LM2596 - Ti PDF
LM2596 - Ti PDF
LM2596 - Ti PDF
Regulator
LM2596
SIMPLE SWITCHER ® Power Converter 150 kHz
3A Step-Down Voltage Regulator
General Description Features
The LM2596 series of regulators are monolithic integrated n 3.3V, 5V, 12V, and adjustable output versions
circuits that provide all the active functions for a step-down n Adjustable version output voltage range, 1.2V to 37V
(buck) switching regulator, capable of driving a 3A load with ± 4% max over line and load conditions
excellent line and load regulation. These devices are avail- n Available in TO-220 and TO-263 packages
able in fixed output voltages of 3.3V, 5V, 12V, and an adjust- n Guaranteed 3A output load current
able output version. n Input voltage range up to 40V
Requiring a minimum number of external components, these n Requires only 4 external components
regulators are simple to use and include internal frequency n Excellent line and load regulation specifications
compensation†, and a fixed-frequency oscillator.
n 150 kHz fixed frequency internal oscillator
The LM2596 series operates at a switching frequency of n TTL shutdown capability
150 kHz thus allowing smaller sized filter components than
n Low power standby mode, IQ typically 80 µA
what would be needed with lower frequency switching regu-
lators. Available in a standard 5-lead TO-220 package with n High efficiency
several different lead bend options, and a 5-lead TO-263 n Uses readily available standard inductors
surface mount package. n Thermal shutdown and current limit protection
A standard series of inductors are available from several
different manufacturers optimized for use with the LM2596 Applications
series. This feature greatly simplifies the design of n Simple high-efficiency step-down (buck) regulator
switch-mode power supplies. n On-card switching regulators
Other features include a guaranteed ± 4% tolerance on out- n Positive to negative converter
put voltage under specified input voltage and output load Note: †Patent Number 5,382,918.
conditions, and ± 15% on the oscillator frequency. External
shutdown is included, featuring typically 80 µA standby cur-
rent. Self protection features include a two stage frequency
reducing current limit for the output switch and an over
temperature shutdown for complete protection under fault
conditions.
01258301
SIMPLE SWITCHER ® and Switchers Made Simple ® are registered trademarks of National Semiconductor Corporation.
01258302 01258303
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LM2596
Absolute Maximum Ratings (Note 1) Human Body Model (Note 2) 2 kV
If Military/Aerospace specified devices are required, Lead Temperature
please contact the National Semiconductor Sales Office/ S Package
Distributors for availability and specifications.
Vapor Phase (60 sec.) +215˚C
Maximum Supply Voltage 45V Infrared (10 sec.) +245˚C
ON /OFF Pin Input Voltage −0.3 ≤ V ≤ +25V T Package (Soldering, 10 sec.) +260˚C
Feedback Pin Voltage −0.3 ≤ V ≤+25V Maximum Junction Temperature +150˚C
Output Voltage to Ground
(Steady State) −1V
Operating Conditions
Power Dissipation Internally limited
Storage Temperature Range −65˚C to +150˚C Temperature Range −40˚C ≤ TJ ≤ +125˚C
LM2596-3.3
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
LM2596-3.3
Units
Symbol Parameter Conditions Typ Limit
(Limits)
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT Output Voltage 4.75V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
η Efficiency VIN = 12V, ILOAD = 3A 73 %
LM2596-5.0
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
LM2596-5.0
Units
Symbol Parameter Conditions Typ Limit
(Limits)
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT Output Voltage 7V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 5.0 V
4.800/4.750 V(min)
5.200/5.250 V(max)
η Efficiency VIN = 12V, ILOAD = 3A 80 %
LM2596-12
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
LM2596-12
Units
Symbol Parameter Conditions Typ Limit
(Limits)
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VOUT Output Voltage 15V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 12.0 V
11.52/11.40 V(min)
12.48/12.60 V(max)
η Efficiency VIN = 25V, ILOAD = 3A 90 %
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LM2596
LM2596-ADJ
Electrical Characteristics
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
LM2596-ADJ
Units
Symbol Parameter Conditions Typ Limit
(Limits)
(Note 3) (Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit Figure 1
VFB Feedback Voltage 4.5V ≤ VIN ≤ 40V, 0.2A ≤ ILOAD ≤ 3A 1.230 V
VOUT programmed for 3V. Circuit of Figure 1 1.193/1.180 V(min)
1.267/1.280 V(max)
η Efficiency VIN = 12V, VOUT = 3V, ILOAD = 3A 73 %
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LM2596
All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for TJ = 25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, VIN = 12V for the 3.3V, 5V, and Adjustable version and VIN = 24V for the 12V ver-
sion. ILOAD = 500 mA
LM2596-XX
Units
Symbol Parameter Conditions Typ Limit
(Limits)
(Note 3) (Note 4)
IH ON /OFF Pin Input Current VLOGIC = 2.5V (Regulator OFF) 5 µA
15 µA(max)
IL VLOGIC = 0.5V (Regulator ON) 0.02 µA
5 µA(max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 3: Typical numbers are at 25˚C and represent the most likely norm.
