LM2596
SIMPLE SWITCHER®Power Converter 150 kHz
3A Step-Down Voltage Regulator
General Description
The LM2596 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving a 3A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, and an adjust-
able output version.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation†, and a fixed-frequency oscillator.
The LM2596 series operates at a switching frequency of
150 kHz thus allowing smaller sized filter components than
what would be needed with lower frequency switching regu-
lators. Available in a standard 5-lead TO-220 package with
several different lead bend options, and a 5-lead TO-263
surface mount package.
A standard series of inductors are available from several dif-
ferent manufacturers optimized for use with the LM2596 se-
ries. This feature greatly simplifies the design of
switch-mode power supplies.
Other features include a guaranteed ±4%tolerance on out-
put voltage under specified input voltage and output load
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 tem-
perature shutdown for complete protection under fault condi-
tions.
Features
n3.3V, 5V, 12V, and adjustable output versions
nAdjustable version output voltage range, 1.2V to 37V
±4%max over line and load conditions
nAvailable in TO-220 and TO-263 packages
nGuaranteed 3A output load current
nInput voltage range up to 40V
nRequires only 4 external components
nExcellent line and load regulation specifications
n150 kHz fixed frequency internal oscillator
nTTL shutdown capability
nLow power standby mode, I
Q
typically 80 µA
nHigh efficiency
nUses readily available standard inductors
nThermal shutdown and current limit protection
Applications
nSimple high-efficiency step-down (buck) regulator
nOn-card switching regulators
nPositive to negative converter
Note: †Patent Number 5,382,918.
Typical Application (Fixed Output Voltage Versions)
SIMPLE SWITCHER®and
Switchers Made Simple
®
are registered trademarks of National Semiconductor Corporation.
DS012583-1
December 1997
LM2596 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator
© 1999 National Semiconductor Corporation DS012583 www.national.com
Connection Diagrams and Ordering Information
Bent and Staggered Leads, Through Hole
Package
5-Lead TO-220 (T)
DS012583-2
Order Number LM2596T-3.3, LM2596T-5.0,
LM2596T-12 or LM2596T-ADJ
See NS Package Number T05D
Surface Mount Package
5-Lead TO-263 (S)
DS012583-3
Order Number LM2596S-3.3, LM2596S-5.0,
LM2596S-12 or LM2596S-ADJ
See NS Package Number TS5B
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Supply Voltage 45V
ON /OFF Pin Input Voltage −0.3 ≤V≤+25V
Feedback Pin Voltage −0.3 ≤V≤+25V
Output Voltage to Ground
(Steady State) −1V
Power Dissipation Internally limited
Storage Temperature Range −65˚C to +150˚C
ESD Susceptibility
Human Body Model (Note 2) 2 kV
Lead Temperature
S Package
Vapor Phase (60 sec.) +215˚C
Infrared (10 sec.) +245˚C
T Package (Soldering, 10 sec.) +260˚C
Maximum Junction Temperature +150˚C
Operating Conditions
Temperature Range −40˚C ≤T
J
≤+125˚C
Supply Voltage 4.5V to 40V
LM2596-3.3
Electrical Characteristics
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
Symbol Parameter Conditions LM2596-3.3 Units
(Limits)
Typ
(Note 3) Limit
(Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
OUT
Output Voltage 4.75V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
ηEfficiency V
IN
=12V, I
LOAD
=3A 73 %
LM2596-5.0
Electrical Characteristics
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
Symbol Parameter Conditions LM2596-5.0 Units
(Limits)
Typ
(Note 3) Limit
(Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
OUT
Output Voltage 7V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 5.0 V
4.800/4.750 V(min)
5.200/5.250 V(max)
ηEfficiency V
IN
=12V, I
LOAD
=3A 80 %
LM2596-12
Electrical Characteristics
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
Symbol Parameter Conditions LM2596-12 Units
(Limits)
Typ
(Note 3) Limit
(Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
OUT
Output Voltage 15V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 12.0 V
11.52/11.40 V(min)
12.48/12.60 V(max)
ηEfficiency V
IN
=12V, I
LOAD
=3A 90 %
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LM2596-ADJ
Electrical Characteristics
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range
Symbol Parameter Conditions LM2596-ADJ Units
(Limits)
Typ
(Note 3) Limit
(Note 4)
SYSTEM PARAMETERS (Note 5) Test Circuit
Figure 1
V
FB
Feedback Voltage 4.5V ≤V
IN
≤40V, 0.2A ≤I
LOAD
≤3A 1.230 V
V
OUT
programmed for 3V. Circuit of
Figure 1
1.193/1.180 V(min)
1.267/1.280 V(max)
ηEfficiency V
IN
=12V, V
OUT
=3V, I
LOAD
=3A 73 %
All Output Voltage Versions
Electrical Characteristics
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
=12V for the 3.3V, 5V, and Adjustable version and V
IN
=24V for the 12V ver-
sion. I
LOAD
=500 mA
Symbol Parameter Conditions LM2596-XX Units
(Limits)
Typ
(Note 3) Limit
(Note 4)
DEVICE PARAMETERS
I
b
Feedback Bias Current Adjustable Version Only, V
FB
=1.3V 10 nA
50/100 nA (max)
f
O
Oscillator Frequency (Note 6) 150 kHz
127/110 kHz(min)
173/173 kHz(max)
V
SAT
Saturation Voltage I
OUT
=3A (Notes 7, 8) 1.16 V
1.4/1.5 V(max)
DC Max Duty Cycle (ON) (Note 8) 100 %
Min Duty Cycle (OFF) (Note 9) 0
I
CL
Current Limit Peak Current (Notes 7, 8) 4.5 A
3.6/3.4 A(min)
6.9/7.5 A(max)
I
L
Output Leakage Current Output =0V (Notes 7, 9) 50 µA(max)
Output =−1V (Note 10) 2 mA
30 mA(max)
I
Q
Quiescent Current (Note 9) 5 mA
10 mA(max)
I
STBY
Standby Quiescent
Current ON/OFF pin =5V (OFF) (Note 10) 80 µA
200/250 µA(max)
θ
JC
Thermal Resistance TO-220 or TO-263 Package, Junction to Case 2 ˚C/W
θ
JA
TO-220 Package, Junction to Ambient (Note 11) 50 ˚C/W
θ
JA
TO-263 Package, Junction to Ambient (Note 12) 50 ˚C/W
θ
JA
TO-263 Package, Junction to Ambient (Note 13) 30 ˚C/W
θ
JA
TO-263 Package, Junction to Ambient (Note 14) 20 ˚C/W
ON/OFF CONTROL Test Circuit
Figure 1
ON /OFF Pin Logic Input 1.3 V
V
IH
Threshold Voltage Low (Regulator ON) 0.6 V(max)
V
IL
High (Regulator OFF) 2.0 V(min)
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All Output Voltage Versions
Electrical Characteristics (Continued)
Specifications with standard type face are for T
J
=25˚C, and those with boldface type apply over full Operating Tempera-
ture Range. Unless otherwise specified, V
IN
=12V for the 3.3V, 5V, and Adjustable version and V
IN
=24V for the 12V ver-
sion. I
LOAD
=500 mA
Symbol Parameter Conditions LM2596-XX Units
(Limits)
Typ
(Note 3) Limit
(Note 4)
ON/OFF CONTROL Test Circuit
Figure 1
I
H
ON /OFF Pin Input Current V
LOGIC
=2.5V (Regulator OFF) 5 µA
15 µA(max)
I
L
V
LOGIC
=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 in-
tended 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%produc-
tion 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 sys-
tem 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. The amount of reduction is determined by the severity of current over-
load.
