LM2576/LM2576HV Series
SIMPLE SWITCHER

3A Step-Down Voltage Regulator

General Description

The LM2576 series of regulators are monolithic integrated
circuits that provide all the active functions for a step-down
(buck) switching regulator, capable of driving 3A load with
excellent line and load regulation. These devices are avail-
able in fixed output voltages of 3.3V, 5V, 12V, 15V, and an
adjustable 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 LM2576 series offers a high-efficiency replacement for
popular three-terminal linear regulators. It substantially re-
duces the size of the heat sink, and in some cases no heat
sink is required.

A standard series of inductors optimized for use with the
LM2576 are available from several different manufacturers.
This feature greatly simplifies the design of switch-mode
power supplies.

Other features include a guaranteed

4% tolerance on out-

put voltage within specified input voltages and output load
conditions, and

10% on the oscillator frequency. External

shutdown is included, featuring 50 µA (typical) standby cur-
rent. The output switch includes cycle-by-cycle current limit-
ing, as well as thermal shutdown for full protection under
fault conditions.

Features

3.3V, 5V, 12V, 15V, and adjustable output versions

Adjustable version output voltage range,

1.23V to 37V (57V for HV version)

4% max over

line and load conditions

Guaranteed 3A output current

Wide input voltage range, 40V up to 60V for

HV version

Requires only 4 external components

52 kHz fixed frequency internal oscillator

TTL shutdown capability, low power standby mode

High efficiency

Uses readily available standard inductors

Thermal shutdown and current limit protection

P+ Product Enhancement tested

Applications

Simple high-efficiency step-down (buck) regulator

Efficient pre-regulator for linear regulators

On-card switching regulators

Positive to negative converter (Buck-Boost)

Typical Application

(Fixed Output Voltage Versions)

SIMPLE SWITCHER

is a registered trademark of National Semiconductor Corporation.

DS011476-1

FIGURE 1.

June 1999

LM2576/LM2576HV

Series

SIMPLE

SWITCHER

Step-Down

oltage

Regulator

DS011476

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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

LM2576

45V

LM2576HV

63V

ON /OFF Pin Input Voltage

-0.3V

Output Voltage to Ground

(Steady State)

-1V

Power Dissipation

Internally Limited

Storage Temperature Range

-65°C to +150°C

Maximum Junction Temperature

150°C

Minimum ESD Rating

(C = 100 pF, R = 1.5 k

)

2 kV

Lead Temperature

(Soldering, 10 Seconds)

260°C

Operating Ratings

Temperature Range

LM2576/LM2576HV

-40°C

+125°C

Supply Voltage

LM2576

40V

LM2576HV

60V

LM2576-3.3, LM2576HV-3.3
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range.

Symbol

Parameter

Conditions

LM2576-3.3

Units

(Limits)

LM2576HV-3.3

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

OUT

Output Voltage

= 12V, I

LOAD

= 0.5A

3.3

Circuit of

Figure 2

3.234

V(Min)

3.366

V(Max)

OUT

Output Voltage

40V, 0.5A

LOAD

3.3

LM2576

Circuit of

Figure 2

3.168/3.135

V(Min)

3.432/3.465

V(Max)

OUT

Output Voltage

60V, 0.5A

LOAD

3.3

LM2576HV

Circuit of

Figure 2

3.168/3.135

V(Min)

3.450/3.482

V(Max)

Efficiency

= 12V, I

LOAD

= 3A

LM2576-5.0, LM2576HV-5.0
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those with Figure 2 boldface type apply over full Operating

Temperature Range.

Symbol

Parameter

Conditions

LM2576-5.0

Units

(Limits)

LM2576HV-5.0

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

OUT

Output Voltage

= 12V, I

LOAD

= 0.5A

5.0

Circuit of

Figure 2

4.900

V(Min)

5.100

V(Max)

OUT

Output Voltage

0.5A

LOAD

3A,

5.0

LM2576

40V

4.800/4.750

V(Min)

Circuit of

Figure 2

5.200/5.250

V(Max)

OUT

Output Voltage

0.5A

LOAD

3A,

5.0

LM2576HV

60V

4.800/4.750

V(Min)

Circuit of

Figure 2

5.225/5.275

V(Max)

Efficiency

= 12V, I

LOAD

= 3A

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LM2576-12, LM2576HV-12
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range.