Note 4: All limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to
calculate Average Outgoing Quality Level (AOQL).
Note 5: External components such as the catch diode, inductor, input and output capacitors, and voltage programming resistors can affect switching regulator
system performance. When the LM2596 is used as shown in the Figure 1 test circuit, system performance will be as shown in system parameters section of Electrical
Characteristics.
Note 6: The switching frequency is reduced when the second stage current limit is activated.
Note 7: No diode, inductor or capacitor connected to output pin.
Note 8: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 9: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and the ADJ. version, and 15V for the 12V version, to force the output transistor
switch OFF.
Note 10: VIN = 40V.
Note 11: Junction to ambient thermal resistance (no external heat sink) for the TO-220 package mounted vertically, with the leads soldered to a printed circuit board
with (1 oz.) copper area of approximately 1 in2.
Note 12: Junction to ambient thermal resistance with the TO-263 package tab soldered to a single printed circuit board with 0.5 in2 of (1 oz.) copper area.
Note 13: 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.
Note 14: 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 LM2596S side of the board, and approximately 16 in2 of copper on the other side of the p-c board. See Application Information in this data sheet and the thermal
model in Switchers Made Simple™ version 4.3 software.
Normalized
Output Voltage Line Regulation Efficiency
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LM2596
Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Switch Saturation
Voltage Switch Current Limit Dropout Voltage
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LM2596
Typical Performance Characteristics (Circuit of Figure 1) (Continued)
Feedback Pin
Bias Current
01258316
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LM2596
Typical Performance Characteristics
Continuous Mode Switching Waveforms Discontinuous Mode Switching Waveforms
VIN = 20V, VOUT = 5V, ILOAD = 2A VIN = 20V, VOUT = 5V, ILOAD = 500 mA
L = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ L = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩ
01258317 01258318
Horizontal Time Base: 2 µs/div. Horizontal Time Base: 2 µs/div.
A: Output Pin Voltage, 10V/div. A: Output Pin Voltage, 10V/div.
B: Inductor Current 1A/div. B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 50 mV/div. C: Output Ripple Voltage, 100 mV/div.
Load Transient Response for Continuous Mode Load Transient Response for Discontinuous Mode
VIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A VIN = 20V, VOUT = 5V, ILOAD = 500 mA to 2A
L = 32 µH, COUT = 220 µF, COUT ESR = 50 mΩ L = 10 µH, COUT = 330 µF, COUT ESR = 45 mΩ
01258320
Horizontal Time Base: 200 µs/div.
01258319
A: Output Voltage, 100 mV/div. (AC)
Horizontal Time Base: 100 µs/div.
B: 500 mA to 2A Load Pulse
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
01258322
CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 220 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 5A, 40V Schottky Rectifier, 1N5825
L1 — 68 µH, L38
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LM2596
Test Circuit and Layout Guidelines (Continued)
01258323
As in any switching regulator, layout is very important. Rap- When using the adjustable version, special care must be
idly switching currents associated with wiring inductance can taken as to the location of the feedback resistors and the
generate voltage transients which can cause problems. For associated wiring. Physically locate both resistors near the
minimal inductance and ground loops, the wires indicated by IC, and route the wiring away from the inductor, especially an
heavy lines should be wide printed circuit traces and open core type of inductor. (See application section for more
should be kept as short as possible. For best results, information.)
external components should be located as close to the
switcher lC as possible using ground plane construction or
single point grounding.
If open core inductors are used, special care must be
taken as to the location and positioning of this type of induc-
tor. Allowing the inductor flux to intersect sensitive feedback,
lC groundpath and COUT wiring can cause problems.
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LM2596
LM2596 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given: Given:
VOUT = Regulated Output Voltage (3.3V, 5V or 12V) VOUT = 5V
VIN(max) = Maximum DC Input Voltage VIN(max) = 12V
ILOAD(max) = Maximum Load Current ILOAD(max) = 3A
1. Inductor Selection (L1) 1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Fig- A. Use the inductor selection guide for the 5V version shown
ures Figure 4, Figure 5, or Figure 6. (Output voltages of 3.3V, in Figure 5.