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 in2of (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 in2of (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 in2of (1 oz.) copper area on
the LM2596S side of the board, and approximately 16 in2of 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.
Typical Performance Characteristics (Circuit of
Figure 1
)
Normalized
Output Voltage
DS012583-4
Line Regulation
DS012583-5
Efficiency
DS012583-6
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Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Switch Saturation
Voltage
DS012583-7
Switch Current Limit
DS012583-8
Dropout Voltage
DS012583-9
Operating
Quiescent Current
DS012583-10
Shutdown
Quiescent Current
DS012583-11
Minimum Operating
Supply Voltage
DS012583-12
ON /OFF Threshold
Voltage
DS012583-13
ON /OFF Pin
Current (Sinking)
DS012583-14
Switching Frequency
DS012583-15
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Typical Performance Characteristics (Circuit of
Figure 1
) (Continued)
Typical Performance Characteristics
Feedback Pin
Bias Current
DS012583-16
Continuous Mode Switching Waveforms
V
IN
=20V, V
OUT
=5V, I
LOAD
=2A
L=32 µH, C
OUT
=220 µF, C
OUT
ESR =50 mΩ
DS012583-17
A: Output Pin Voltage, 10V/div.
B: Inductor Current 1A/div.
C: Output Ripple Voltage, 50 mV/div.
Horizontal Time Base: 2 µs/div.
Discontinuous Mode Switching Waveforms
V
IN
=20V, V
OUT
=5V, I
LOAD
=500 mA
L=10 µH, C
OUT
=330 µF, C
OUT
ESR =45 mΩ
DS012583-18
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.5A/div.
C: Output Ripple Voltage, 100 mV/div.
Horizontal Time Base: 2 µs/div.
Load Transient Response for Continuous Mode
V
IN
=20V, V
OUT
=5V, I
LOAD
=500 mA to 2A
L=32 µH, C
OUT
=220 µF, C
OUT
ESR =50 mΩ
DS012583-19
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Horizontal Time Base: 100 µs/div.
Load Transient Response for Discontinuous Mode
V
IN
=20V, V
OUT
=5V, I
LOAD
=500 mA to 2A
L=10 µH, C
OUT
=330 µF, C
OUT
ESR =45 mΩ
DS012583-20
A: Output Voltage, 100 mV/div. (AC)
B: 500 mA to 2A Load Pulse
Horizontal Time Base: 200 µs/div.
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Test Circuit and Layout Guidelines
As in any switching regulator, layout is very important. Rap-
idly 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 should be wide printed circuit traces and
should be kept as short as possible. For best results, ex-
ternal 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 C
OUT
wiring can cause problems.
When using the adjustable version, special care must be
taken as to the location of the feedback resistors and the as-
sociated wiring. Physically locate both resistors near the IC,
and route the wiring away from the inductor, especially an
open core type of inductor. (See application section for more
information.)
Fixed Output Voltage Versions
DS012583-22
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
Adjustable Output Voltage Versions
DS012583-23
where VREF =1.23V
Select R1to be approximately 1 kΩ,usea1
%resistor for best stability.
CIN — 470 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 220 µF, 35V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 5A, 40V Schottky Rectifier, 1N5825
L1 — 68 µH, L38
R1 — 1 kΩ,1
%
C
FF — See Application Information Section
FIGURE 1. Standard Test Circuits and Layout Guides
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LM2596 Series Buck Regulator Design Procedure (Fixed Output)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
Given:
V
OUT
=Regulated Output Voltage (3.3V, 5V or 12V)
V
IN
(max) =Maximum DC Input Voltage
I
LOAD
(max) =Maximum Load Current
Given:
V
OUT
=5V
V
IN
(max) =12V
I
LOAD
(max) =3A
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from Fig-
ures
Figure 4
,
Figure 5
,or
Figure 6
. (Output voltages of 3.3V,
5V, or 12V respectively.) For all other voltages, see the de-
sign procedure for the adjustable version.
B. From the inductor value selection guide, identify the induc-
tance 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).
C. Select an appropriate inductor from the four manufactur-
er’s part numbers listed in
Figure 8
.
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version shown
in
Figure 5
.
B. From the inductor value selection guide shown in
Figure 5
,
the inductance region intersected by the 12V horizontal line
and the 3A vertical line is 33 µH, and the inductor code is
L40.
C. The inductance value required is 33 µH. From the table in
Figure 8
, go to the L40 line and choose an inductor part num-
ber from any of the four manufacturers shown. (In most in-
stance, both through hole and surface mount inductors are
available.)
2. Output Capacitor Selection (C
OUT
)
A. 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
should be located close to the IC using short capacitor leads
and short copper traces. Do not use capacitors larger than
820 µF .
For additional information, see section on output capaci-
tors in application information section.
B. To simplify the capacitor selection procedure, refer to the
quick design component selection table shown in
Figure 2
.
This table contains different input voltages, output voltages,
and load currents, and lists various inductors and output ca-
pacitors that will provide the best design solutions.
