output Ripple Voltage = (ΔIIND) × (ESR of COUT = 0.62 A × 0.1 Ω = 62 mVp-p
4.
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 induces a voltage into any wire or PCB copper trace that comes
within the inductor's magnetic field. 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 placed 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 corrects 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. 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.
9.2 Typical Applications
9.2.1 Fixed Output Voltage Version
Component values shown are for VIN = 15 V,
VOUT = 5 V, ILOAD = 3 A.
CIN = –70-μF, –50-V, aluminum electrolytic Nichicon PL Series
COUT = 220-μF, 25-V aluminum electrolytic Nichicon PL Series
D1 = 5-A, 40-V Schottky Rectifier, 1N5825
L1 = 68, L38
Typical Values
CSS = 0.1 μF
CDELAY = 0.1 μF
RPullup = 47k
Figure 40. Typical Application for Fixed Output Voltage Versions
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Typical Applications (continued)
9.2.1.1 Design Requirements
Table 2 lists the design parameters of this example.
Table 2. 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)
3A
9.2.1.2 Detailed Design Procedure
9.2.1.2.1 Inductor Selection (L1)
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.
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 3-A vertical line is 33 μH, and the inductor code is L40.
Select an appropriate inductor from the four manufacturer's part numbers listed in Table 1. The inductance value
required is 33 μH. See row L30 of Table 3 and choose an inductor part number from any of the manufactures (in
most instance, both through-hole and surface-mount inductors are available).
9.2.1.2.2 Output Capacitor Selection (COUT)
In the majority of applications, low ESR (Equivalent Series Resistance) electrolytic capacitors between 82 μF and
820 μF and low ESR solid tantalum capacitors between 10 μF and 470 μF provide the best results. This
capacitor must be placed close to the IC using short capacitor leads and short copper traces. Do not use
capacitors larger than 820 μF. For additional information, see Output Capacitor (COUT).
To simplify the capacitor selection procedure, see Table 3. This table contains different input voltages, output
voltages, and load currents, and lists various inductors and output capacitors that provides the best design
solutions.
From the quick design component selection table shown in Table 3, locate the 5-V output voltage section. In the
load current column, choose the load current line that is closest to the current needed in your application, for this
example, use the 3-A line. In the maximum input voltage column, select the line that covers the input voltage
needed in your application, in this example, use the 15-V line. Continuing on this line are recommended
inductors and capacitors that provides the best overall performance.
The capacitor list contains both through-hole electrolytic and surface-mount tantalum capacitors from four
different capacitor manufacturers. TI recommends that both the manufacturers and the manufacturer's series that
are listed in the table be used. In this example, aluminum electrolytic capacitors from several different
manufacturers are available with the range of ESR numbers needed:
• 330-μF, 35-V Panasonic HFQ series
• 330-μF, 35-V Nichicon PL series
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Table 3. LM2599 Fixed Voltage Quick Design Component Selection Table
CONDITIONS
OUTPUT
VOLTAGE
(V)
LOAD
CURRENT
(A)
3
3.3
2
3
5
2
3
12
2
OUTPUT CAPACITOR
INDUCTOR
THROUGH-HOLE ELECTROLYTIC
SURFACE-MOUNT TANTALUM
MAX INPUT
VOLTAGE (V)
INDUCTANCE
(μH)
INDUCTOR
(#)
PANASONIC HFQ
SERIES (μF/V)
NICHICON PL
SERIES (μF/V)
AVX TPS
SERIES (μF/V)
SPRAGUE 595D
SERIES (μF/V)
5
22
L41
470/25
560/16
330/6.3
390/6.3
7
22
L41
560/35
560/35
330/6.3
390/6.3
10
22
L41
680/35
680/35
330/6.3
390/6.3
40
33
L40
560/35
470/35
330/6.3
390/6.3
6
22
L33
470/25
470/35
330/6.3
390/6.3
10
33
L32
330/35
330/35
330/6.3
390/6.3
40
47
L39
330/35
270/50
220/10
330/10
8
22
L41
470/25
560/16
220/10
330/10
10
22
L41
560/25
560/25
220/10
330/10
15
33
L40
330/35
330/35
220/10
330/10
40
47
L39
330/35
270/35
220/10
330/10
9
22
L33
470/25
560/16
220/10
330/10
20
68
L38
180/35
180/35
100/10
270/10
40
68
L38
180/35
180/35
100/10
270/10
15
22
L41
470/25
470/25
100/16
180/16
18
33
L40
330/25
330/25
100/16
180/16
30
68
L44
180/25
180/25
100/16
120/20
40
68
L44
180/35
180/35
100/16
120/20
15
33
L32
330/25
330/25
100/16
180/16
20
68
L38
180/25
180/25
100/16
120/20
40
150
L42
82/25
82/25
68/20
68/25
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 needed 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 needed. But 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 value or 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)
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 LM2599. The most stressful condition for this diode is an overload or shorted output
condition. See Table 4. In this example, a 5-A, 20-V, 1N5823 Schottky diode provides the best performance, and
is not overstressed even for a shorted output.