5V, or 12V respectively.) For all other voltages, see the design B. From the inductor value selection guide shown in Figure 5,
procedure for the adjustable version. the inductance region intersected by the 12V horizontal line
B. From the inductor value selection guide, identify the induc- and the 3A vertical line is 33 µH, and the inductor code is L40.
tance region intersected by the Maximum Input Voltage line C. The inductance value required is 33 µH. From the table in
and the Maximum Load Current line. Each region is identified Figure 8, go to the L40 line and choose an inductor part
by an inductance value and an inductor code (LXX). number from any of the four manufacturers shown. (In most
C. Select an appropriate inductor from the four manufacturer’s instance, both through hole and surface mount inductors are
part numbers listed in Figure 8. available.)
2. Output Capacitor Selection (COUT) 2. Output Capacitor Selection (COUT)
A. In the majority of applications, low ESR (Equivalent Series A. See section on output capacitors in application infor-
Resistance) electrolytic capacitors between 82 µF and 820 µF mation section.
and low ESR solid tantalum capacitors between 10 µF and B. From the quick design component selection table shown in
470 µF provide the best results. This capacitor should be Figure 2, locate the 5V output voltage section. In the load
located close to the IC using short capacitor leads and short current column, choose the load current line that is closest to
copper traces. Do not use capacitors larger than 820 µF . the current needed in your application, for this example, use
For additional information, see section on output capaci- the 3A line. In the maximum input voltage column, select the
tors in application information section. line that covers the input voltage needed in your application, in
B. To simplify the capacitor selection procedure, refer to the this example, use the 15V line. Continuing on this line are
quick design component selection table shown in Figure 2. recommended inductors and capacitors that will provide the
This table contains different input voltages, output voltages, best overall performance.
and load currents, and lists various inductors and output ca- The capacitor list contains both through hole electrolytic and
pacitors that will provide the best design solutions. surface mount tantalum capacitors from four different capaci-
C. The capacitor voltage rating for electrolytic capacitors tor manufacturers. It is recommended that both the manufac-
should be at least 1.5 times greater than the output voltage, turers and the manufacturer’s series that are listed in the table
and often much higher voltage ratings are needed to satisfy be used.
the low ESR requirements for low output ripple voltage. In this example aluminum electrolytic capacitors from several
D. For computer aided design software, see Switchers Made different manufacturers are available with the range of ESR
Simple™ version 4.3 or later. numbers needed.
330 µF 35V Panasonic HFQ Series
330 µF 35V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V or
more is needed. But even a low ESR, switching grade, 220 µF
10V aluminum electrolytic capacitor would exhibit approxi-
mately 225 mΩ of ESR (see the curve in Figure 14 for the ESR
vs voltage rating). This amount of ESR would result in rela-
tively 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) should be selected. A
16V or 25V capacitor will reduce the ripple voltage by approxi-
mately half.
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LM2596
LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1) 3. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3 times A. Refer to the table shown in Figure 11. In this example, a 5A,
greater than the maximum load current. Also, if the power 20V, 1N5823 Schottky diode will provide the best perfor-
supply design must withstand a continuous output short, the mance, and will not be overstressed even for a shorted output.
diode should have a current rating equal to the maximum
current limit of the LM2596. The most stressful condition for
this diode is an overload or shorted output condition.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and
must be located close to the LM2596 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 should be the first
choice, especially in low output voltage applications. Ultra-fast
recovery, or High-Efficiency rectifiers also provide good re-
sults. Ultra-fast recovery diodes typically have reverse recov-
ery times of 50 ns or less. Rectifiers such as the 1N5400
series are much too slow and should not be used.
4. Input Capacitor (CIN) 4. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the Input capacitor are the input
between the input pin and ground pin to prevent large voltage voltage rating and the RMS current rating. With a nominal
transients from appearing at the input. This capacitor should input voltage of 12V, an aluminum electrolytic capacitor with a
be located close to the IC using short leads. In addition, the voltage rating greater than 18V (1.5 x VIN) would be needed.
RMS current rating of the input capacitor should be selected to The next higher capacitor voltage rating is 25V.
be at least 1⁄2 the DC load current. The capacitor manufactur- The RMS current rating requirement for the input capacitor in
ers data sheet must be checked to assure that this current a buck regulator is approximately 1⁄2 the DC load current. In
rating is not exceeded. The curve shown in Figure 13 shows this example, with a 3A load, a capacitor with a RMS current
typical RMS current ratings for several different aluminum rating of at least 1.5A is needed. The curves shown in Figure
electrolytic capacitor values. 13 can be used to select an appropriate input capacitor. From
For an aluminum electrolytic, the capacitor voltage rating the curves, locate the 35V line and note which capacitor
should be approximately 1.5 times the maximum input volt- values have RMS current ratings greater than 1.5A. A 680 µF/
age. Caution must be exercised if solid tantalum capacitors 35V capacitor could be used.