C. The capacitor voltage rating for electrolytic capacitors
should 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.
D. For computer aided design software, see
Switchers Made
Simple
™version 4.3 or later.
2. Output Capacitor Selection (C
OUT
)
A. See section on output capacitors in application infor-
mation section.
B. From the quick design component selection table shown
in
Figure 2
, locate the 5V 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 3A line. In the maximum input voltage column, select the
line that covers the input voltage needed in your application,
in this example, use the 15V line. Continuing on this line are
recommended inductors and capacitors that will provide the
best overall performance.
The capacitor list contains both through hole electrolytic and
surface mount tantalum capacitors from four different capaci-
tor manufacturers. It is recommended that both the manufac-
turers 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 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 ap-
proximately 225 mΩof ESR (see the curve in
Figure 14
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) should be
selected. A 16V or 25V capacitor will reduce the ripple volt-
age by approximately half.
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LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
PROCEDURE (Fixed Output Voltage Version) EXAMPLE (Fixed Output Voltage Version)
3. Catch Diode Selection (D1)
A. 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 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 results. Ultra-fast recovery diodes typically have re-
verse recovery times of 50 ns or less. Rectifiers such as the
1N5400 series are much too slow and should not be used.
3. Catch Diode Selection (D1)
A. Refer to the table shown in
Figure 11
. In this example, a
5A, 20V, 1N5823 Schottky diode will provide the best perfor-
mance, and will not be overstressed even for a shorted out-
put.
4. Input Capacitor (C
IN
)
Alow ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground pin to prevent large volt-
age transients from appearing at the input. This capacitor
should be located close to the IC using short leads. In addi-
tion, the RMS current rating of the input capacitor should be
selected to be at least
1
⁄
2
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
Fig-
ure 13
shows typical RMS current ratings for several different
aluminum electrolytic capacitor values.
For an aluminum electrolytic, the capacitor voltage rating
should be approximately 1.5 times the maximum input volt-
age. Caution must be exercised if solid tantalum capacitors
are used (see Application Information on input capacitor).
The tantalum capacitor voltage rating should be 2 times the
maximum input voltage and it is recommended that they be
surge current tested by the manufacturer.
Use caution when using ceramic capacitors for input bypass-
ing, because it may cause severe ringing at the V
IN
pin.
For additional information, see section on input capaci-
tors in Application Information section.
4. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the in-
put voltage rating and the RMS current rating. With a nominal
input voltage of 12V, an aluminum electrolytic capacitor with
a voltage rating greater than 18V (1.5 x V
IN
) would be
needed. The next higher capacitor voltage rating is 25V.
The RMS current rating requirement for the input capacitor in
a buck regulator is approximately
1
⁄
2
the DC load current. In
this example, with a 3A load, a capacitor with a RMS current
rating of at least 1.5A is needed. The curves shown in
Figure
13
can be used to select an appropriate input capacitor.
From the curves, locate the 35V line and note which capaci-
tor values have RMS current ratings greater than 1.5A. A
680 µF/35V capacitor could be used.
For a through hole design, a 680 µF/35V electrolytic capaci-
tor (Panasonic HFQ series or Nichicon PL series or equiva-
lent) would be adequate. other types or other manufacturers
capacitors can be used provided the RMS ripple current rat-
ings are adequate.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the ca-
pacitor 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
current tested.
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LM2596 Series Buck Regulator Design Procedure (Fixed Output) (Continued)
Conditions Inductor Output Capacitor
Through Hole Electrolytic Surface Mount Tantalum
Output Load Max Input Inductance Inductor Panasonic Nichicon AVX TPS Sprague
Voltage Current Voltage (µH) (#) HFQ Series PL Series Series 595D Series
(V) (A) (V) (µF/V) (µF/V) (µF/V) (µF/V)
3.3 3 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
2 10 33 L32 330/35 330/35 330/6.3 390/6.3
40 47 L39 330/35 270/50 220/10 330/10
53 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
2 20 68 L38 180/35 180/35 100/10 270/10
40 68 L38 180/35 180/35 100/10 270/10
12 3 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
2 20 68 L38 180/25 180/25 100/16 120/20
40 150 L42 82/25 82/25 68/20 68/25
FIGURE 2. LM2596 Fixed Voltage Quick Design Component Selection Table
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LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
Given:
V
OUT
=Regulated Output Voltage
V
IN
(max) =Maximum Input Voltage
I
LOAD
(max) =Maximum Load Current
F=Switching Frequency
(Fixed at a nominal 150 kHz).
Given:
V
OUT
=20V
V
IN
(max) =28V
I
LOAD
(max) =3A
F=Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in
Figure 1
)
Use the following formula to select the appropriate resistor
values.
Select a value for R
1
between 240Ωand 1.5 kΩ. The lower
resistor values minimize noise pickup in the sensitive feed-
back pin. (For the lowest temperature coefficient and the best
stability with time, use 1%metal film resistors.)
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in
Figure 1
)
Select R
1
to be 1 kΩ,1
%
. Solve for R
2
.
R
2
=1k (16.26 − 1) =15.26k, closest 1%value is 15.4 kΩ.
R
2
=15.4 kΩ.
2. Inductor Selection (L1)
A. Calculate the inductor Volt •microsecond constant E •T
(V •µs), from the following formula:
where V
SAT
=internal switch saturation voltage =1.16V
and V
D
=diode forward voltage drop =0.5V
B. 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 7
.
C. on the horizontal axis, select the maximum load current.
D. 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).
E. Select an appropriate inductor from the four manufactur-
er’s part numbers listed in
Figure 8
.
2. Inductor Selection (L1)
A. Calculate the inductor Volt •microsecond constant
(E •T),
B. E•T=34.2 (V •µs)
C. I
LOAD
(max) =3A
D. From the inductor value selection guide shown in
Figure 7
,
the inductance region intersected by the 34 (V •µs) horizon-
tal line and the 3A vertical line is 47 µH, and the inductor
code is L39.
E. From the table in
Figure 8,
locate line L39, and select an
inductor part number from the list of manufacturers part num-
bers.
www.national.com 12
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
3. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR electrolytic or solid
tantalum capacitors between 82 µF and 820 µF provide the
best results. This capacitor should be located close to the IC
using short capacitor leads and short copper traces. Do not
use capacitors larger than 820 µF. For additional informa-
tion, see section on output capacitors in application in-
formation section.
