LM2597/LM2597HV
SIMPLE SWITCHER®Power Converter 150 kHz 0.5A
Step-Down Voltage Regulator, with Features
General Description
The LM2597/LM2597HV series of regulators are monolithic
integrated circuits that provide all the active functions for a
step-down (buck) switching regulator, capable of driving a
0.5A load with excellent line and load regulation. These
devices are available in fixed output voltages of 3.3V, 5V,
12V, and an adjustable output version, and are packaged in
an 8-lead DIP and an 8-lead surface mount package.
This series of switching regulators is similar to the LM2594
series, with additional supervisory and performance features
added.
Requiring a minimum number of external components, these
regulators are simple to use and include internal frequency
compensation†, improved line and load specifications,
fixed-frequency oscillator, Shutdown /Soft-start, error flag
delay and error flag output.
The LM2597/LM2597HV series operates at a switching fre-
quency of 150 kHz thus allowing smaller sized filter compo-
nents than what would be needed with lower frequency
switching regulators. Because of its high efficiency, the cop-
per traces on the printed circuit board are normally the only
heat sinking needed.
A standard series of inductors (both through hole and sur-
face mount types) are available from several different manu-
facturers optimized for use with the LM2597/LM2597HV 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 all conditions of input voltage and output
load conditions, and ±15% on the oscillator frequency. Ex-
ternal shutdown is included, featuring typically 85 µA
standby current. Self protection features include a two stage
current limit for the output switch and an over temperature
shutdown for complete protection under fault conditions.
The LM2597HV is for use in applications requiring and input
voltage up to 60V.
Features
n3.3V, 5V, 12V, and adjustable output versions
nAdjustable version output voltage range, 1.2V to 37V
(57V for HV version)±4% max over line and load
conditions
nGuaranteed 0.5A output current
nAvailable in 8-pin surface mount and DIP-8 package
nInput voltage range up to 60V
n150 kHz fixed frequency internal oscillator
nShutdown /Soft-start
nOut of regulation error flag
nError output delay
nBias Supply Pin (V
BS
) for internal circuitry improves
efficiency at high input voltages
nLow power standby mode, I
Q
typically 85 µA
nHigh Efficiency
nUses readily available standard inductors
nThermal shutdown and current limit protection
Applications
nSimple high-efficiency step-down (buck) regulator
nEfficient pre-regulator for linear regulators
nOn-card switching regulators
nPositive to Negative converter
Typical Application (Fixed Output Voltage Versions)
SIMPLE SWITCHER®and
Switchers Made Simple
®are registered trademarks of National Semiconductor Corporation.
DS012440-1
†Patent Number 5,382,918.
December 2000
LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
Regulator, with Features
© 2001 National Semiconductor Corporation DS012440 www.national.com
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 (V
IN
)
LM2597 45V
LM2597HV 60V
SD /SS Pin Input Voltage (Note 2) 6V
Delay Pin Voltage (Note 2) 1.5V
Flag Pin Voltage −0.3 ≤V≤45V
Bias Supply Voltage (V
BS
) −0.3 ≤V≤30V
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 3) 2 kV
Lead Temperature
M8 Package
Vapor Phase (60 sec.) +215˚C
Infrared (15 sec.) +220˚C
N Package (Soldering, 10 sec.) +260˚C
Maximum Junction Temperature +150˚C
Operating Conditions
Temperature Range −40˚C ≤T
J
+125˚C
Supply VoltageLM2597 4.5V to 40V
LM2597HV 4.5V to 60V
LM2597/LM2597HV-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.V
INmax
=40V for the LM2597 and 60V for the LM2597HV
Symbol Parameter Conditions LM2597/LM2597HV-3.3 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 12
V
OUT
Output Voltage 4.75V ≤V
IN
≤V
INmax
, 0.1A ≤I
LOAD
≤0.5A 3.3 V
3.168/3.135 V(min)
3.432/3.465 V(max)
ηEfficiency V
IN
= 12V, I
LOAD
= 0.5A 80 %
LM2597/LM2597HV-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.V
INmax
=40V for the LM2597 and 60V for the LM2597HV
Symbol Parameter Conditions LM2597/LM2597HV-5.0 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 12
V
OUT
Output Voltage 7V ≤V
IN
≤V
INmax
, 0.1A ≤I
LOAD
≤0.5A 5 V
4.800/4.750 V(min)
5.200/5.250 V(max)
ηEfficiency V
IN
= 12V, I
LOAD
= 0.5A 82 %
LM2597/LM2597HV
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LM2597/LM2597HV-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.V
INmax
=40V for the LM2597 and 60V for the LM2597HV
Symbol Parameter Conditions LM2597/LM2597HV-12 Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 12
V
OUT
Output Voltage 15V ≤V
IN
≤V
INmax
, 0.1A ≤I
LOAD
≤0.5A 12 V
11.52/11.40 V(min)
12.48/12.60 V(max)
ηEfficiency V
IN
= 25V, I
LOAD
= 0.5A 88 %
LM2597/LM2597HV-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.V
INmax
=40V for the LM2597 and 60V for the LM2597HV
Symbol Parameter Conditions LM2597/LM2597HV-ADJ Units
(Limits)
Typ Limit
(Note 4) (Note 5)
SYSTEM PARAMETERS (Note 6) Test Circuit
Figure 12
V
FB
Feedback Voltage 4.5V ≤V
IN
≤V
INmax
, 0.1A ≤I
LOAD
≤0.5A 1.230 V
V
OUT
programmed for 3V. Circuit of
Figure 12
. 1.193/1.180 V(min)
1.267/1.280 V(max)
ηEfficiency V
IN
= 12V, V
OUT
= 3V, I
LOAD
= 0.5A 80 %
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
= 100 mA.
Symbol Parameter Conditions LM2597/LM2597HV-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
I
b
Feedback Bias Current Adjustable Version Only, V
FB
= 1.235V 10 50/100 nA
f
O
Oscillator Frequency (Note 7) 150 kHz
127/110 kHz(min)
173/173 kHz(max)
V
SAT
Saturation Voltage I
OUT
= 0.5A (Notes 8 and 9) 0.9 V
1.1/1.2 V(max)
DC Max Duty Cycle (ON) (Note 9) 100 %
Min Duty Cycle (OFF) (Note 10) 0
I
CL
Current Limit Peak Current, (Notes 8 and 9) 0.8 A
0.65/0.58 A(min)
1.3/1.4 A(max)
I
L
Output Leakage Current (Notes 8, 10 and 11) Output = 0V 50 µA(max)
Output = −1V 2 mA
15 mA(max)
I
Q
Operating Quiescent SD /SS Pin Open, V
BS
Pin Open(Note 10) 5mA
LM2597/LM2597HV
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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
= 100 mA.