are used (see Application Information on input capacitor). The For a through hole design, a 680 µF/35V electrolytic capacitor
tantalum capacitor voltage rating should be 2 times the maxi- (Panasonic HFQ series or Nichicon PL series or equivalent)
mum input voltage and it is recommended that they be surge would be adequate. other types or other manufacturers ca-
current tested by the manufacturer. pacitors can be used provided the RMS ripple current ratings
Use caution when using ceramic capacitors for input bypass- are adequate.
ing, because it may cause severe ringing at the VIN pin. For surface mount designs, solid tantalum capacitors can be
For additional information, see section on input capaci- used, but caution must be exercised with regard to the capaci-
tors in Application Information section. tor surge current rating (see Application Information on input
capacitors in this data sheet). The TPS series available from
AVX, and the 593D series from Sprague are both surge cur-
rent tested.
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LM2596
LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
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LM2596
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given: Given:
VOUT = Regulated Output Voltage VOUT = 20V
VIN(max) = Maximum Input Voltage VIN(max) = 28V
ILOAD(max) = Maximum Load Current ILOAD(max) = 3A
F = Switching Frequency (Fixed at a nominal 150 kHz). F = Switching Frequency (Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R1 and R2, as 1. Programming Output Voltage (Selecting R1 and R2, as
shown in Figure 1 ) shown in Figure 1 )
Use the following formula to select the appropriate resistor Select R1 to be 1 kΩ, 1%. Solve for R2.
values.
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LM2596
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
3. Output Capacitor Selection (COUT) 3. Output Capacitor SeIection (COUT)
A. In the majority of applications, low ESR electrolytic or solid A. See section on COUT in Application Information section.
tantalum capacitors between 82 µF and 820 µF provide the B. From the quick design table shown in Figure 3, locate the
best results. This capacitor should be located close to the IC output voltage column. From that column, locate the output
using short capacitor leads and short copper traces. Do not voltage closest to the output voltage in your application. In this
use capacitors larger than 820 µF. For additional informa- example, select the 24V line. Under the output capacitor sec-
tion, see section on output capacitors in application in- tion, select a capacitor from the list of through hole electrolytic
formation section. or surface mount tantalum types from four different capacitor
B. To simplify the capacitor selection procedure, refer to the manufacturers. It is recommended that both the manufactur-
quick design table shown in Figure 3. This table contains ers and the manufacturers series that are listed in the table be
different output voltages, and lists various output capacitors used.
that will provide the best design solutions. In this example, through hole aluminum electrolytic capacitors
C. The capacitor voltage rating should be at least 1.5 times from several different manufacturers are available.
greater than the output voltage, and often much higher voltage 220 µF/35V Panasonic HFQ Series
ratings are needed to satisfy the low ESR requirements
150 µF/35V Nichicon PL Series
needed for low output ripple voltage.
C. For a 20V output, a capacitor rating of at least 30V or more
is needed. In this example, either a 35V or 50V capacitor
would work. A 35V rating was chosen, although a 50V rating
could also be used if a lower output ripple voltage is needed.
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. Refer to
the capacitor manufacturers data sheet for this information.
4. Feedforward Capacitor (CFF) (See Figure 1) 4. Feedforward Capacitor (CFF)
For output voltages greater than approximately 10V, an addi- The table shown in Figure 3 contains feed forward capacitor
tional capacitor is required. The compensation capacitor is values for various output voltages. In this example, a 560 pF
typically between 100 pF and 33 nF, and is wired in parallel capacitor is needed.
with the output voltage setting resistor, R2. It provides addi-
tional stability for high output voltages, low input-output volt-
ages, and/or very low ESR output capacitors, such as solid
tantalum capacitors.
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LM2596
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
5. Catch Diode Selection (D1) 5. Catch Diode Selection (D1)
A. The catch diode current rating must be at least 1.3 times A. Refer to the table shown in Figure 11. Schottky diodes
greater than the maximum load current. Also, if the power provide the best performance, and in this example a 5A, 40V,
supply design must withstand a continuous output short, the 1N5825 Schottky diode would be a good choice. The 5A diode
diode should have a current rating equal to the maximum rating is more than adequate and will not be overstressed
current limit of the LM2596. The most stressful condition for even for a shorted output.
this diode is an overload or shorted output condition.
B. The reverse voltage rating of the diode should be at least
1.25 times the maximum input voltage.
C. This diode must be fast (short reverse recovery time) and
must be located close to the LM2596 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 should 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. Recti-
fiers such as the 1N4001 series are much too slow and should
not be used.