B. To simplify the capacitor selection procedure, refer to the
quick design table shown in
Figure 3
. This table contains dif-
ferent output voltages, and lists various output capacitors
that will provide the best design solutions.
C. The capacitor voltage rating should be at least 1.5 times
greater than the output voltage, and often much higher volt-
age ratings are needed to satisfy the low ESR requirements
needed for low output ripple voltage.
3. Output Capacitor SeIection (C
OUT
)
A. See section on C
OUT
in Application Information section.
B. From the quick design table shown in
Figure 3
, 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 24V line. Under the output capacitor
section, select a capacitor from the list of through hole elec-
trolytic or surface mount tantalum types from four different
capacitor manufacturers. It is recommended that both the
manufacturers and the manufacturers series that are listed in
the table be used.
In this example, through hole aluminum electrolytic capaci-
tors from several different manufacturers are available.
220 µF/35V Panasonic HFQ Series
150 µF/35V Nichicon PL Series
C. For a 20V output, a capacitor rating of at least 30V or
more is needed. In this example, either a 35V or 50V capaci-
tor would work.A 35V rating was chosen, although a 50V rat-
ing 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. Re-
fer to the capacitor manufacturers data sheet for this informa-
tion.
4. Feedforward Capacitor (C
FF
)(See
Figure 1
)
For output voltages greater than approximately 10V, an addi-
tional 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, R
2
. 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.
This capacitor type can be ceramic, plastic, silver mica, etc.
(Because of the unstable characteristics of ceramic capaci-
tors made with Z5U material, they are not recommended.)
4. Feedforward Capacitor (C
FF
)
The table shown in
Figure 3
contains feed forward capacitor
values for various output voltages. In this example, a 560 pF
capacitor is needed.
www.national.com13
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
(Continued)
PROCEDURE (Adjustable Output Voltage Version) EXAMPLE (Adjustable Output Voltage Version)
5. Catch Diode Selection (D1)
A. 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 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 are also a
good choice, but some types with an abrupt turn-off charac-
teristic may cause instability or EMl problems. Ultra-fast re-
covery diodes typically have reverse recovery times of 50 ns
or less. Rectifiers such as the 1N4001 series are much too
slow and should not be used.
5. Catch Diode Selection (D1)
A. Refer to the table shown in
Figure 11
. Schottky diodes
provide the best performance, and in this example a 5A, 40V,
1N5825 Schottky diode would be a good choice. The 5A di-
ode rating is more than adequate and will not be over-
stressed even for a shorted output.
6. Input Capacitor (C
IN
)
Alow 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 should be selected to be
at least
1
⁄
2
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 13
shows typical
RMS current ratings for several different aluminum electro-
lytic capacitor values.
This capacitor should be located close to the IC using short
leads and the voltage rating should be approximately 1.5
times the maximum input voltage.
If solid tantalum input capacitors are used, it is recomended
that they be surge current tested by the manufacturer.
Use caution when using a high dielectric constant ceramic
capacitor for input bypassing, because it may cause severe
ringing at the V
IN
pin.
For additional information, see section on input capaci-
tors in application information section.
6. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the in-
put voltage rating and the RMS current rating. With a nominal
input voltage of 28V, an aluminum electrolytic aluminum elec-
trolytic capacitor with a voltage rating greater than 42V (1.5 x
V
IN
) would be needed. Since the the next higher capacitor
voltage rating is 50V, a 50V capacitor should be used. The
capacitor voltage rating of (1.5 x V
IN
) is a conservative guide-
line, and can be modified somewhat if desired.
The RMS current rating requirement for the input capacitor of
a buck regulator is approximately
1
⁄
2
the DC load current. In
this example, with a 3A load, a capacitor with a RMS current
rating of at least 1.5A is needed.
The curves shown in
Figure 13
can be used to select an ap-
propriate input capacitor. From the curves, locate the 50V
line and note which capacitor values have RMS current rat-
ings greater than 1.5A. Either a 470 µF or 680 µF, 50V ca-
pacitor could be used.
For a through hole design, a 680 µF/50V electrolytic capaci-
tor (Panasonic HFQ series or Nichicon PL series or equiva-
lent) would be adequate. Other types or other manufacturers
capacitors can be used provided the RMS ripple current rat-
ings are adequate.
For surface mount designs, solid tantalum capacitors can be
used, but caution must be exercised with regard to the ca-
pacitor surge current rting (see Application Information or in-
put capacitors in this data sheet). The TPS series available
from AVX, and the 593D series from Sprague are both surge
current 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 switch-
ing regulators. Switchers Made Simple (version 4.3 or later)
is available on a 3
1
⁄
2
" diskette for IBM compatible computers.