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Table 4. Diode Selection Table
3-A DIODES
VR
SURFACE-MOUNT
SCHOTTKY
20 V
ULTRA FAST
RECOVERY
SK32
4-A TO 6-A DIODES
THROUGH-HOLE
SCHOTTKY
ULTRA FAST
RECOVERY
SURFACE-MOUNT
SCHOTTKY
ULTRA FAST
RECOVERY
30 V
SK33
All of these diodes
are rated to at least
50 V
SR502
SR302
1N5823
1N5821
MBR330
50WQ03
All of these diodes
are rated to at least
50 V
50 V
or
more
50WQ04
SR504
MBR340
30WQ04
MURS320
31DQ04
SK35
30WF10
SR305
MBRS360
MBR350
30WQ05
31DQ05
All of these diodes
are rated to at least
50 V
SB530
SR304
MBRS340
SR503
1N5824
1N5822
40 V
ULTRA FAST
RECOVERY
SB520
All of these diodes
are rate to at least
50 V
31DQ03
SK34
SCHOTTKY
1N5820
MBR320
30WQ03
THROUGH-HOLE
1N5825
MUR320
MURS620
SB540
50WF10
50WQ05
MUR620
HER601
SB550
50SQ080
The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
This diode must be fast (short reverse recovery time) and must be placed close to the LM2599 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. Ultra-fast recovery diodes
typically have reverse recovery times of 50 ns or less. Rectifiers such as the IN5400 series are much too slow
and must not be used.
9.2.1.2.4 Input Capacitor (CIN)
A low-ESR aluminum or tantalum bypass capacitor is needed 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. The curve shown in Figure 31 shows typical RMS current ratings
for several different aluminum electrolytic capacitor values.
This capacitor must be placed 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 that 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) would be needed. 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 3-A load, a capacitor with a RMS current rating of at least 1.5 A is needed. The
curves shown in Figure 31 can be used to select an appropriate input capacitor. From the curves, locate the 35-V
line and note which capacitor values have RMS current ratings greater than 1.5 A. A 680-μF, 35-V capacitor
could be used.
For a through-hole design, a 680-μF, 35-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).
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9.2.1.3 Application Curves
Horizontal time base: 50 μs/div,
VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA to 2 A,
L = 32 μH, COUT = 220 μF, COUT ESR = 50 mΩ
A: Output Voltage, 100 mV/div. (AC)
B: 500-mA to 2-A load pulse
Horizontal time base: 2 μs/div,
VIN = 20 V, VOUT = 5 V, ILOAD = 2 A
L = 32 μH, COUT = 220 μF, COUT, ESR = 50 mΩ
A: Output pin voltage, 10 V/div.
B: Inductor current, 1 A/div.
C: Output ripple voltage, 50 mV/div.
Figure 41. Continuous Mode Switching Waveforms
Figure 42. Load Transient Response for Continuous Mode
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9.2.2 Adjustable Output Voltage Version
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 = 3 A.
CIN = 470-μF, 35-V, aluminum electrolytic Nichicon PL Series
COUT = 220-μF, 35-V, aluminum electrolytic Nichicon PL Series
D1 = 5-A, 30-V Schottky Rectifier, 1N5824
L1 = 68 μH, L38
R1 = 1 kΩ, 1%
R2 = 7.15k, 1%
CFF = 3.3 nF
RFF = 3 kΩ
Typical Values
CSS = 0.1 μF
CDELAY = 0.1 μF
RPULLUP = 4.7k
Figure 43. Typical Application for Adjustable Output Voltage Versions
9.2.2.1 Design Requirements
Table 5 lists the design parameters of this example.
Table 5. Design Parameters
PARAMETER
EXAMPLE VALUE
Regulated output voltage, VOUT
20 V
Maximum DC Input voltage, VIN(max)
28 V
Maximum load current, ILOAD(max)
3A
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.
Use Equation 1 to select the appropriate resistor values.
(1)
Select a value for R1 between 240 Ω and 1.5 kΩ using Equation 2. 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).
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(2)
Select R1 to be 1 kΩ, 1%. Solve for R2 with Equation 3.
(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) from Equation 4.
where
•
•
VSAT = internal switch saturation voltage = 1.16 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 38.
E × T = 34.2 (V × μs)
3. On the horizontal axis, select the maximum load current.
ILOAD(max) = 3 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 38, the inductance region intersected by the 34 (V × μs) horizontal line and the 3-A
vertical line is 47 μH, and the inductor code is L39.