6. Input Capacitor (CIN) 6. Input Capacitor (CIN)
A low ESR aluminum or tantalum bypass capacitor is needed The important parameters for the Input capacitor are the input
between the input pin and ground to prevent large voltage voltage rating and the RMS current rating. With a nominal
transients from appearing at the input. In addition, the RMS input voltage of 28V, an aluminum electrolytic aluminum elec-
current rating of the input capacitor should be selected to be at trolytic capacitor with a voltage rating greater than 42V (1.5 x
least 1⁄2 the DC load current. The capacitor manufacturers VIN) would be needed. Since the the next higher capacitor
data sheet must be checked to assure that this current rating voltage rating is 50V, a 50V capacitor should be used. The
is not exceeded. The curve shown in Figure 13 shows typical capacitor voltage rating of (1.5 x VIN) is a conservative guide-
RMS current ratings for several different aluminum electrolytic line, and can be modified somewhat if desired.
capacitor values. The RMS current rating requirement for the input capacitor of
This capacitor should be located close to the IC using short a buck regulator is approximately 1⁄2 the DC load current. In
leads and the voltage rating should be approximately 1.5 this example, with a 3A load, a capacitor with a RMS current
times the maximum input voltage. rating of at least 1.5A is needed.
If solid tantalum input capacitors are used, it is recomended The curves shown in Figure 13 can be used to select an
that they be surge current tested by the manufacturer. appropriate input capacitor. From the curves, locate the 50V
Use caution when using a high dielectric constant ceramic line and note which capacitor values have RMS current ratings
capacitor for input bypassing, because it may cause severe greater than 1.5A. Either a 470 µF or 680 µF, 50V capacitor
ringing at the VIN pin. could be used.
For additional information, see section on input capaci- For a through hole design, a 680 µF/50V electrolytic capacitor
tors in application information section. (Panasonic HFQ series or Nichicon PL series or equivalent)
would be adequate. Other types or other manufacturers ca-
pacitors 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 capaci-
tor surge current rting (see Application Information or input
capacitors in this data sheet). The TPS series available from
AVX, and the 593D series from Sprague are both surge cur-
rent tested.
To further simplify the buck regulator design procedure, Na-
tional Semiconductor is making available computer design
software to be used with the Simple Switcher line ot switching
regulators. Switchers Made Simple (version 4.3 or later) is
available on a 31⁄2" diskette for IBM compatible computers.
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LM2596
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
Output Through Hole Output Capacitor Surface Mount Output Capacitor
Voltage Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
(V) HFQ Series Series Capacitor Series 595D Series Capacitor
(µF/V) (µF/V) (µF/V) (µF/V)
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
01258324 01258326
01258325 01258327
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LM2596
LM2596 Series Buck Regulator Design Procedure (Continued)
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LM2596
LM2596 Series Buck Regulator Design Procedure (Continued)
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LM2596
Block Diagram
01258321
FIGURE 12.
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LM2596
Application Information (Continued) RMS ripple current rating, voltage rating, and capacitance
value. For the output capacitor, the ESR value is the most
A graph shown in Figure 13 shows the relationship between important parameter.
an electrolytic capacitor value, its voltage rating, and the The output capacitor requires an ESR value that has an
RMS current it is rated for. These curves were obtained from upper and lower limit. For low output ripple voltage, a low
the Nichicon “PL” series of low ESR, high reliability electro- ESR value is needed. This value is determined by the maxi-
lytic capacitors designed for switching regulator applications. mum allowable output ripple voltage, typically 1% to 2% of
Other capacitor manufacturers offer similar types of capaci- the output voltage. But if the selected capacitor’s ESR is
tors, but always check the capacitor data sheet. extremely low, there is a possibility of an unstable feedback
“Standard” electrolytic capacitors typically have much higher loop, resulting in an oscillation at the output. Using the
ESR numbers, lower RMS current ratings and typically have capacitors listed in the tables, or similar types, will provide
a shorter operating lifetime. design solutions under all conditions.