www.national.com 14
LM2596 Series Buck Regulator Design Procedure (Adjustable Output)
LM2596 Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
Output
Voltage
(V)
Through Hole Output Capacitor Surface Mount Output Capacitor
Panasonic Nichicon PL Feedforward AVX TPS Sprague Feedforward
HFQ Series Series Capacitor Series 595D Series Capacitor
(µF/V) (µF/V) (µF/V) (µF/V)
2820/35 820/35 33 nF 330/6.3 470/4 33 nF
4560/35 470/35 10 nF 330/6.3 390/6.3 10 nF
6470/25 470/25 3.3 nF 220/10 330/10 3.3 nF
9330/25 330/25 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
FIGURE 3. Output Capacitor and Feedforward Capacitor Selection Table
DS012583-24
FIGURE 4. LM2596-3.3
DS012583-25
FIGURE 5. LM2596-5.0
DS012583-26
FIGURE 6. LM2596-12
DS012583-27
FIGURE 7. LM2596-ADJ
www.national.com15
LM2596 Series Buck Regulator Design Procedure (Continued)
Inductance
(µH) Cur-
rent
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
L15 22 0.99 67148350 67148460 RL-1284-22-43 RL1500-22 PE-53815 PE-53815-S DO3308-223
L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DO3316-683
L22 47 1.17 67144080 67144460 RL-5471-6 — PE-53822 PE-53822-S DO3316-473
L23 33 1.40 67144090 67144470 RL-5471-7 — PE-53823 PE-53823-S DO3316-333
L24 22 1.70 67148370 67148480 RL-1283-22-43 — PE-53824 PE-53825-S DO3316-223
L25 15 2.10 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 DO5022P-334
L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S DO5022P-224
L28 150 1.20 67144120 67144500 RL-5471-3 — PE-53828 PE-53828-S DO5022P-154
L29 100 1.47 67144130 67144510 RL-5471-4 — PE-53829 PE-53829-S DO5022P-104
L30 68 1.78 67144140 67144520 RL-5471-5 — PE-53830 PE-53830-S DO5022P-683
L31 47 2.20 67144150 67144530 RL-5471-6 — PE-53831 PE-53831-S DO5022P-473
L32 33 2.50 67144160 67144540 RL-5471-7 — PE-53932 PE-53932-S DO5022P-333
L33 22 3.10 67148390 67148500 RL-1283-22-43 — PE-53933 PE-53933-S DO5022P-223
L34 15 3.40 67148400 67148790 RL-1283-15-43 — PE-53934 PE-53934-S DO5022P-153
L35 220 1.70 67144170 — RL-5473-1 — PE-53935 PE-53935-S —
L36 150 2.10 67144180 — RL-5473-4 — PE-54036 PE-54036-S —
L37 100 2.50 67144190 — RL-5472-1 — PE-54037 PE-54037-S —
L38 68 3.10 67144200 — RL-5472-2 — PE-54038 PE-54038-S —
L39 47 3.50 67144210 — RL-5472-3 — PE-54039 PE-54039-S —
L40 33 3.50 67144220 67148290 RL-5472-4 — PE-54040 PE-54040-S —
L41 22 3.50 67144230 67148300 RL-5472-5 — PE-54041 PE-54041-S —
L42 150 2.70 67148410 — RL-5473-4 — PE-54042 PE-54042-S —
L43 100 3.40 67144240 — RL-5473-2 — PE-54043 —
L44 68 3.40 67144250 — RL-5473-3 — PE-54044 —
FIGURE 8. Inductor Manufacturers Part Numbers
Coilcraft Inc. Phone (800) 322-2645
FAX (708) 639-1469
Coilcraft Inc., Europe Phone +11 1236 730 595
FAX +44 1236 730 627
Pulse Engineering Inc. Phone (619) 674-8100
FAX (619) 674-8262
Pulse Engineering
Inc., Phone +353 93 24 107
Europe FAX +353 93 24 459
Renco Electronics Inc. Phone (800) 645-5828
FAX (516) 586-5562
Schott Corp. Phone (612) 475-1173
FAX (612) 475-1786
FIGURE 9. Inductor Manufacturers Phone Numbers
www.national.com 16
LM2596 Series Buck Regulator Design Procedure (Continued)
Nichicon Corp. Phone (708) 843-7500
FAX (708) 843-2798
Panasonic Phone (714) 373-7857
FAX (714) 373-7102
AVX Corp. Phone (803) 448-9411
FAX (803) 448-1943
Sprague/Vishay Phone (207) 324-4140
FAX (207) 324-7223
FIGURE 10. Capacitor Manufacturers Phone Numbers
VR 3A Diodes 4A–6A Diodes
Surface Mount Through Hole Surface Mount Through Hole
Schottky Ultra Fast Schot-
tky Ultra Fast Schot-
tky Ultra Fast Schot-
tky Ultra Fast
Recovery Recovery Recovery Recovery
20V All of
these
diodes
are
rated to
at least
50V.
1N5820 All of
these
diodes
are
rated to
at least
50V.
All of
these
diodes
are
rated to
at least
50V.
SR502 All of
these
diodes
are
rated to
at least
50V.
SK32 SR302 1N5823
MBR320 SB520
30V 30WQ03 1N5821
SK33 MBR330 50WQ03 SR503
31DQ03 1N5824
1N5822 SB530
40V SK34 SR304 50WQ04 SR504
MBRS340 MBR340 1N5825
30WQ04 MURS320 31DQ04 MUR320 MURS620 SB540 MUR620
50V SK35 30WF10 SR305 50WF10 HER601
or MBRS360 MBR350 50WQ05 SB550
More 30WQ05 31DQ05 50SQ080
FIGURE 11. Diode Selection Table
www.national.com17
Block Diagram
Application Information
PIN FUNCTIONS
+V
IN
—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.
Ground —Circuit ground.
Output —Internal switch. The voltage at this pin switches
between (+V
IN
−V
SAT
) and approximately −0.5V, with a duty
cycle of approximately V
OUT
/V
IN
. To minimize coupling to
sensitive circuitry, the PC board copper area connected to
this pin should be kept to a minimum.
Feedback —Senses the regulated output voltage to com-
plete the feedback loop.
ON /OFF — Allows the switching regulator circuit to be shut
down using logic level signals thus dropping the total input
supply current to approximately 80 µA. Pulling this pin below
a threshold voltage of approximately 1.3V turns the regulator
on, and pulling this pin above 1.3V (up to a maximum of 25V)
shuts the regulator down. If this shutdown feature is not
needed, the ON /OFF pin can be wired to the ground pin or
it can be left open, in either case the regulator will be in the
ON condition.
EXTERNAL COMPONENTS
INPUT CAPACITOR
C
IN
—A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground pin. It must be lo-
cated near the regulator using short leads. This capacitor
prevents large voltage transients from appearing at the in-
put, 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 buck regulator’s in-
put capacitor, this capacitor should be chosen for its RMS
current rating rather than its capacitance or voltage ratings,
although the capacitance value and voltage rating are di-
rectly related to the RMS current rating.
The RMS current rating of a capacitor could be viewed as a
capacitor’s power rating. The RMS current flowing through
the capacitors internal ESR produces power which causes
the internal temperature of the capacitor to rise. The RMS
current rating of a capacitor is determined by the amount of
current required to raise the internal temperature approxi-
mately 10˚C above an ambient temperature of 105˚C. The
ability of the capacitor to dissipate this heat to the surround-
ing air will determine the amount of current the capacitor can
safely sustain. Capacitors that are physically large and have
a large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage electro-
lytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the sur-
rounding air, and therefore will have a higher RMS current
rating.
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 ca-
pacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manu-
facturers data sheet for maximum allowable RMS ripple cur-
rent. For a maximum ambient temperature of 40˚C, a gen-
eral 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 sat-
isfy the RMS current requirements.