Symbol Parameter Conditions LM2597/LM2597HV-XX Units
(Limits)
Typ Limit
(Note 4) (Note 5)
DEVICE PARAMETERS
Current 10 mA(max)
I
STBY
Standby Quiescent SD /SS pin = 0V (Note 10)LM2597 85 µA
Current 200/250 µA(max)
LM2597HV 140 250/300 µA(max)
θ
JA
Thermal Resistance N Package, Junction to Ambient (Note 12) 95 ˚C/W
M Package, Junction to Ambient (Note 12) 150
SHUTDOWN/SOFT-START CONTROL Test Circuit of
Figure 12
V
SD
Shutdown Threshold 1.3 V
Voltage Low, (Shutdown Mode) 0.6 V(max)
High, (Soft-start Mode) 2V(min)
V
SS
Soft-start Voltage V
OUT
= 20% of Nominal Output Voltage 2 V
V
OUT
= 100% of Nominal Output Voltage 3
I
SD
Shutdown Current V
SHUTDOWN
= 0.5V 5µA
10 µA(max)
I
SS
Soft-start Current V
Soft-start
= 2.5V 1.6 µA
5 µA(max)
FLAG/DELAY CONTROL Test Circuit of
Figure 12
Regulator Dropout Low (Flag ON) 96 %
Detector 92 %(min)
Threshold Voltage 98 %(max)
VF
SAT
Flag Output Saturation I
SINK
= 3 mA 0.3 V
Voltage V
DELAY
= 0.5V 0.7/1.0 V(max)
IF
L
Flag Output Leakage
Current V
FLAG
= 40V 0.3 µA
Delay Pin Threshold 1.25 V
Voltage Low (Flag ON) 1.21 V(min)
High (Flag OFF) and V
OUT
Regulated 1.29 V(max)
Delay Pin Source V
DELAY
= 0.5V 3 µA
Current 6 µA(max)
Delay Pin Saturation Low (Flag ON) 55 mV
350/400 mV(max)
BIAS SUPPLY
I
BS
Bias Supply Pin Current V
BS
= 2V (Note 10) 120 µA
400 µA(max)
V
BS
= 4.4V (Note 10) 4 mA
10 mA(max)
I
Q
Operating Quiescent
Current V
BS
= 4.4V , V
in
pin current(Note 10) 1 2 mA
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: Voltage internally clamped. If clamp voltage is exceeded, limit current to a maximum of 1 mA.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 4: Typical numbers are at 25˚C and represent the most likely norm.
LM2597/LM2597HV
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All Output Voltage Versions
Electrical Characteristics (Continued)
Note 5: 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 6: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the
LM2597/LM2597HV is used as shown in the
Figure 12
test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.
Note 7: The switching frequency is reduced when the second stage current limit is activated. The amount of reduction is determined by the severity of current
overload.
Note 8: No diode, inductor or capacitor connected to output pin.
Note 9: Feedback pin removed from output and connected to 0V to force the output transistor switch ON.
Note 10: Feedback pin removed from output and connected to 12V for the 3.3V, 5V, and theADJ. version, and 15V for the 12V version, to force the output transistor
switch OFF.
Note 11: VIN = 40V for the LM2597 and 60V for the LM2597HV.
Note 12: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will
lower thermal resistance further. See application hints in this data sheet and the thermal model in Switchers Made Simple ™software.
Typical Performance Characteristics
Normalized
Output Voltage
DS012440-2
Line Regulation
DS012440-3
Efficiency
DS012440-4
Switch Saturation
Voltage
DS012440-5
Switch Current Limit
DS012440-6
Dropout Voltage
DS012440-7
LM2597/LM2597HV
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Typical Performance Characteristics (Continued)
Quiescent Current
DS012440-8
Standby
Quiescent Current
DS012440-9
Minimum Operating
Supply Voltage
DS012440-10
Feedback Pin
Bias Current
DS012440-11
Flag Saturation
Voltage
DS012440-12
Switching Frequency
DS012440-13
Soft-start
DS012440-14
Shutdown /Soft-start
Current
DS012440-15
Delay Pin Current
DS012440-16
LM2597/LM2597HV
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Typical Performance Characteristics (Continued)
V
IN
and V
BS
Current vs
V
BS
and Temperature
DS012440-17
Soft-start Response
DS012440-18
Shutdown /Soft-start
Threshold Voltage
DS012440-25
Continuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
=5V,I
LOAD
= 400 mA
L = 100 µH, C
OUT
= 120 µF, C
OUT
ESR = 140 mΩ
DS012440-19
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.2A/div.
C: Output Ripple Voltage, 20 mV/div.
Horizontal Time Base: 2 µs/div.
Discontinuous Mode Switching Waveforms
V
IN
= 20V, V
OUT
=5V,I
LOAD
= 200 mA
L = 33 µH, C
OUT
= 220 µF, C
OUT
ESR=60mΩ
DS012440-20
A: Output Pin Voltage, 10V/div.
B: Inductor Current 0.2A/div.
C: Output Ripple Voltage, 20 mV/div.
Horizontal Time Base: 2 µs/div.
Load Transient Response for Continuous Mode
V
IN
= 20V, V
OUT
=5V,I
LOAD
= 200 mA to 500 mA
L = 100 µH, C
OUT
= 120 µF, C
OUT
ESR = 140 mΩ
DS012440-21
A: Output Voltage, 50 mV/div. (AC)
B: 200 mA to 500 mA Load Pulse
Horizontal Time Base: 50 µs/div.
Load Transient Response for Discontinuous Mode
V
IN
= 20V, V
OUT
=5V,I
LOAD
= 100 mA to 200 mA
L = 33 µH, C
OUT
= 220 µF, C
OUT
ESR=60mΩ
DS012440-22
A: Output Voltage, 50 mV/div. (AC)
B: 100 mA to 200 mA Load Pulse
Horizontal Time Base: 200 µs/div.
LM2597/LM2597HV
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Connection Diagrams and Ordering Information
8–Lead DIP (N)
DS012440-23
Top View
Order Number LM2597N-3.3,
LM2597N-5.0, LM2597N-12 or
LM2597N-ADJ
LM2597HVN-3.3, LM2597HVN-5.0,
LM2597HVN-12 or LM2597HVN-ADJ
See NS Package Number N08E
8–Lead Surface Mount (M)
DS012440-24
Top View
Order Number LM2597M-3.3,
LM2597M-5.0, LM2597M-12 or
LM2597M-ADJ
LM2597HVM-3.3, LM2597HVM-5.0,
LM2597HVM-12 or LM2597HVM-ADJ
See NS Package Number M08A
LM2597/LM2597HV
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LM2597/LM2597HV 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) = 0.4A
1. Inductor Selection (L1)
A. Select the correct inductor value selection guide from
Figure 3
,
Figure 4
,or
Figure 5
. (Output voltages of 3.3V, 5V, or
12V respectively.) For all other voltages, see the design pro-
cedure 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 manufacturer’s
part numbers listed in
Figure 7
.
1. Inductor Selection (L1)
A. Use the inductor selection guide for the 5V version shown
in
Figure 4
.
B. From the inductor value selection guide shown in
Figure 4
,
the inductance region intersected by the 12V horizontal line
and the 0.4A vertical line is 100 µH, and the inductor code is
L20.