Because of their small size and excellent performance, sur- If very low output ripple voltage (less than 15 mV) is re-
face mount solid tantalum capacitors are often used for input quired, refer to the section on Output Voltage Ripple and
bypassing, but several precautions must be observed. A Transients for a post ripple filter.
small percentage of solid tantalum capacitors can short if the An aluminum electrolytic capacitor’s ESR value is related to
inrush current rating is exceeded. This can happen at turn on the capacitance value and its voltage rating. In most cases,
when the input voltage is suddenly applied, and of course, higher voltage electrolytic capacitors have lower ESR values
higher input voltages produce higher inrush currents. Sev- (see Figure 14 ). Often, capacitors with much higher voltage
eral capacitor manufacturers do a 100% surge current test- ratings may be needed to provide the low ESR values re-
ing on their products to minimize this potential problem. If quired for low output ripple voltage.
high turn on currents are expected, it may be necessary to
The output capacitor for many different switcher designs
limit this current by adding either some resistance or induc-
often can be satisfied with only three or four different capaci-
tance before the tantalum capacitor, or select a higher volt-
tor values and several different voltage ratings. See the
age capacitor. As with aluminum electrolytic capacitors, the
quick design component selection tables in Figure 2 and 4
RMS ripple current rating must be sized to the load current.
for typical capacitor values, voltage ratings, and manufactur-
ers capacitor types.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version) Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem-
CFF — A Feedforward Capacitor CFF, shown across R2 in
peratures and typically rises 3X @ −25˚C and as much as
Figure 1 is used when the ouput voltage is greater than 10V
10X at −40˚C. See curve shown in Figure 15.
or when COUT has a very low ESR. This capacitor adds lead
compensation to the feedback loop and increases the phase Solid tantalum capacitors have a much better ESR spec for
margin for better loop stability. For CFF selection, see the cold temperatures and are recommended for temperatures
design procedure section. below −25˚C.
01258329
01258328
FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
FIGURE 13. RMS Current Ratings for Low ESR
Electrolytic Capacitors (Typical)
CATCH DIODE
OUTPUT CAPACITOR Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
COUT — An output capacitor is required to filter the output
a fast diode and must be located close to the LM2596 using
and provide regulator loop stability. Low impedance or low
short leads and short printed circuit traces.
ESR Electrolytic or solid tantalum capacitors designed for
switching regulator applications must be used. When select- Because of their very fast switching speed and low forward
ing an output capacitor, the important capacitor parameters voltage drop, Schottky diodes provide the best performance,
are; the 100 kHz Equivalent Series Resistance (ESR), the especially in low output voltage applications (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
www.national.com 20
LM2596
Application Information (Continued)
01258331
21 www.national.com
LM2596
Application Information (Continued) voltage, the ESR of the output capacitor must be low, how-
ever, caution must be exercised when using extremely low
DISCONTINUOUS MODE OPERATION ESR capacitors because they can affect the loop stability,
The selection guide chooses inductor values suitable for resulting in oscillation problems. If very low output ripple
continuous mode operation, but for low current applications voltage is needed (less than 20 mV), a post ripple filter is
and/or high input voltages, a discontinuous mode design recommended. (See Figure 1.) The inductance required is
may be a better choice. It would use an inductor that would typically between 1 µH and 5 µH, with low DC resistance, to
be physically smaller, and would need only one half to one maintain good load regulation. A low ESR output filter ca-
third the inductance value needed for a continuous mode pacitor is also required to assure good dynamic load re-
design. The peak switch and inductor currents will be higher sponse and ripple reduction. The ESR of this capacitor may
in a discontinuous design, but at these low load currents (1A be as low as desired, because it is out of the regulator
and below), the maximum switch current will still be less than feedback loop. The photo shown in Figure 17 shows a
the switch current limit. typical output ripple voltage, with and without a post ripple
filter.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The When observing output ripple with a scope, it is essential
output pin (switch) waveform can have some damped sinu- that a short, low inductance scope probe ground connection
soidal ringing present. (See Typical Performance Character- be used. Most scope probe manufacturers provide a special
istics photo titled Discontinuous Mode Switching Wave- probe terminator which is soldered onto the regulator board,
forms) This ringing is normal for discontinuous operation, preferable at the output capacitor. This provides a very short
and is not caused by feedback loop instabilities. In discon- scope ground thus eliminating the problems associated with
tinuous operation, there is a period of time where neither the the 3 inch ground lead normally provided with the probe, and
switch or the diode are conducting, and the inductor current provides a much cleaner and more accurate picture of the
has dropped to zero. During this time, a small amount of ripple voltage waveform.
energy can circulate between the inductor and the switch/ The voltage spikes are caused by the fast switching action of
diode parasitic capacitance causing this characteristic ring- the output switch and the diode, and the parasitic inductance
ing. Normally this ringing is not a problem, unless the ampli- of the output filter capacitor, and its associated wiring. To
tude becomes great enough to exceed the input voltage, and minimize these voltage spikes, the output capacitor should
even then, there is very little energy present to cause dam- be designed for switching regulator applications, and the
age. lead lengths must be kept very short. Wiring inductance,
Different inductor types and/or core materials produce differ- stray capacitance, as well as the scope probe used to evalu-
ent amounts of this characteristic ringing. Ferrite core induc- ate these transients, all contribute to the amplitude of these
tors have very little core loss and therefore produce the most spikes.