A graph shown in
Figure 13
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 electro-
DS012583-21
FIGURE 12.
www.national.com 18
Application Information (Continued)
lytic capacitors designed for switching regulator applications.
Other capacitor manufacturers offer similar types of capaci-
tors, 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, sur-
face 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 turn on
when the input voltage is suddenly applied, and of course,
higher input voltages produce higher inrush currents. Sev-
eral capacitor manufacturers do a 100%surge current test-
ing on their products to minimize this potential problem. If
high turn on currents are expected, it may be necessary to
limit this current by adding either some resistance or induc-
tance before the tantalum capacitor, or select a higher volt-
age capacitor. As with aluminum electrolytic capacitors, the
RMS ripple current rating must be sized to the load current.
FEEDFORWARD CAPACITOR
(Adjustable Output Voltage Version)
C
FF
—A Feedforward Capacitor C
FF
, shown across R2 in
Figure 1
is used when the ouput voltage is greater than 10V
or when C
OUT
has a very low ESR. This capacitor adds lead
compensation to the feedback loop and increases the phase
margin for better loop stability. For C
FF
selection, see the de-
sign procedure section.
OUTPUT CAPACITOR
C
OUT
—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 select-
ing 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 up-
per 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 out-
put voltage. But if the selected capacitor’s ESR is extremely
low, there is a possibility of an unstable feedback loop, re-
sulting in an oscillation at the output. Using the capacitors
listed in the tables, or similar types, will provide design solu-
tions under all conditions.
If very low output ripple voltage (less than 15 mV) is re-
quired, refer to the section on 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 14
). Often, capacitors with much higher voltage
ratings may be needed to provide the low ESR values re-
quired for low output ripple voltage.
The output capacitor for many different switcher designs of-
ten can be satisfied with only three or four different capacitor
values and several different voltage ratings. See the quick
design component selection tables in
Figure 2
and
4
for typi-
cal capacitor values, voltage ratings, and manufacturers ca-
pacitor types.
Electrolytic capacitors are not recommended for tempera-
tures below −25˚C. The ESR rises dramatically at cold tem-
peratures and typically rises 3X @−25˚C and as much as
10X at −40˚C. See curve shown in
Figure 15
.
Solid tantalum capacitors have a much better ESR spec for
cold temperatures and are recommended for temperatures
below −25˚C.
CATCH DIODE
Buck regulators require a diode to provide a return path for
the inductor current when the switch turns off. This must be
a fast diode and must be located close to the LM2596 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 (5V and lower).
Ultra-fast recovery, or High-Efficiency rectifiers are also a
good choice, but some types with an abrupt turnoff charac-
teristic may cause instability or EMI problems. Ultra-fast re-
covery diodes typically have reverse recovery times of 50 ns
or less. Rectifiers such as the 1N5400 series are much too
slow and should not be used.
DS012583-28
FIGURE 13. RMS Current Ratings for Low ESR
Electrolytic Capacitors (Typical)
DS012583-29
FIGURE 14. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
www.national.com19
Application Information (Continued)
INDUCTOR SELECTION
All switching regulators have two basic modes of operation;
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulators
performance and requirements. Most switcher designs will
operate in the discontinuous mode when the load current is
low.
The LM2596 (or any of the Simple Switcher family) can be
used for both continuous or discontinuous modes of opera-
tion.
In many cases the preferred mode of operation is the con-
tinuous mode. It offers greater output power, lower peak
switch, inductor and diode currents, and can have lower out-
put ripple voltage. But it does require 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 selec-
tion guide (nomograph) was designed (see
Figure 4
through
8
). This guide assumes that the regulator is operating in the
continuous mode, and selects an inductor that will allow a
peak-to-peak inductor ripple current to be a certain percent-
age 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 16
.)
By allowing the percentage of inductor ripple current to in-
crease 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, etc., as well as different core ma-
terials, such as ferrites and powdered iron. The least expen-
sive, the bobbin, rod or stick core, consists of wire wound on
a ferrite bobbin. This type of construction makes for an inex-
pensive inductor, but since the magnetic flux is not com-
pletely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus caus-
ing problems with both the switching regulator operation and
nearby sensitive circuitry, and can give incorrect scope read-
ings because of induced voltages in the scope probe. Also
see section on Open Core Inductors.
When multiple switching regulators are located on the same
PC board, open core magnetics can cause interference be-
tween two or more of the regulator circuits, especially at high
currents.A torroid or E-core inductor (closed magnetic struc-
ture) should 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 Engineer-
ing.
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 overheat-
ing of the inductor and/or the LM2596. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
The inductor manufacturer’s data sheets include current and
energy limits to avoid inductor saturation.
DS012583-30
FIGURE 15. Capacitor ESR Change vs Temperature
DS012583-31
FIGURE 16. (∆I
IND
) Peak-to-Peak Inductor
Ripple Current (as a Percentage of the Load Current)
vs Load Current
www.national.com 20
Application Information (Continued)
DISCONTINUOUS MODE OPERATION
The selection guide chooses inductor values suitable for
continuous mode operation, but for low current applications
and/or high input voltages, a discontinuous mode design
may be a better choice. It 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 de-
sign. The peak switch and inductor currents will be higher in
a discontinuous design, but at these low load currents (1A
and below), the maximum switch current will still be less than
the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design.The out-
put pin (switch) waveform can have some damped sinusoi-
dal ringing present. (See Typical Performance Characteris-
tics photo titled Discontinuous Mode Switching Waveforms)
This ringing is normal for discontinuous operation, and is not
caused by feedback loop instabilities. In discontinuous op-
eration, there is a period of time where neither the switch or
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/diode
parasitic capacitance causing this characteristic ringing. Nor-
mally this ringing is not a problem, unless the amplitude be-
comes great enough to exceed the input voltage, and even
then, there is very little energy present to cause damage.
Different inductor types and/or core materials produce differ-
ent amounts of this characteristic ringing. Ferrite core induc-
tors have very little core loss and therefore produce the most
ringing. The higher core loss of powdered iron inductors pro-
duce less ringing. If desired, a series RC could be placed in
parallel with the inductor to dampen the ringing. The com-
puter aided design software
Switchers Made Simple
(ver-
sion 4.3) will provide all component values for continuous
and discontinuous modes of operation.