C. The inductance value required is 100 µH. From the table in
Figure 7
, go to the L20 line and choose an inductor part
number from any of the four manufacturers shown. (In most
instance, both through hole and surface mount inductors are
available.)
2. Output Capacitor Selection (C
OUT
)
A. In the majority of applications, low ESR (Equivalent Series
Resistance) electrolytic capacitors between 82 µF and 220 µF
and low ESR solid tantalum capacitors between 15 µF and
100 µF provide the best results. This capacitor should be
located close to the IC using short capacitor leads and short
copper traces. Do not use capacitors larger than 220 µF.
For additional information, see 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 1
.
This table contains different input voltages, output voltages,
and load currents, and lists various inductors and output
capacitors that will provide the best design solutions.
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 4.1 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 1
, 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 0.5Aline. 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.
120 µF 25V Panasonic HFQ Series
120 µF 25V Nichicon PL Series
C. For a 5V output, a capacitor voltage rating at least 7.5V or
more is needed. But, in this example, even a low ESR,
switching grade, 120 µF 10V aluminum electrolytic capacitor
would exhibit approximately 400 mΩof ESR (see the curve in
Figure 17
for the ESR vs voltage rating). This amount of ESR
would result in relatively high output ripple voltage. To reduce
the ripple to 1% of the output voltage, or less, a capacitor with
a higher voltage rating (lower ESR) should be selected.A 16V
or 25V capacitor will reduce the ripple voltage by approxi-
mately half.
LM2597/LM2597HV
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LM2597/LM2597HV 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 LM2597. 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 LM2597 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and 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 1N4001
series are much too slow and should not be used.
3. Catch Diode Selection (D1)
A. Refer to the table shown in
Figure 10
. In this example, a
1A, 20V, 1N5817 Schottky diode will provide the best perfor-
mance, and will not be overstressed even for a shorted out-
put.
4. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. In addition, the RMS
current rating of the input capacitor 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 16
shows typical
RMS current ratings for several different aluminum electrolytic
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 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 input
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 400 mA load, a capacitor with a RMS
current rating of at least 200 mAis needed. The curves shown
in
Figure 16
can be used to select an appropriate input
capacitor. From the curves, locate the 25V line and note which
capacitor values have RMS current ratings greater than 200
mA. Either a 47 µF or 68 µF, 25V capacitor could be used.
For a through hole design, a 68 µF/25V electrolytic capacitor
(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 are
recommended. The TPS series available from AVX, and the
593D series from Sprague are both surge current tested.
LM2597/LM2597HV
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LM2597/LM2597HV Series Buck Regulator Design Procedure (Fixed
Output) (Continued)
Conditions Inductor Output Capacitor
Through Hole Surface Mount
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 0.5 5 33 L14 220/16 220/16 100/16 100/6.3
7 47 L13 120/25 120/25 100/16 100/6.3
10 68 L21 120/25 120/25 100/16 100/6.3
40 100 L20 120/35 120/35 100/16 100/6.3
6 68 L4 120/25 120/25 100/16 100/6.3
0.2 10 150 L10 120/16 120/16 100/16 100/6.3
40 220 L9 120/16 120/16 100/16 100/6.3
50.5 8 47 L13 180/16 180/16 100/16 33/25
10 68 L21 180/16 180/16 100/16 33/25
15 100 L20 120/25 120/25 100/16 33/25
40 150 L19 120/25 120/25 100/16 33/25
9 150 L10 82/16 82/16 100/16 33/25
0.2 20 220 L9 120/16 120/16 100/16 33/25
40 330 L8 120/16 120/16 100/16 33/25
12 0.5 15 68 L21 82/25 82/25 100/16 15/25
18 150 L19 82/25 82/25 100/16 15/25
30 220 L27 82/25 82/25 100/16 15/25
40 330 L26 82/25 82/25 100/16 15/25
15 100 L11 82/25 82/25 100/16 15/25
0.2 20 220 L9 82/25 82/25 100/16 15/25
40 330 L17 82/25 82/25 100/16 15/25
FIGURE 1. LM2597/LM2597HV Fixed Voltage Quick Design Component Selection Table
LM2597/LM2597HV
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LM2597/LM2597HV 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) = 0.5A
F = Switching Frequency
(Fixed at a nominal 150 kHz).
1. Programming Output Voltage (Selecting R
1
and R
2
,as
shown in
Figure 12
)
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 12
)
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 = 0.9V
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 6
.
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 manufacturer’s
part numbers listed in
Figure 7
.
2. Inductor Selection (L1)
A. Calculate the inductor Volt •microsecond constant (E •T),
B. E•T = 35.2 (V •µs)
C. I
LOAD
(max) = 0.5A
D. From the inductor value selection guide shown in
Figure 6
,
the inductance region intersected by the 35 (V •µs) horizontal
line and the 0.5Avertical line is 150 µH, and the inductor code
is L19.
E. From the table in
Figure 7
, locate line L19, and select an
inductor part number from the list of manufacturers part num-
bers.
LM2597/LM2597HV
www.national.com 12
LM2597/LM2597HV 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 220 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 220 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 2
. This table contains
different 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 2
, 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 capacitors
from several different manufacturers are available.
82 µF 50V Panasonic HFQ Series
120 µF 50V 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 capacitor
would work. A 50V rating was chosen because it has a lower
ESR which provides a lower output ripple voltage.
Other manufacturers or other types of capacitors may also be
used, provided the capacitor specifications (especially the
100 kHz ESR) closely match the types listed in the table.
Refer to the capacitor manufacturers data sheet for this infor-
mation.
4. Feedforward Capacitor (C
FF
)(See
Figure 12
)
For output voltages greater than approximately 10V, an addi-
tional capacitor is required. The compensation capacitor is
typically between 50 pF and 10 nF, and is wired in parallel with
the output voltage setting resistor, R
2
. It provides additional
stability for high output voltages, low input-output voltages,
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 capacitors
made with Z5U material, they are not recommended.)
4. Feedforward Capacitor (C
FF
)
The table shown in
Figure 2
contains feed forward capacitor
values for various output voltages. In this example,a1nF
capacitor is needed.
LM2597/LM2597HV
www.national.com13
LM2597/LM2597HV 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 LM2597. 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 LM2597 using short leads and
short printed circuit traces. Because of their fast switching
speed and low forward voltage drop, Schottky diodes provide
the best performance and efficiency, and 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.
5. Catch Diode Selection (D1)
A. Refer to the table shown in
Figure 10
. Schottky diodes
provide the best performance, and in this example a 1A, 40V,
1N5819 Schottky diode would be a good choice. The 1Adiode
rating is more than adequate and will not be overstressed
even for a shorted output.
6. Input Capacitor (C
IN
)
A low ESR aluminum or tantalum bypass capacitor is needed
between the input pin and ground to prevent large voltage
transients from appearing at the input. In addition, the RMS
current rating of the input capacitor 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 16
shows typical
RMS current ratings for several different aluminum electrolytic
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 ceramic capacitors for input bypass-
ing, because it may cause severe ringing at the V
IN
pin.