Symbol

Parameter

Conditions

LM2576-12

Units

(Limits)

LM2576HV-12

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

OUT

Output Voltage

= 25V, I

LOAD

= 0.5A

Circuit of

Figure 2

11.76

V(Min)

12.24

V(Max)

OUT

Output Voltage

0.5A

LOAD

3A,

LM2576

15V

40V

11.52/11.40

V(Min)

Circuit of

Figure 2

12.48/12.60

V(Max)

OUT

Output Voltage

0.5A

LOAD

3A,

LM2576HV

15V

60V

11.52/11.40

V(Min)

Circuit of

Figure 2

12.54/12.66

V(Max)

Efficiency

= 15V, I

LOAD

= 3A

LM2576-15, LM2576HV-15
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range.

Symbol

Parameter

Conditions

LM2576-15

Units

(Limits)

LM2576HV-15

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

OUT

Output Voltage

= 25V, I

LOAD

= 0.5A

Circuit of

Figure 2

14.70

V(Min)

15.30

V(Max)

OUT

Output Voltage

0.5A

LOAD

3A,

LM2576

18V

40V

14.40/14.25

V(Min)

Circuit of

Figure 2

15.60/15.75

V(Max)

OUT

Output Voltage

0.5A

LOAD

3A,

LM2576HV

18V

60V

14.40/14.25

V(Min)

Circuit of

Figure 2

15.68/15.83

V(Max)

Efficiency

= 18V, I

LOAD

= 3A

LM2576-ADJ, LM2576HV-ADJ
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range.

Symbol

Parameter

Conditions

LM2576-ADJ

Units

(Limits)

LM2576HV-ADJ

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

OUT

Feedback Voltage

= 12V, I

LOAD

= 0.5A

1.230

OUT

= 5V,

1.217

V(Min)

Circuit of

Figure 2

1.243

V(Max)

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LM2576-ADJ, LM2576HV-ADJ
Electrical Characteristics

(Continued)

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range.

Symbol

Parameter

Conditions

LM2576-ADJ

Units

(Limits)

LM2576HV-ADJ

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

OUT

Feedback Voltage

0.5A

LOAD

3A,

1.230

LM2576

40V

1.193/1.180

V(Min)

OUT

= 5V, Circuit of Figure 2

1.267/1.280

V(Max)

OUT

Feedback Voltage

0.5A

LOAD

3A,

1.230

LM2576HV

60V

1.193/1.180

V(Min)

OUT

= 5V, Circuit of Figure 2

1.273/1.286

V(Max)

Efficiency

= 12V, I

LOAD

= 3A, V

OUT

= 5V

All Output Voltage Versions
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range. Unless otherwise specified, V

= 12V for the 3.3V, 5V, and Adjustable version, V

= 25V for the 12V version, and

= 30V for the 15V version. I

LOAD

= 500 mA.

Symbol

Parameter

Conditions

LM2576-XX

Units

(Limits)

LM2576HV-XX

Typ

Limit

(Note 2)

DEVICE PARAMETERS

Feedback Bias Current

OUT

= 5V (Adjustable Version Only)

100/500

Oscillator Frequency

(Note 11)

kHz

47/42

kHz

(Min)

58/63

kHz

(Max)

SAT

Saturation Voltage

OUT

= 3A (Note 4)

1.4

1.8/2.0

V(Max)

Max Duty Cycle (ON)

(Note 5)

%(Min)

Current Limit

(Notes 4, 11)

5.8

4.2/3.5

A(Min)

6.9/7.5

A(Max)

Output Leakage Current

(Notes 6, 7):

Output = 0V

mA(Max)

Output = -1V

7.5

Output = -1V

mA(Max)

Quiescent Current

(Note 6)

mA(Max)

STBY

Standby Quiescent

ON /OFF Pin = 5V (OFF)

µA

Current

200

µA(Max)

Thermal Resistance

T Package, Junction to Ambient (Note 8)

T Package, Junction to Ambient (Note 9)

°C/W

T Package, Junction to Case

S Package, Junction to Ambient (Note 10)

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All Output Voltage Versions
Electrical Characteristics

(Continued)

Specifications with standard type face are for T

= 25°C, and those with boldface type apply over full Operating Temperature

Range. Unless otherwise specified, V

= 12V for the 3.3V, 5V, and Adjustable version, V

= 25V for the 12V version, and

= 30V for the 15V version. I

LOAD

= 500 mA.