5. Select an appropriate inductor from the four manufacturer's part numbers listed in Table 1. From the table in
Table 1, locate line L39, and select an inductor part number from the list of manufacturers part numbers.
9.2.2.2.3 Output Capacitor Selection (COUT)
In the majority of applications, low-ESR electrolytic or solid tantalum capacitors between 82 μF and 820 μF
provide the best results. This capacitor must be placed close to the IC using short capacitor leads and short
copper traces. Do not use capacitors larger than 820 μF. For additional information, see Output Capacitor (COUT).
To simplify the capacitor selection procedure, see Table 6. This table contains different output voltages, and lists
various output capacitors that provides the best design solutions.
From the quick design table shown in 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. 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 the table be used.
In this example, through-hole aluminum electrolytic capacitors from several different manufacturers are available.
• 220-V, 35-A Panasonic HFQ series
• 150-V, 35-A Nichicon PL series
The capacitor voltage rating must be at least 1.5 times greater than the output voltage, and often much higher
voltage ratings are needed to satisfy the low ESR requirements needed for low output ripple voltage.
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For a 20-V output, a capacitor rating of at least 30 V or more is needed. 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 the table. See 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 100 pF and 33 nF, and is wired in parallel with the output voltage setting resistor,
R2. It provides additional stability for high output voltages, low input-output voltages, and/or very low ESR output
capacitors, such as solid tantalum capacitors.
(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, they are not recommended).
Table 6 contains feedforward capacitor values for various output voltages. In this example, a 560-pF capacitor is
needed.
Table 6. Output Capacitor and Feedforward Capacitor Selection Table
THROUGH-HOLE OUTPUT CAPACITOR
OUTPUT
VOLTAGE PANASONIC HFQ
NICHICON PL
FEEDFORWARD
(V)
SERIES (μF/V)
SERIES (μF/V)
CAPACITOR
SURFACE-MOUNT OUTPUT CAPACITOR
AVX TPS
SERIES (μF/V)
SPRAGUE 595D
SERIES (μF/V)
FEEDFORWARD
CAPACITOR
2
820/35
820/35
33 nF
330/6.3
470/4
33 nF
4
560/35
470/35
10 nF
330/6.3
390/6.3
10 nF
6
470/25
470/25
3.3 nF
220/10
330/10
3.3 nF
9
330/25
330/25
1.5 nF
100/16
180/16
1.5 nF
12
330/25
330/25
1 nF
100/16
180/16
1 nF
15
220/35
220/35
680 pF
68/20
120/20
680 pF
24
220/35
150/35
560 pF
33/25
33/25
220 pF
28
100/50
100/50
390 pF
10/35
15/50
220 pF
9.2.2.2.5 Catch Diode Selection (D1)
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 LM2599. The most stressful condition for this diode is an overload or shorted output
condition (see Table 4). Schottky diodes provide the best performance, and in this example a 3-A, 40-V, 1N5825
Schottky diode would be a good choice. The 3-A diode rating is more than adequate and is not overstressed
even for a shorted output.
The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
This diode must be fast (short reverse recovery time) and must be placed close to the LM2599 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 a good choice, but some types with an
abrupt turn-off 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 are much too slow and must not be
used.
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9.2.2.2.6 Input Capacitor (CIN)
A low-ESR aluminum or tantalum bypass capacitor is needed 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. The curve shown in Figure 31 shows typical RMS current ratings
for several different aluminum electrolytic capacitor values.
This capacitor must be placed 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 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.
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) would be needed. 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 3-A load, a capacitor with a RMS current rating of at least 1.5 A is needed.
The curves shown in Figure 31 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 1.5 A. Either a 470-μF or 680μF, 50-V capacitor could be used.
For a through-hole design, a 680-μF/50V 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 sure current rating. The TPS series available from AVX, and the 593D series from Sprague are
both surge current tested. For additional information, see Input Capacitor (CIN).
9.2.2.3 Application Curves
Horizontal Time Base: 200 μs/div
VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA to 2 A
L = 10 μH, COUT = 330 μF, COUT ESR = 45 mΩ
A: Output voltage, 100 mV/div. (AC)
B: 500-mA to 2-A load pulse
Horizontal time base: 2 μs/div
VIN = 20 V, VOUT = 5 V, ILOAD = 500 mA,
L = 10 μH, COUT = 330 μF, COUT ESR = 45 mΩ
A: Output pin voltage, 10 V/div.
B: Inductor current, 0.5 A/div.
C: Output ripple voltage, 100 mV/div.