For additional information, see section on input capacitor
in application information section.
6. Input Capacitor (C
IN
)
The important parameters for the Input capacitor are the input
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 400 mA load, a capacitor with a RMS
current rating of at least 200 mA is needed.
The curves shown in
Figure 16
can be used to select an
appropriate input capacitor. From the curves, locate the 50V
line and note which capacitor values have RMS current rat-
ings greater than 200 mA. A 47 µF/50V low ESR electrolytic
capacitor capacitor is needed.
For a through hole design, a 47 µF/50V electrolytic capacitor
(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 are
recommended. 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 switching
regulators. Switchers Made Simple (version 4.1 or later) is
available at National’s web site, www.national.com.
LM2597/LM2597HV
www.national.com 14
LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable
Output) (Continued)
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)
1.2 220/25 220/25 0 220/10 220/10 0
4180/25 180/25 4.7 nF 100/10 120/10 4.7 nF
682/25 82/25 4.7 nF 100/10 120/10 4.7 nF
982/25 82/25 3.3 nF 100/16 100/16 3.3 nF
12 82/25 82/25 2.2 nF 100/16 100/16 2.2 nF
15 82/25 82/25 1.5 nF 68/20 100/20 1.5 nF
24 82/50 120/50 1 nF 10/35 15/35 220 pF
28 82/50 120/50 820 pF 10/35 15/35 220 pF
FIGURE 2. Output Capacitor and Feedforward Capacitor Selection Table
LM2597/LM2597HV
www.national.com15
LM2597/LM2597HV Series Buck Regulator Design Procedure
INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)
DS012440-57
FIGURE 3. LM2597/LM2597HV-3.3
DS012440-30
FIGURE 4. LM2597/LM2597HV-5.0
DS012440-58
FIGURE 5. LM2597/LM2597HV-12 DS012440-32
FIGURE 6. LM2597/LM2597HV-ADJ
LM2597/LM2597HV
www.national.com 16
LM2597/LM2597HV Series Buck Regulator Design Procedure (Continued)
Induc-
tance
(µH)
Cur-
rent
(A)
Schott Renco Pulse Engineering Coilcraft
Through Surface Through Surface Through Surface Surface
Hole Mount Hole Mount Hole Mount Mount
L1 220 0.18 67143910 67144280 RL-5470-3 RL1500-220 PE-53801 PE-53801-S DO1608-224
L2 150 0.21 67143920 67144290 RL-5470-4 RL1500-150 PE-53802 PE-53802-S DO1608-154
L3 100 0.26 67143930 67144300 RL-5470-5 RL1500-100 PE-53803 PE-53803-S DO1608-104
L4 68 0.32 67143940 67144310 RL-1284-68 RL1500-68 PE-53804 PE-53804-S DO1608-68
L5 47 0.37 67148310 67148420 RL-1284-47 RL1500-47 PE-53805 PE-53805-S DO1608-473
L6 33 0.44 67148320 67148430 RL-1284-33 RL1500-33 PE-53806 PE-53806-S DO1608-333
L7 22 0.60 67148330 67148440 RL-1284-22 RL1500-22 PE-53807 PE-53807-S DO1608-223
L8 330 0.26 67143950 67144320 RL-5470-2 RL1500-330 PE-53808 PE-53808-S DO3308-334
L9 220 0.32 67143960 67144330 RL-5470-3 RL1500-220 PE-53809 PE-53809-S DO3308-224
L10 150 0.39 67143970 67144340 RL-5470-4 RL1500-150 PE-53810 PE-53810-S DO3308-154
L11 100 0.48 67143980 67144350 RL-5470-5 RL1500-100 PE-53811 PE-53811-S DO3308-104
L12 68 0.58 67143990 67144360 RL-5470-6 RL1500-68 PE-53812 PE-53812-S DO1608-683
L13 47 0.70 67144000 67144380 RL-5470-7 RL1500-47 PE-53813 PE-53813-S DO3308-473
L14 33 0.83 67148340 67148450 RL-1284-33 RL1500-33 PE-53814 PE-53814-S DO1608-333
L15 22 0.99 67148350 67148460 RL-1284-22 RL1500-22 PE-53815 PE-53815-S DO1608-223
L16 15 1.24 67148360 67148470 RL-1284-15 RL1500-15 PE-53816 PE-53816-S DO1608-153
L17 330 0.42 67144030 67144410 RL-5471-1 RL1500-330 PE-53817 PE-53817-S DO3316-334
L18 220 0.55 67144040 67144420 RL-5471-2 RL1500-220 PE-53818 PE-53818-S DO3316-224
L19 150 0.66 67144050 67144430 RL-5471-3 RL1500-150 PE-53819 PE-53819-S DO3316-154
L20 100 0.82 67144060 67144440 RL-5471-4 RL1500-100 PE-53820 PE-53820-S DO3316-104
L21 68 0.99 67144070 67144450 RL-5471-5 RL1500-68 PE-53821 PE-53821-S DDO3316-683
L26 330 0.80 67144100 67144480 RL-5471-1 — PE-53826 PE-53826-S —
L27 220 1.00 67144110 67144490 RL-5471-2 — PE-53827 PE-53827-S —
FIGURE 7. Inductor Manufacturers Part Numbers
Coilcraft Inc. Phone (800) 322-2645
FAX (708) 639-1469
Coilcraft Inc., Europe Phone +44 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 8. Inductor Manufacturers Phone Numbers
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) 324-7223
FAX (207) 324-4140
FIGURE 9. Capacitor Manufacturers Phone Numbers
LM2597/LM2597HV
www.national.com17
LM2597/LM2597HV Series Buck Regulator Design Procedure (Continued)
Block Diagram
VR 1A Diodes
Surface Mount Through Hole
Schottky Ultra Fast Schottky Ultra Fast
Recovery Recovery
20V All of these diodes are rated to 1N5817 All of these diodes are rated to
at least 60V. SR102 at least 60V.
MBRS130 1N5818
30V SR103
11DQ03
MBRS140 MURS120 1N5819 MUR120
40V 10BQ040 10BF10 SR104 HER101
10MQ040 11DQ04 11DF1
50V
or
more
MBRS160 SR105
10BQ050 MBR150
10MQ060 11DQ05
MBRS1100 MBR160
10MQ090 SB160
SGL41-60 11DQ10
SS16
FIGURE 10. Diode Selection Table
DS012440-26
FIGURE 11.
LM2597/LM2597HV
www.national.com 18
Typical Circuit and Layout Guidelines
Fixed Output Voltage Versions
DS012440-27
Component Values shown are for VIN = 15V, VOUT =5V,I
LOAD = 500 mA.