Symbol

Parameter

Conditions

LM2576-XX

Units

(Limits)

LM2576HV-XX

Typ

Limit

(Note 2)

ON /OFF CONTROL Test Circuit

Figure 2

ON /OFF Pin

OUT

= 0V

1.4

2.2/2.4

V(Min)

Logic Input Level

OUT

= Nominal Output Voltage

1.2

1.0/0.8

V(Max)

ON /OFF Pin Input

ON /OFF Pin = 5V (OFF)

µA

Current

µA(Max)

ON /OFF Pin = 0V (ON)

µA

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: 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.

Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2576/
LM2576HV is used as shown in the

Figure 2 test circuit, system performance will be as shown in system parameters section of Electrical Characteristics.

Note 4: Output pin sourcing current. No diode, inductor or capacitor connected to output.

Note 5: Feedback pin removed from output and connected to 0V.

Note 6: Feedback pin removed from output and connected to +12V for the Adjustable, 3.3V, and 5V versions, and +25V for the 12V and 15V versions, to force the
output transistor OFF.

Note 7: V

= 40V (60V for high voltage version).

Note 8: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with

inch leads in a socket, or on a PC

board with minimum copper area.

Note 9: Junction to ambient thermal resistance (no external heat sink) for the 5 lead TO-220 package mounted vertically, with

inch leads soldered to a PC board

containing approximately 4 square inches of copper area surrounding the leads.

Note 10: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using
0.5 square inches of copper area,

is 50°C/W, with 1 square inch of copper area,

is 37°C/W, and with 1.6 or more square inches of copper area,

is 32°C/W.

Note 11: The oscillator frequency reduces to approximately 11 kHz in the event of an output short or an overload which causes the regulated output voltage to drop
approximately 40% from the nominal output voltage. This self protection feature lowers the average power dissipation of the IC by lowering the minimum duty cycle
from 5% down to approximately 2%.

Typical Performance Characteristics

(Circuit of

Figure 2)

Normalized Output Voltage

DS011476-27

Line Regulation

DS011476-28

Dropout Voltage

DS011476-29

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LM2576 Series Buck Regulator Design Procedure

PROCEDURE (Fixed Output Voltage Versions)

EXAMPLE (Fixed Output Voltage Versions)

Given:

OUT

= Regulated Output Voltage

(3.3V, 5V, 12V, or 15V)

(Max) = Maximum Input Voltage

LOAD

(Max) = Maximum Load Current

Given:

OUT

= 5V

(Max) = 15V

LOAD

(Max) = 3A

1. Inductor Selection (L1)

A. Select the correct Inductor value selection guide from
Figures 3, 4, 5 or Figure 6. (Output voltages of 3.3V, 5V,
12V or 15V respectively). For other output voltages, see
the design procedure for the adjustable version.

B. From the inductor value selection guide, identify the in-
ductance region intersected by V

(Max) and I

LOAD

(Max),

and note the inductor code for that region.

C. Identify the inductor value from the inductor code, and
select an appropriate inductor from the table shown in
Figure 3. Part numbers are listed for three inductor manu-
facturers. The inductor chosen must be rated for opera-
tion at the LM2576 switching frequency (52 kHz) and for a
current rating of 1.15 x I

LOAD

. For additional inductor in-

formation, see the inductor section in the Application
Hints section of this data sheet.

1. Inductor Selection (L1)

A. Use the selection guide shown in

Figure 4.

B. From the selection guide, the inductance area inter-
sected by the 15V line and 3A line is L100.

C. Inductor value required is 100 µH. From the table in
Figure 3. Choose AIE 415-0930, Pulse Engineering
PE92108, or Renco RL2444.