ringing. The higher core loss of powdered iron inductors When a switching regulator is operating in the continuous
produce less ringing. If desired, a series RC could be placed mode, the inductor current waveform ranges from a triangu-
in parallel with the inductor to dampen the ringing. The lar to a sawtooth type of waveform (depending on the input
computer aided design software Switchers Made Simple voltage). For a given input and output voltage, the
(version 4.3) will provide all component values for continu- peak-to-peak amplitude of this inductor current waveform
ous and discontinuous modes of operation. remains constant. As the load current increases or de-
creases, 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 will reach zero, and the
switcher will smoothly change from a continuous to a discon-
tinuous mode of operation. Most switcher designs (irregard-
less how large the inductor value is) will be forced to run
discontinuous if the output is lightly loaded. This is a per-
fectly acceptable mode of operation.
01258332
www.national.com 22
LM2596
Application Information (Continued) 1. Peak Inductor or peak switch current
01258333
23 www.national.com
LM2596
Application Information (Continued) 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
THERMAL CONSIDERATIONS the board.
The LM2596 is available in two packages, a 5-pin TO-220
(T) and a 5-pin surface mount TO-263 (S).
The TO-220 package needs a heat sink under most condi-
tions. The size of the heatsink depends on the input voltage,
the output voltage, the load current and the ambient tem-
perature. The curves in Figure 19 show the LM2596T junc-
tion temperature rises above ambient temperature for a 3A
load and different input and output voltages. The data for
these curves was taken with the LM2596T (TO-220 pack-
age) operating as a buck switching regulator in an ambient
temperature of 25˚C (still air). These temperature rise num-
bers are all approximate and there are many factors that can
affect these temperatures. Higher ambient temperatures re-
quire 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 01258334
other heat producing components, such as the catch diode
and inductor. The PC board copper area that the package is
soldered to should be at least 0.4 in2, and ideally should Circuit Data for Temperature Rise Curve
have 2 or more square inches of 2 oz. (0.0028) in) copper. TO-220 Package (T)
Additional copper area improves the thermal characteristics,
Capacitors Through hole electrolytic
but with copper areas greater than approximately 6 in2, only
small improvements in heat dissipation are realized. If fur- Inductor Through hole, Renco
ther thermal improvements are needed, double sided, mul- Diode Through hole, 5A 40V, Schottky
tilayer PC board with large copper areas and/or airflow are
PC board 3 square inches single sided 2 oz. copper
recommended.
(0.0028")
The curves shown in Figure 20 show the LM2596S (TO-263
package) junction temperature rise above ambient tempera- FIGURE 19. Junction Temperature Rise, TO-220
ture with a 2A 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 PC board 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 2A are
used, double sided or multilayer PC boards with large cop-
per areas and/or airflow might be needed, especially for high
ambient temperatures and high output voltages.
For the best thermal performance, wide copper traces and
generous amounts of printed circuit board copper should be
used in the board layout. (One exception to this is the output
(switch) pin, which should 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 01258335
numbers are all approximate, and there are many factors
that will affect these numbers. Some of these factors include
board size, shape, thickness, position, location, and even Circuit Data for Temperature Rise Curve
board temperature. Other factors are, trace width, total TO-263 Package (S)
printed circuit copper area, copper thickness, single- or Capacitors Surface mount tantalum, molded “D” size
double-sided, multilayer board and the amount of solder on
Inductor Surface mount, Pulse Engineering, 68 µH
the board. The effectiveness of the PC board to dissipate
heat also depends on the size, quantity and spacing of other Diode Surface mount, 5A 40V, Schottky
components on the board, as well as whether the surround- PC board 9 square inches single sided 2 oz. copper
ing air is still or moving. Furthermore, some of these com- (0.0028")
ponents such as the catch diode will add heat to the PC
board and the heat can vary as the input voltage changes. FIGURE 20. Junction Temperature Rise, TO-263
For the inductor, depending on the physical size, type of core
www.national.com 24
LM2596
Application Information (Continued) tures a constant threshold voltage for turn on and turn off
(zener voltage plus approximately one volt). If hysteresis is
needed, the circuit in Figure 24 has a turn ON voltage which
is different than the turn OFF voltage. The amount of hyster-
esis is approximately equal to the value of the output volt-
age. If zener voltages greater than 25V are used, an addi-
tional 47 kΩ resistor is needed from the ON /OFF pin to the
ground pin to stay within the 25V maximum limit of the ON
/OFF pin.