CIN — 47 µF, 50V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 1A, 30V Schottky Rectifier, 1N5818
L1 — 100 µH, L20
Typical Values
CSS — 0.1 µF
CDELAY — 0.1 µF
RPull Up — 4.7k
*Use Bias Supply pin for 5V and 12V Versions
Adjustable Output Voltage Versions
DS012440-56
Select R1to be approximately 1 kΩ, use a 1% resistor for best stability.
Component Values shown are for VIN = 20V,
VOUT = 10V, ILOAD = 500 mA.
CIN — 68 µF, 35V, Aluminum Electrolytic Nichicon “PL Series”
COUT — 120 µF, 25V Aluminum Electrolytic, Nichicon “PL Series”
D1 — 1A, 30V Schottky Rectifier, 1N5818
L1 — 150 µH, L19
R1—1kΩ,1%
R
2— 7.15k, 1%
CFF — 3.3 nF, See Application Information Section
Typical Values
CSS — 0.1 µF
CDELAY — 0.1 µF
RPULL UP — 4.7k
*For output voltages between 4V and 20V
FIGURE 12. Typical Circuits and Layout Guides
LM2597/LM2597HV
www.national.com19
Typical Circuit and Layout
Guidelines (Continued)
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,
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 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
associated 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.)
Application Information
PIN FUNCTIONS
+V
IN
(Pin 7)—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 (Pin 6)—Circuit ground.
Output (Pin 8)—Internal switch. The voltage at this pin
switches between (+V
IN
−V
SAT
) and approximately −0.5V,
with a duty cycle of V
OUT
/V
IN
. To minimize coupling to sen-
sitive circuitry, the PC board copper area connected to this
pin should be kept to a minimum.
Feedback (Pin 4)—Senses the regulated output voltage to
complete the feedback loop.
Shutdown /Soft-start (Pin 5)—This dual function pin pro-
vides the following features: (a) Allows the switching regula-
tor circuit to be shut down using logic level signals thus
dropping the total input supply current to approximately
80 µA. (b) Adding a capacitor to this pin provides a soft-start
feature which minimizes startup current and provides a con-
trolled ramp up of the output voltage.
Error Flag (Pin 1)—Open collector output that provides a
low signal (flag transistor ON) when the regulated output
voltage drops more than 5% from the nominal output volt-
age. On start up, Error Flag is low until V
OUT
reaches 95% of
the nominal output voltage and a delay time determined by
the Delay pin capacitor. This signal can be used as a reset to
a microprocessor on power-up.
Delay (Pin 2)—At power-up, this pin can be used to provide
a time delay between the time the regulated output voltage
reaches 95% of the nominal output voltage, and the time the
error flag output goes high.
Bias Supply (Pin 3)—This feature allows the regulators
internal circuitry to be powered from the regulated output
voltage or an external supply, instead of the input voltage.
This results in increased efficiency under some operating
conditions, such as low output current and/or high input
voltage.
Special Note If any of the above four features (Shutdown
/Soft-start, Error Flag, Delay, or Bias Supply) are not used,
the respective pins should be left open.
EXTERNAL COMPONENTS
SOFT-START CAPACITOR
C
SS
—A capacitor on this pin provides the regulator with a
Soft-start feature (slow start-up). When the DC input voltage
is first applied to the regulator, or when the Shutdown
/Soft-start pin is allowed to go high, a constant current
(approximately 5 µA begins charging this capacitor). As the
capacitor voltage rises, the regulator goes through four op-
erating regions (See the bottom curve in
Figure 13
).
1. Regulator in Shutdown.
When the SD /SS pin voltage is
between 0V and 1.3V, the regulator is in shutdown, the
output voltage is zero, and the IC quiescent current is ap-
proximately 85 µA.
2. Regulator ON, but the output voltage is zero.
With the
SD /SS pin voltage between approximately 1.3V and 1.8V,
the internal regulatory circuitry is operating, the quiescent
current rises to approximately 5 mA, but the output voltage is
still zero. Also, as the 1.3V threshold is exceeded, the
Soft-start capacitor charging current decreases from 5 µA
down to approximately 1.6 µA. This decreases the slope of
capacitor voltage ramp.
3. Soft-start Region.
When the SD /SS pin voltage is be-
tween 1.8V and 2.8V (@25˚C), the regulator is in a Soft-start
condition. The switch (Pin 8) duty cycle initially starts out
very low, with narrow pulses and gradually get wider as the
capacitor SD /SS pin ramps up towards 2.8V. As the duty
cycle increases, the output voltage also increases at a con-
trolled ramp up. See the center curve in
Figure 13
. The input
supply current requirement also starts out at a low level for
the narrow pulses and ramp up in a controlled manner. This
is a very useful feature in some switcher topologies that
require large startup currents (such as the inverting configu-
ration) which can load down the input power supply.
Note: The lower curve shown in
Figure 13
shows the Soft-start region from
0% to 100%. This is not the duty cycle percentage, but the output
voltage percentage. Also, the Soft-start voltage range has a negative
temperature coefficient associated with it. See the Soft-start curve in
the electrical characteristics section.
4. Normal operation.
Above 2.8V, the circuit operates as a
standard Pulse Width Modulated switching regulator. The
capacitor will continue to charge up until it reaches the
internal clamp voltage of approximately 7V. If this pin is
driven from a voltage source, the current must be limited to
about 1 mA.
If the part is operated with an input voltage at or below the
internal soft-start clamp voltage of approximately 7V, the
voltage on the SD/SS pin tracks the input voltage and can be
disturbed by a step in the voltage. To maintain proper func-
tion under these conditions, it is strongly recommended that
the SD/SS pin be clamped externally between the 3V maxi-
mum soft-start threshold and the 4.5V minimum input volt-
age.
Figure 15
is an example of an external 3.7V (approx.)
clamp that prevents a line-step related glitch but does not
interfere with the soft-start behavior of the device.
LM2597/LM2597HV
www.national.com 20
Application Information (Continued)
DS012440-33
FIGURE 13. Soft-start, Delay, Error, Output
LM2597/LM2597HV
www.national.com21
Application Information (Continued)
DELAY CAPACITOR
C
DELAY
—Provides delay for the error flag output. See the
upper curve in
Figure 13
, and also refer to timing diagrams in
Figure 14
. A capacitor on this pin provides a time delay
between the time the regulated output voltage (when it is
increasing in value) reaches 95% of the nominal output
voltage, and the time the error flag output goes high. A 3 µA
constant current from the delay pin charges the delay ca-
pacitor resulting in a voltage ramp. When this voltage
reaches a threshold of approximately 1.3V, the open collec-
tor error flag output (or power OK) goes high. This signal can
be used to indicate that the regulated output has reached the
correct voltage and has stabilized.
If, for any reason, the regulated output voltage drops by 5%
or more, the error output flag (Pin 1) immediately goes low
(internal transistor turns on). The delay capacitor provides
very little delay if the regulated output is dropping out of
regulation. The delay time for an output that is decreasing is
approximately a 1000 times less than the delay for the rising
output. For a 0.1 µF delay capacitor, the delay time would be
approximately 50 ms when the output is rising and passes
through the 95% threshold, but the delay for the output
dropping would only be approximately 50 µs.