OUTPUT VOLTAGE RIPPLE AND TRANSIENTS
The output voltage of a switching power supply operating in
the continuous mode will contain a sawtooth ripple voltage at
the switcher frequency, and may also contain short voltage
spikes at the peaks of the sawtooth waveform.
The output ripple voltage is a function of the inductor saw-
tooth 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 volt-
age, the ESR of the output capacitor must be low, however,
caution must be exercised when using extremely low ESR
capacitors because they can affect the loop stability, result-
ing in oscillation problems. If very low output ripple voltage is
needed (less than 20 mV), a post ripple filter is recom-
mended. (See
Figure 1
.) The inductance required is typically
between 1 µH and 5 µH, with low DC resistance, to maintain
good load regulation.Alow 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 de-
sired, because it is out of the regulator feedback loop. The
photo shown in
Figure 17
shows a typical output ripple volt-
age, 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,
preferable 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 and the diode, and the parasitic inductance
of the output filter capacitor, and its associated wiring. To
minimize these voltage spikes, the output capacitor should
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 evalu-
ate these transients, all contribute to the amplitude of these
spikes.
When a switching regulator is operating in the continuous
mode, the inductor current waveform ranges from a triangu-
lar 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 re-
mains 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 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 dis-
continuous 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 (∆I
IND
) can be useful for
determining a number of other circuit parameters. Param-
eters such as, peak inductor or peak switch current, mini-
DS012583-32
FIGURE 17. Post Ripple Filter Waveform
DS012583-33
FIGURE 18. Peak-to-Peak Inductor
Ripple Current vs Load Current
www.national.com21
Application Information (Continued)
mum load current before the circuit becomes discontinuous,
output ripple voltage and output capacitor ESR can all be
calculated from the peak-to-peak ∆I
IND
. When the inductor
nomographs shown in
Figure 4
through
8
are used to select
an inductor value, the peak-to-peak inductor ripple current
can immediately be determined. The curve shown in
Figure
18
shows the range of (∆I
IND
) that can be expected for differ-
ent load currents. The curve also shows how the
peak-to-peak inductor ripple current (∆I
IND
) changes as you
go from the lower border to the upper border (for a given load
current) within an inductance region. The upper border rep-
resents a higher input voltage, while the lower border repre-
sents a lower input voltage (see Inductor Selection Guides).
These curves are only correct for continuous mode opera-
tion, and only if the inductor selection guides are used to se-
lect the inductor value
Consider the following example:
V
OUT
=5V, maximum load current of 2.5A
V
IN
=12V, nominal, varying between 10V and 16V.
The selection guide in
Figure 5
shows that the vertical line
for a 2.5Aload current, and the horizontal line for the 12V in-
put voltage intersect approximately midway between the up-
per and lower borders of the 33 µH inductance region.
A 33 µH inductor will allow a peak-to-peak inductor current
(∆I
IND
) to flow that will be a percentage of the maximum load
current. Referring to
Figure 18
, follow the 2.5A line approxi-
mately midway into the inductance region, and read the
peak-to-peak inductor ripple current (∆I
IND
) on the left hand
axis (approximately 620 mA p-p).
As the input voltage increases to 16V, it approaches the up-
per border of the inductance region, and the inductor ripple
current increases. Referring to the curve in
Figure 18
,itcan
be seen that for a load current of 2.5A, the peak-to-peak in-
ductor ripple current (∆I
IND
) is 620 mA with 12V in, and can
range from 740 mAat the upper border (16V in) to 500 mA at
the lower border (10V in).
Once the ∆I
IND
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 dis-
continuous
3. Output Ripple Voltage =(∆I
IND
)x(ESR of C
OUT
)
=0.62Ax0.1Ω=62 mV p-p
4.
OPEN CORE INDUCTORS
Another possible source of increased output ripple voltage or
unstable operation is from an open core inductor. Ferrite
bobbin or stick inductors have magnetic lines of flux flowing
through the air from one end of the bobbin to the other end.
These magnetic lines of flux will induce a voltage into any
wire or PC board copper trace that comes within the induc-
tor’s magnetic field. The strength of the magnetic field, the
orientation and location of the PC copper trace to the mag-
netic 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 PC board copper trace as one
turn of a transformer (secondary) with the inductor winding
as the primary. Many millivolts can be generated in a copper
trace located near an open core inductor which can cause
stability problems or high output ripple voltage problems.
If unstable operation is seen, and an open core inductor is
used, it’s possible that the location of the inductor with re-
spect 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 op-
eration. If the circuit now operates correctly, then the mag-
netic flux from the open core inductor is causing the problem.
Substituting a closed core inductor such as a torroid or
E-core will correct the problem, or re-arranging the PC layout
may be necessary. Magnetic flux cutting the IC device
ground trace, feedback trace, or the positive or negative
traces of the output capacitor should be minimized.
Sometimes, locating a trace directly beneath a bobbin in-
ductor will provide good results, provided it is exactly in the
center of the inductor (because the induced voltages cancel
themselves out), but if it is off center one direction or the
other, then problems could arise. 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 the
user, but to alert the user on what kind of problems to watch
out for when using them. Open core bobbin or “stick” induc-
tors are an inexpensive, simple way of making a compact ef-
ficient inductor, and they are used by the millions in many dif-
ferent applications.
THERMAL CONSIDERATIONS
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
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 in
2
, and ideally should
have 2 or more square inches of 2 oz. (0.0028) in) copper.
Additional copper area improves the thermal characteristics,
www.national.com 22
Application Information (Continued)
but with copper areas greater than approximately 6 in
2
, only
small improvements in heat dissipation are realized. If fur-
ther thermal improvements are needed, double sided, multi-
layer PC board with large copper areas and/or airflow are
recommended.
The curves shown in
Figure 20
show the LM2596S (TO-263
package) junction temperature rise above ambient tempera-
ture with a 2Aload 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 junc-
tion 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
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
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 PC board to dissipate
heat also depends on the size, quantity and spacing of other
components on the board, as well as whether the surround-
ing air is still or moving. Furthermore, some of these compo-
nents such as the catch diode will add heat to the PC board
and the heat can vary as the input voltage changes. For the
inductor, depending on the physical size, type of core mate-
rial 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.