INVERTING REGULATOR
The circuit in Figure 25 converts a positive input voltage to a
01258336
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator’s ground pin to the
FIGURE 21. Delayed Startup negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
01258337
01258338
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 22. Undervoltage Lockout
for Buck Regulator
FIGURE 23. Undervoltage Lockout
for Inverting Regulator
DELAYED STARTUP
This example uses the LM2596-5.0 to generate a −5V out-
The circuit in Figure 21 uses the the ON /OFF pin to provide put, but other output voltages are possible by selecting other
a time delay between the time the input voltage is applied output voltage versions, including the adjustable version.
and the time the output voltage comes up (only the circuitry Since this regulator topology can produce an output voltage
pertaining to the delayed start up is shown). As the input that is either greater than or less than the input voltage, the
voltage rises, the charging of capacitor C1 pulls the ON /OFF maximum output current greatly depends on both the input
pin high, keeping the regulator off. Once the input voltage and output voltage. The curve shown in Figure 26 provides a
reaches its final value and the capacitor stops charging, and guide as to the amount of output load current possible for the
resistor R2 pulls the ON /OFF pin low, thus allowing the different input and output voltage conditions.
circuit to start switching. Resistor R1 is included to limit the
The maximum voltage appearing across the regulator is the
maximum voltage applied to the ON /OFF pin (maximum of
absolute sum of the input and output voltage, and this must
25V), reduces power supply noise sensitivity, and also limits
be limited to a maximum of 40V. For example, when convert-
the capacitor, C1, discharge current. When high input ripple
ing +20V to −12V, the regulator would see 32V between the
voltage exists, avoid long delay time, because this ripple can
input pin and ground pin. The LM2596 has a maximum input
be coupled into the ON /OFF pin and cause problems.
voltage spec of 40V.
This delayed startup feature is useful in situations where the
Additional diodes are required in this regulator configuration.
input power source is limited in the amount of current it can
Diode D1 is used to isolate input voltage ripple or noise from
deliver. It allows the input voltage to rise to a higher voltage
coupling through the CIN capacitor to the output, under light
before the regulator starts operating. Buck regulators require
or no load conditions. Also, this diode isolation changes the
less input current at higher input voltages.
topology to closley resemble a buck configuration thus pro-
UNDERVOLTAGE LOCKOUT viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
Some applications require the regulator to remain off until drop) but for higher input voltages, a fast recovery diode
the input voltage reaches a predetermined voltage. An und- could be used.
ervoltage lockout feature applied to a buck regulator is
shown in Figure 22, while Figure 23 and 24 applies the same Without diode D3, when the input voltage is first applied, the
feature to an inverting circuit. The circuit in Figure 23 fea- charging current of CIN can pull the output positive by sev-
eral volts for a short period of time. Adding D3 prevents the
output from going positive by more than a diode voltage.
25 www.national.com
LM2596
Application Information (Continued)
01258339
This circuit has hysteresis
Regulator starts switching at VIN = 13V
Regulator stops switching at VIN = 8V
01258340
CIN — 68 µF/25V Tant. Sprague 595D
470 µF/50V Elec. Panasonic HFQ
COUT — 47 µF/20V Tant. Sprague 595D
220 µF/25V Elec. Panasonic HFQ
www.national.com 26
LM2596
Application Information (Continued) OFF. With the inverting configuration, some level shifting is
required, because the ground pin of the regulator is no
INVERTING REGULATOR SHUTDOWN METHODS longer at ground, but is now setting at the negative output
To use the ON /OFF pin in a standard buck configuration is voltage level. Two different shutdown methods for inverting
simple, pull it below 1.3V ( @25˚C, referenced to ground) to regulators are shown in Figure 27 and 28.
turn regulator ON, pull it above 1.3V to shut the regulator
01258342
01258343
FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device
27 www.national.com
LM2596
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED
01258344
CIN — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”
COUT — 330 µF, 35V, Aluminum Electrolytic Panasonic, “HFQ Series”
D1 — 5A, 40V Schottky Rectifier, 1N5825
L1 — 47 µH, L39, Renco, Through Hole
Thermalloy Heat Sink #7020
www.national.com 28
LM2596
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED
01258345
CIN — 470 µF, 50V, Aluminum Electrolytic Panasonic, “HFQ Series”
COUT — 220 µF, 35V Aluminum Electrolytic Panasonic, “HFQ Series”
D1 — 5A, 40V Schottky Rectifier, 1N5825
L1 — 47 µH, L39, Renco, Through Hole
R1 — 1 kΩ, 1%
R2 — Use formula in Design Procedure
CFF — See Figure 3.
Thermalloy Heat Sink #7020
29 www.national.com
LM2596
Physical Dimensions inches (millimeters)
unless otherwise noted
www.national.com 30
LM2596 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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