R
Pull Up
—The error flag output, (or power OK) is the col-
lector of a NPN transistor, with the emitter internally
grounded. To use the error flag, a pullup resistor to a positive
voltage is needed. The error flag transistor is rated up to a
maximum of 45V and can sink approximately 3 mA. If the
error flag is not used, it can be left open.
INPUT CAPACITOR
C
IN
—A low ESR aluminum or tantalum bypass capacitor is
needed between the input pin and ground pin. It must be
located 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
input capacitor, this capacitor should be chosen for its RMS
DS012440-34
FIGURE 14. Timing Diagram for 5V Output
DS012440-75
FIGURE 15. External 3.7V Soft-Start Clamp
LM2597/LM2597HV
www.national.com 22
Application Information (Continued)
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 elec-
trolytic capacitor will be physically larger than a lower voltage
capacitor, and thus be able to dissipate more heat to the
surrounding air, and therefore will have a higher RMS cur-
rent 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
capacitor’s electrolyte, resulting in eventual failure.
Selecting an input capacitor requires consulting the manu-
facturers data sheet for maximum allowable RMS ripple
current. For a maximum ambient temperature of 40˚C, a
general guideline would be to select a capacitor with a ripple
current rating of approximately 50% of the DC load current.
For ambient temperatures up to 70˚C, a current rating of
75% of the DC load current would be a good choice for a
conservative design. The capacitor voltage rating must be at
least 1.25 times greater than the maximum input voltage,
and often a much higher voltage capacitor is needed to
satisfy the RMS current requirements.
A graph shown in
Figure 16
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-
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.
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
upper and lower limit. For low output ripple voltage, a low
ESR value is needed. This value is determined by the maxi-
mum allowable output ripple voltage, typically 1% to 2% of
the output voltage. But if the selected capacitor’s ESR is
extremely low, there is a possibility of an unstable feedback
loop, resulting in an oscillation at the output. Using the
capacitors listed in the tables, or similar types, will provide
design solutions 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 17
). 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
often can be satisfied with only three or four different capaci-
tor values and several different voltage ratings. See the
DS012440-28
FIGURE 16. RMS Current Ratings for Low
ESR Electrolytic Capacitors (Typical)
DS012440-29
FIGURE 17. Capacitor ESR vs Capacitor Voltage Rating
(Typical Low ESR Electrolytic Capacitor)
LM2597/LM2597HV
www.national.com23
Application Information (Continued)
quick design component selection tables in
Figure 1
and
Figure 2
for typical capacitor values, voltage ratings, and
manufacturers capacitor 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 18
.
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 LM2594 using
short leads and short printed circuit traces.
Because of their very fast switching speed and low forward
voltage drop, Schottky diodes provide the best performance,
especially in low output voltage applications (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
recovery 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.
INDUCTOR SELECTION
All switching regulators have two basic modes of operation;
continuous and discontinuous. The difference between the
two types relates to the inductor current, whether it is flowing
continuously, or if it drops to zero for a period of time in the
normal switching cycle. Each mode has distinctively different
operating characteristics, which can affect the regulators
performance and requirements. Most switcher designs will
operate in the discontinuous mode when the load current is
low.
The LM2597 (or any of the Simple Switcher family) can be
used for both continuous or discontinuous modes of 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
output 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 volt-
ages.
To simplify the inductor selection process, an inductor selec-
tion guide (nomograph) was designed (see
Figure 3
through
Figure 6
). 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
percentage of the maximum design load current. This
peak-to-peak inductor ripple current percentage is not fixed,
but is allowed to change as different design load currents are
selected. (See
Figure 19
.)
By allowing the percentage of inductor ripple current to
increase for low load currents, the inductor value and size
can be kept relatively low.
When operating in the continuous mode, the inductor current
waveform ranges from a triangular to a sawtooth type of
waveform (depending on the input voltage), with the average
value of this current waveform equal to the DC output load
current.
Inductors are available in different styles such as pot core,
toroid, E-core, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least
expensive, the bobbin, rod or stick core, consists of wire
wrapped on a ferrite bobbin. This type of construction makes
for a inexpensive inductor, but since the magnetic flux is not
completely contained within the core, it generates more
Electro-Magnetic Interference (EMl). This magnetic flux can
induce voltages into nearby printed circuit traces, thus 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.
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 LM2597. Different inductor
types have different saturation characteristics, and this
should be kept in mind when selecting an inductor.
DS012440-37
FIGURE 18. Capacitor ESR Change vs Temperature
DS012440-31
FIGURE 19. (∆I
IND
) Peak-to-Peak Inductor
Ripple Current (as a Percentage
of the Load Current) vs Load Current
LM2597/LM2597HV
www.national.com 24
Application Information (Continued)
The inductor manufacturers data sheets include current and
energy limits to avoid inductor saturation.
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
design. The peak switch and inductor currents will be higher
in a discontinuous design, but at these low load currents
(200 mA and below), the maximum switch current will still be
less than the switch current limit.
Discontinuous operation can have voltage waveforms that
are considerable different than a continuous design. The
output pin (switch) waveform can have some damped sinu-
soidal ringing present. (See photo titled; Discontinuous
Mode Switching Waveforms) This ringing is normal for dis-
continuous operation, and is not caused by feedback loop
instabilities. In discontinuous operation, 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. Normally this ringing is not a prob-
lem, unless the amplitude becomes great enough to exceed
the input voltage, and even then, there is very little energy
present to cause damage.
Different inductor types 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
produce less ringing. If desired, a series RC could be placed
in parallel with the inductor to dampen the ringing. The
computer aided design software
Switchers Made Simple
(version 4.1) will provide all component values for continu-
ous 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
voltage, the ESR of the output capacitor must be low, how-
ever, caution must be exercised when using extremely low
ESR capacitors because they can affect the loop stability,
resulting in oscillation problems. If very low output ripple
voltage is needed (less than 15 mV), a post ripple filter is
recommended. (See
Figure 12
.) The inductance required is
typically between 1 µH and 5 µH, with low DC resistance, to
maintain good load regulation. A low ESR output filter ca-
pacitor is also required to assure good dynamic load re-
sponse and ripple reduction. The ESR of this capacitor may
be as low as desired, because it is out of the regulator
feedback loop. The photo shown in
Figure 20
shows a
typical output ripple voltage, with and without a post ripple
filter.
When observing output ripple with a scope, it is essential
that a short, low inductance scope probe ground connection
be used. Most scope probe manufacturers provide a special
probe terminator which is soldered onto the regulator board,
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, the diode, and the parasitic inductance of
the output filter capacitor, and its associated wiring. To mini-
mize 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 evaluate
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
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-
DS012440-39
FIGURE 20. Post Ripple Filter Waveform
DS012440-40
FIGURE 21. Peak-to-Peak Inductor
Ripple Current vs Load Current
LM2597/LM2597HV
www.national.com25
Application Information (Continued)
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.