Figure 44. Discontinuous Mode Switching Waveforms
Figure 45. Load Transient Response
for Discontinuous Mode
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10 Power Supply Recommendations
The LM2599 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 to select 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 to select 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 = 470-μF, 50-V, aluminum electrolytic Panasonic HFQ Series
COUT = 330-μF, 35-V, aluminum electrolytic Panasonic HFQ Series
D1 = 5-A, 40-V Schottky rectifier, 1N5825
L1 = 47-μH, L39, Renco through hole
RPULL UP = 10k
CDELAY = 0.1 μF
CSD/SS = 0.1 μF
Thermalloy heat sink #7020
Figure 46. Typical Through-Hole PCB Layout, Fixed Output (1x Size), Double-Sided
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SNVS123D – APRIL 1998 – REVISED MAY 2016
Layout Examples (continued)
CIN = 470-μF, 50-V, aluminum electrolytic Panasonic, HFQ Series
COUT = 220-μF, 35-V, aluminum electrolytic Panasonic, HFQ Series
D1 = 5-A, 40-V Schottky Rectifier, 1N5825
L1 = 47-μH, L39, Renco, through-hole
R1 = 1 kΩ, 1%
R2 = Use formula in Detailed Design Procedure
CFF = See Figure 35
RFF = See Feedforward Capacitor (CFF) for Adjustable Output Voltage Version Only
RPULL UP = 10k
CDELAY = 0.1 μF
CSD/SS= 0.1 μF
Thermalloy heat sink #7020
Figure 47. Typical Through-Hole PCB Layout, Adjustable Output (1x Size), Double-Sided
11.3 Thermal Considerations
The LM2599 is available in two packages, a 7-pin TO-220 and a 7-pin surface-mount TO-263.
The TO-220 package needs a heat sink under most conditions. The size of the heat sink depends on the input
voltage, the output voltage, the load current and the ambient temperature. The curves in Figure 48 show the
LM2599T junction temperature rises above ambient temperature for a 3-A load and different input and output
voltages. The data for these curves was taken with the LM2599T (TO-220 package) operating as a buck
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
more heat sinking.
The TO-263 surface-mount package tab is designed to be soldered to the copper on a printed-circuit board. 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 6 in2, only small improvements in heat
dissipation are realized. If further thermal improvements are needed, double-sided, multilayer PCBs with large
copper areas or airflow are recommended.
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Thermal Considerations (continued)
The curves shown in Figure 49 show the LM2599S (TO-263 package) junction temperature rise above ambient
temperature with a 2-A 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. When
load currents higher than 2 A are used, double-sided or multilayer PCBs with large copper areas or airflow might
be needed, especially for high ambient temperatures and high output voltages.
CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-220 PACKAGE (NDZ)
Capacitors
Through-hole electrolytic
Inductor
Through-hole Renco
Diode
Through-hole, 5-A, 40-V Schottky
PCB
3-square inch, single-sided, 2-oz copper (0.0028″)
Figure 48. Junction Temperature Rise, TO-220
36
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Thermal Considerations (continued)
CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-263 PACKAGE (KTW)
Capacitors
Surface-mount, tantalum molded D size
Inductor
Surface-mount, Pulse engineering, 68 μH
Diode
Surface-mount, 5-A, 40-V, Schottky
PCB
9-square inch, single-sided, 2-oz copper (0.0028″)
Figure 49. Junction Temperature Rise, TO-263
For the best thermal performance, wide copper traces and generous amounts of printed-circuit board 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 affects 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.
38
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PACKAGE OPTION ADDENDUM
www.ti.com
30-Sep-2021
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)
LM2599S-12/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-12 P+
LM2599S-3.3/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-3.3 P+
LM2599S-5.0
NRND
DDPAK/
TO-263
KTW
7
45
Non-RoHS
& Green
Call TI
Level-3-235C-168 HR
-40 to 125
LM2599S
-5.0 P+
LM2599S-5.0/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-5.0 P+
LM2599S-ADJ/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
45
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-ADJ P+
LM2599SX-12/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-12 P+
LM2599SX-3.3/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-3.3 P+
LM2599SX-5.0/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-5.0 P+
LM2599SX-ADJ/NOPB
ACTIVE
DDPAK/
TO-263
KTW
7
500
RoHS-Exempt
& Green
SN
Level-3-245C-168 HR
-40 to 125
LM2599S
-ADJ P+
LM2599T-12/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-12 P+
LM2599T-5.0/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-5.0 P+
LM2599T-ADJ
NRND
TO-220
NDZ
7
45
Non-RoHS
& Green
Call TI
Level-1-NA-UNLIM
-40 to 125
LM2599T
-ADJ P+
LM2599T-ADJ/NOPB
ACTIVE
TO-220
NDZ
7
45
RoHS & Green
SN
Level-1-NA-UNLIM
-40 to 125
LM2599T
-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.
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
30-Sep-2021
(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