DS012583-34
Circuit Data for Temperature Rise Curve
TO-220 Package (T)
Capacitors Through hole electrolytic
Inductor Through hole, Renco
Diode Through hole, 5A 40V, Schottky
PC board 3 square inches single sided 2 oz. copper
(0.0028")
FIGURE 19. Junction Temperature Rise, TO-220
DS012583-35
Circuit Data for Temperature Rise Curve
TO-263 Package (S)
Capacitors Surface mount tantalum, molded “D” size
Inductor Surface mount, Pulse Engineering, 68 µH
Diode Surface mount, 5A 40V, Schottky
PC board 9 square inches single sided 2 oz. copper
(0.0028")
FIGURE 20. Junction Temperature Rise, TO-263
DS012583-36
FIGURE 21. Delayed Startup
www.national.com23
Application Information (Continued)
DELAYED STARTUP
The circuit in
Figure 21
uses the the ON /OFF pin to provide
a time delay between the time the input voltage is applied
and the time the output voltage comes up (only the circuitry
pertaining to the delayed start up is shown).As the input volt-
age rises, the charging of capacitor C1 pulls the ON /OFF pin
high, keeping the regulator off. Once the input voltage
reaches its final value and the capacitor stops charging, and
resistor R
2
pulls the ON /OFF pin low, thus allowing the cir-
cuit to start switching. Resistor R
1
is included to limit the
maximum voltage applied to the ON /OFF pin (maximum of
25V), reduces power supply noise sensitivity, and also limits
the capacitor, C1, discharge current. When high input ripple
voltage exists, avoid long delay time, because this ripple can
be coupled into the ON /OFF pin and cause problems.
This delayed startup feature is useful in situations where the
input power source is limited in the amount of current it can
deliver. It allows the input voltage to rise to a higher voltage
before the regulator starts operating. Buck regulators require
less input current at higher input voltages.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage. An und-
ervoltage lockout feature applied to a buck regulator is
shown in
Figure 22
, while
Figure 23
and
24
applies the same
feature to an inverting circuit. The circuit in
Figure 23
fea-
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 kΩresistor 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
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulator’s ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
This example uses the LM2596-5.0 to generate a −5V out-
put, but other output voltages are possible by selecting other
output voltage versions, including the adjustable version.
Since 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 26
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 40V. For example, when convert-
ing +20V to −12V, the regulator would see 32V between the
input pin and ground pin. The LM2596 has a maximum input
voltage spec of 40V.
Additional diodes are required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C
IN
capacitor to the output, under light
or no load conditions. Also, this diode isolation changes the
topology to closley resemble a buck configuration thus pro-
viding good closed loop stability. A Schottky diode is recom-
mended for low input voltages, (because of its lower voltage
drop) but for higher input voltages, a fast recovery diode
could be used.
Without diode D3, when the input voltage is first applied, the
charging current of C
IN
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.
DS012583-37
FIGURE 22. Undervoltage Lockout
for Buck Regulator
DS012583-38
This circuit has an ON/OFF threshold of approximately 13V.
FIGURE 23. Undervoltage Lockout
for Inverting Regulator
www.national.com 24
Application Information (Continued)
Because of differences in the operation of the inverting regu-
lator, the standard design procedure is not used to select the
inductor value. In the majority of designs, a 33 µH, 3.5A in-
ductor is the best choice. Capacitor selection can also be
narrowed down to just a few values. Using the values shown
in
Figure 25
will provide good results in the majority of invert-
ing 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 LM2596 current limit (ap-
prox 4.5A) are needed for at least 2 ms or more, until the out-
put reaches its nominal output voltage. The actual time de-
pends 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 rela-
tively high startup currents required by the inverting topology,
the delayed startup feature (C1, R
1
and R
2
) shown in
Figure
25
is recommended. By delaying the regulator startup, the
input capacitor is allowed to charge up to a higher voltage
before the switcher begins operating.Aportion of the high in-
put current needed for startup is now supplied by the input
capacitor (C
IN
). For severe start up conditions, the input ca-
pacitor can be made much larger than normal.
INVERTING REGULATOR SHUTDOWN METHODS
To use the ON /OFF pin in a standard buck configuration is
simple, pull it below 1.3V (@25˚C, referenced to ground) to
turn regulator ON, pull it above 1.3V to shut the regulator
OFF. With the inverting configuration, some level shifting is
required, because the ground pin of the regulator is no
longer at ground, but is now setting at the negative output
voltage level. Two different shutdown methods for inverting
regulators are shown in
Figure 27
and
28.
DS012583-39
This circuit has hysteresis
Regulator starts switching at VIN =13V
Regulator stops switching at VIN =8V
FIGURE 24. Undervoltage Lockout with Hysteresis for Inverting Regulator
DS012583-40
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
FIGURE 25. Inverting −5V Regulator with Delayed Startup
DS012583-41
FIGURE 26. Inverting Regulator Typical Load Current
www.national.com25
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, FIXED OUTPUT (1X SIZE), DOUBLE SIDED
DS012583-42
FIGURE 27. Inverting Regulator Ground Referenced Shutdown
DS012583-43
FIGURE 28. Inverting Regulator Ground Referenced Shutdown using Opto Device
DS012583-44
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 26
Application Information (Continued)
TYPICAL THROUGH HOLE PC BOARD LAYOUT, ADJUSTABLE OUTPUT (1X SIZE), DOUBLE SIDED
DS012583-45
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—1kΩ,1
%
R
2— Use formula in Design Procedure
CFF — See
Figure 3
.
Thermalloy Heat Sink #7020
FIGURE 29. PC Board Layout
www.national.com27
Physical Dimensions inches (millimeters) unless otherwise noted
5-Lead TO-220 (T)
Order Number LM2596T-3.3, LM2596T-5.0,
LM2596T-12 or LM2596T-ADJ
NS Package Number T05D
www.national.com 28
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
National Semiconductor
Corporation
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Tel: 1-800-272-9959
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www.national.com
5-Lead TO-263 Surface Mount Package (S)
Order Number LM2596S-3.3, LM2596S-5.0,
LM2596S-12 or LM2596S-ADJ
NS Package Number TS5B
LM2596 SIMPLE SWITCHER Power Converter 150 kHz 3A Step-Down Voltage Regulator
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.