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-
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 3
through
Figure 6
are used to
select an inductor value, the peak-to-peak inductor ripple
current can immediately be determined. The curve shown in
Figure 21
shows the range of (∆I
IND
) that can be expected
for different 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
select the inductor value
Consider the following example:
V
OUT
= 5V, maximum load current of 300 mA
V
IN
= 15V, nominal, varying between 11V and 20V.
The selection guide in
Figure 4
shows that the vertical line
for a 0.3A load current, and the horizontal line for the 15V
input voltage intersect approximately midway between the
upper and lower borders of the 150 µH inductance region. A
150 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 21
, follow the 0.3A 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 150 mA p-p).
As the input voltage increases to 20V, it approaches the
upper border of the inductance region, and the inductor
ripple current increases. Referring to the curve in
Figure 21
,
it can be seen that for a load current of 0.3A, the
peak-to-peak inductor ripple current (∆I
IND
) is 150 mA with
15V in, and can range from 175 mAat the upper border (20V
in) to 120 mA at the lower border (11V 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
)
= 0.150Ax0.240Ω=36mVp-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
respect to other PC traces may be the problem. To deter-
mine if this is the problem, temporarily raise the inductor
away from the board by several inches and then check
circuit operation. If the circuit now operates correctly, then
the magnetic flux from the open core inductor is causing the
problem. Substituting a closed core inductor such as a tor-
roid 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 nega-
tive 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
efficient inductor, and they are used by the millions in many
different applications.
LM2597/LM2597HV
www.national.com 26
Application Information (Continued) THERMAL CONSIDERATIONS
The LM2597/LM2597HV is available in two packages, an
8-pin through hole DIP (N) and an 8-pin surface mount SO-8
(M). Both packages are molded plastic with a copper lead
frame. When the package is soldered to the PC board, the
copper and the board are the heat sink for the LM2597 and
the other heat producing components.
For best thermal performance, wide copper traces should be
used. Pins should be soldered to generous amounts of
printed circuit board copper, (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
even double-sided or multilayer boards provide a better heat
path to the surrounding air. Unless power levels are small,
sockets are not recommended because of the added ther-
mal resistance it adds and the resultant higher junction
temperatures.
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect the junction temperature. Some of these fac-
tors include board size, shape, thickness, position, location,
and even board temperature. Other factors are, trace width,
printed circuit copper area, copper thickness, single- or
double-sided, multilayer board, and the amount of solder on
the board. The effectiveness of the PC board to dissipate
heat also depends on the size, quantity and spacing of other
components on the board. Furthermore, some of these com-
ponents 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
material and the DC resistance, it could either act as a heat
sink taking heat away from the board, or it could add heat to
the board.
The curves shown in
Figure 22
and
Figure 23
show the
LM2597 junction temperature rise above ambient tempera-
ture with a 500 mA load for various input and output volt-
ages. The Bias Supply pin was not used (left open) for these
curves. Connecting the Bias Supply pin to the output voltage
would reduce the junction temperature by approximately 5˚C
to 15˚C, depending on the input and output voltages, and the
load current. This data was taken with the circuit operating
as a buck switcher with all components mounted on a PC
board to simulate the junction temperature under actual
operating conditions. This curve is typical, and can be used
for a quick check on the maximum junction temperature for
various conditions, but keep in mind that there are many
factors that can affect the junction temperature.
BIAS SUPPLY FEATURE
The bias supply (V
BS
) pin allows the LM2597’s internal
circuitry to be powered from a power source, other than V
IN
,
typically the output voltage. This feature can increase effi-
ciency and lower junction temperatures under some operat-
ing conditions. The greatest increase in efficiency occur with
light load currents, high input voltage and low output voltage
(4V to 12V). See efficiency curves shown in
Figure 24
and
Figure 25
. The curves with solid lines are with the V
BS
pin
connected to the regulated output voltage, while the curves
with dashed lines are with the V
BS
pin open.
The bias supply pin requires a minimum of approximately
3.5V at room temperature (4V @−40˚C), and can be as high
as 30V, but there is little advantage of using the bias supply
feature with voltages greater than 15V or 20V. The current
required for the V
IN
pin is typically 4 mA.
DS012440-41
Circuit Data for Temperature Rise Curve (DIP-8)
Capacitors Through hole electrolytic
Inductor Through hole, Schott, 100 µH
Diode Through hole, 1A 40V, Schottky
PC board 4 square inches single sided 2 oz. copper
(0.0028")
FIGURE 22. Junction Temperature Rise, DIP-8
DS012440-42
Circuit Data for Temperature Rise Curve (Surface
Mount)
Capacitors Surface mount tantalum, molded “D” size
Inductor Surface mount, Coilcraft DO33, 100 µH
Diode Surface mount, 1A 40V, Schottky
PC board 4 square inches single sided 2 oz. copper
(0.0028")
FIGURE 23. Junction Temperature Rise, SO-8
LM2597/LM2597HV
www.national.com27
Application Information (Continued)
To use the bias supply feature with output voltages between
4V and 15V, wire the bias pin to the regulated output. Since
the V
BS
pin requires a minimum of 4V to operate, the 3.3V
part cannot be used this way. When the V
BS
pin is left open,
the intemal regulator circuitry is powered from the input
voltage.
SHUTDOWN /SOFT-START
The circuit shown in
Figure 28
is a standard buck regulator
with 24V in, 12V out, 100 mA load, and using a 0.068 µF
Soft-start capacitor. The photo in
Figure 26
and
Figure 27
show the effects of Soft-start on the output voltage, the input
current, with, and without a Soft-start capacitor.
Figure 26
also shows the error flag output going high when the output
voltage reaches 95% of the nominal output voltage. The
reduced input current required at startup is very evident
when comparing the two photos. The Soft-start feature re-
duces the startup current from 700 mAdown to 160 mA, and
delays and slows down the output voltage rise time.
This reduction in start up current is useful in situations where
the input power source is limited in the amount of current it
can deliver. In some applications Soft-start can be used to
replace undervoltage lockout or delayed startup functions.
If a very slow output voltage ramp is desired, the Soft-start
capacitor can be made much larger. Many seconds or even
minutes are possible.
If only the shutdown feature is needed, the Soft-start capaci-
tor can be eliminated.
DS012440-43
FIGURE 24. Effects of Bias Supply Feature on 5V
Regulator Efficiency
DS012440-45
FIGURE 25. Effects of Bias Supply Feature on 12V
Regulator Efficiency
DS012440-44
FIGURE 26. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
DS012440-46
FIGURE 27. Output Voltage, Input Current, at Start-Up,
WITHOUT Soft-start
LM2597/LM2597HV
www.national.com 28
Application Information (Continued)
lNVERTING REGULATOR
The circuit in
Figure 29
converts a positive input voltage to a
negative output voltage with a common ground. The circuit
operates by bootstrapping the regulators ground pin to the
negative output voltage, then grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it.
This example uses the LM2597-5 to generate a −5V output,
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 30
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. In this example, when
converting +20V to −5V, the regulator would see 25V be-
tween the input pin and ground pin. The LM2597 has a
maximum input voltage rating of 40V (60V for the
LM2597HV).
DS012440-47
FIGURE 28. Typical Circuit Using Shutdown /Soft-start and Error Flag Features
DS012440-48
FIGURE 29. Inverting −5V Regulator With Shutdown and Soft-start
LM2597/LM2597HV
www.national.com29
Application Information (Continued)
An additional diode is required in this regulator configuration.
Diode D1 is used to isolate input voltage ripple or noise from
coupling through the C
IN
capacitor to the output, under light
or no load conditions. Also, this diode isolation changes the
topology to closely 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 1N4001 diode could be
used.
Because of differences in the operation of the inverting
regulator, the standard design procedure is not used to
select the inductor value. In the majority of designs, a 100
µH, 1 Amp inductor is the best choice. Capacitor selection
can also be narrowed down to just a few values. Using the
values shown in
Figure 29
will provide good results in the
majority of inverting designs.
This type of inverting regulator can require relatively large
amounts of input current when starting up, even with light
loads. Input currents as high as the LM2597 current limit
(approximately 0.8A) are needed for 1 ms or more, until the
output reaches its nominal output voltage. The actual time
depends on the output voltage and the size of the output
capacitor. Input power sources that are current limited or
sources that can not deliver these currents without getting
loaded down, may not work correctly. Because of the rela-
tively high startup currents required by the inverting topology,
the Soft-start feature shown in
Figure 29
is recommended.
Also shown in
Figure 29
are several shutdown methods for
the inverting configuration. With the inverting configuration,
some level shifting is required, because the ground pin of the
regulator is no longer at ground, but is now at the negative
output voltage. The shutdown methods shown accept
ground referenced shutdown signals.
UNDERVOLTAGE LOCKOUT
Some applications require the regulator to remain off until
the input voltage reaches a predetermined voltage.
Figure
31
contains a undervoltage lockout circuit for a buck configu-
ration, while
Figure 32
and
Figure 33
are for the inverting
types (only the circuitry pertaining to the undervoltage lock-
out is shown).
Figure 31
uses a zener diode to establish the
threshold voltage when the switcher begins operating. When
the input voltage is less than the zener voltage, resistors R1
and R2 hold the Shutdown /Soft-start pin low, keeping the
regulator in the shutdown mode. As the input voltage ex-
ceeds the zener voltage, the zener conducts, pulling the
Shutdown /Soft-start pin high, allowing the regulator to begin
switching. The threshold voltage for the undervoltage lockout
feature is approximately 1.5V greater than the zener voltage.
Figure 32
and
Figure 33
apply the same feature to an
inverting circuit.
Figure 32
features a constant threshold
voltage for turn on and turn off (zener voltage plus approxi-
mately one volt). If hysteresis is needed, the circuit in
Figure
33
has a turn ON voltage which is different than the turn OFF
voltage. The amount of hysteresis is approximately equal to
the value of the output voltage. Since the SD /SS pin has an
internal 7V zener clamp, R2 is needed to limit the current into
this pin to approximately 1 mA when Q1 is on.
NEGATIVE VOLTAGE CHARGE PUMP
Occasionally a low current negative voltage is needed for
biasing parts of a circuit. A simple method of generating a
negative voltage using a charge pump technique and the
switching waveform present at the OUT pin, is shown in
Figure 34
. This unregulated negative voltage is approxi-
mately equal to the positive input voltage (minus a few volts),
and can supply up to a 100 mA of output current. There is a
requirement however, that there be a minimum load of sev-
DS012440-49
FIGURE 30. Maximum Load Current for Inverting
Regulator Circuit
DS012440-50
FIGURE 31. Undervoltage Lockout for a Buck
Regulator
DS012440-52
FIGURE 32. Undervoltage Lockout Without
Hysteresis for an Inverting Regulator
DS012440-53
FIGURE 33. Undervoltage Lockout With
Hysteresis for an Inverting Regulator
LM2597/LM2597HV
www.national.com 30
Application Information (Continued)
eral hundred mA on the regulated positive output for the
charge pump to work correctly. Also, resistor R1 is required
to limit the charging current of C1 to some value less than
the LM2597 current limit (typically 800 mA).
This method of generating a negative output voltage without
an additional inductor can be used with other members of
the Simple Switcher Family, using either the buck or boost
topology.
DS012440-51
FIGURE 34. Charge Pump for Generating a
Low Current, Negative Output Voltage
LM2597/LM2597HV
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Application Information (Continued)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, FIXED OUTPUT (2X size)
DS012440-54
CIN — 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
COUT — 100 µF, 10V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
D1 — 1A, 40V Surface Mount Schottky Rectifier
L1 — Surface Mount Inductor, Coilcraft DO33
CSS — Soft-start Capacitor (surface mount ceramic chip capacitor)
CD— Delay Capacitor (surface mount ceramic chip capacitor)
R3 — Error Flag Pullup Resistor (surface mount chip resistor)
TYPICAL SURFACE MOUNT PC BOARD LAYOUT, ADJUSTABLE OUTPUT (2X size)
DS012440-55
CIN — 10 µF, 35V, Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
COUT — 68 µF, 20V Solid Tantalum, AVX, “TPS Series” (surface mount, “D” size)
D1 — 1A, 40V Surface Mount Schottky Rectifier
L1 — Surface Mount Inductor, Coilcraft DO33
CSS — Soft-start Capacitor (surface mount ceramic chip capacitor)
CD— Delay Capacitor (surface mount ceramic chip capacitor)
CFF — Feedforward Capacitor (surface mount ceramic chip capacitor)
R1 — Output Voltage Program Resistor (surface mount chip resistor)
R2 — Output Voltage Program Resistor (surface mount chip resistor)
R3 — Error Flag Pullup Resistor (surface mount chip resistor)
FIGURE 35. 2X Printed Circuit Board Layout
LM2597/LM2597HV
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Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package,
Order Number LM2597M-3.3, LM2597M-5.0,
LM2597M-12 or LM2597M-ADJ
LM2597HVM-3.3, LM2597HVM-5.0,
LM2597HVM-12 or LM2597HVM-ADJ
NS Package Number M08A
LM2597/LM2597HV
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
LIFE SUPPORT POLICY
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
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com
National Semiconductor
Europe Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
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Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
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Response Group
Tel: 65-2544466
Fax: 65-2504466
Email: ap.support@nsc.com
National Semiconductor
Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507
www.national.com
8-Lead (0.300" Wide) Molded Dual-In-Line Package,
Order Number LM2597N-3.3, LM2597N-5.0, LM2597N-12 or LM2597N-ADJ
LM2597HVN-3.3, LM2597HVN-5.0, LM2597HVN-12 or LM2597HVN-ADJ
NS Package Number N08E
LM2597/LM2597HV SIMPLE SWITCHER Power Converter 150 kHz 0.5A Step-Down Voltage
Regulator, with Features
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.