2. Output Capacitor Selection (C

OUT

)

A. The value of the output capacitor together with the in-
ductor defines the dominate pole-pair of the switching
regulator loop. For stable operation and an acceptable
output ripple voltage, (approximately 1% of the output
voltage) a value between 100 µF and 470 µF is recom-
mended.

B. The capacitor's voltage rating should be at least 1.5
times greater than the output voltage. For a 5V regulator,
a rating of at least 8V is appropriate, and a 10V or 15V
rating is recommended.

Higher voltage electrolytic capacitors generally have
lower ESR numbers, and for this reason it may be neces-
sary to select a capacitor rated for a higher voltage than
would normally be needed.

2. Output Capacitor Selection (C

OUT

)

A. C

OUT

= 680 µF to 2000 µF standard aluminum electro-

lytic.

B.Capacitor voltage rating = 20V.

3. Catch Diode Selection (D1)

A.The catch-diode current rating must be at least 1.2
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 LM2576. 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.

3. Catch Diode Selection (D1)

A.For this example, a 3A current rating is adequate.

B. Use a 20V 1N5823 or SR302 Schottky diode, or any of
the suggested fast-recovery diodes shown in

Figure 8.

4. Input Capacitor (C

)

An aluminum or tantalum electrolytic bypass capacitor lo-
cated close to the regulator is needed for stable opera-
tion.

4. Input Capacitor (C

)

A 100 µF, 25V aluminum electrolytic capacitor located
near the input and ground pins provides sufficient
bypassing.

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LM2576 Series Buck Regulator Design Procedure

(Continued)

PROCEDURE (Adjustable Output Voltage Versions)

EXAMPLE (Adjustable Output Voltage Versions)

2. Inductor Selection (L1)

A. Calculate the inductor Volt

· microsecond constant,

µs),

from

the

following

formula:

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 cur-
rent.

D. Identify the inductance region intersected by the E

· T

value and the maximum load current value, and note the
inductor code for that region.

E. Identify the inductor value from the inductor code, and
select an appropriate inductor from the table shown in
Figure 9. Part numbers are listed for three inductor manu-
facturers. The inductor chosen must be rated for opera-
tion at the LM2576 switching frequency (52 kHz) and for a
current rating of 1.15 x I

LOAD

. For additional inductor in-

formation, see the inductor section in the application hints
section of this data sheet.

2. Inductor Selection (L1)

A. Calculate E

· T (V · µs)

B. E

· T = 115 V · µs

C. I

LOAD

(Max) = 3A

D. Inductance Region = H150

E. Inductor Value = 150 µH

Choose from AIE part

#415-0936 Pulse Engineering part #PE-531115, or
Renco part #RL2445.

3. Output Capacitor Selection (C

OUT

)

A. The value of the output capacitor together with the in-
ductor defines the dominate pole-pair of the switching
regulator loop. For stable operation, the capacitor must
satisfy the following requirement:

The above formula yields capacitor values between 10 µF
and 2200 µF that will satisfy the loop requirements for
stable operation. But to achieve an acceptable output
ripple voltage, (approximately 1% of the output voltage)
and transient response, the output capacitor may need to
be several times larger than the above formula yields.

B. The capacitor's voltage rating should be at last 1.5
times greater than the output voltage. For a 10V regulator,
a rating of at least 15V or more is recommended. Higher
voltage electrolytic capacitors generally have lower ESR
numbers, and for this reason it may be necessary to se-
lect a capacitor rate for a higher voltage than would nor-
mally be needed.

3. Output Capacitor Selection (C

OUT

)

However, for acceptable output ripple voltage select

OUT

680 µF

OUT

= 680 µF electrolytic capacitor

4. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.2
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 LM2576. The most stressful
condition for this diode is an overload or shorted output.
See diode selection guide in

Figure 8.

B. The reverse voltage rating of the diode should be at
least 1.25 times the maximum input voltage.

4. Catch Diode Selection (D1)

A. For this example, a 3.3A current rating is adequate.

B. Use a 30V 31DQ03 Schottky diode, or any of the sug-
gested fast-recovery diodes in

Figure 8.

5. Input Capacitor (C

)

An aluminum or tantalum electrolytic bypass capacitor lo-
cated close to the regulator is needed for stable opera-
tion.

5. Input Capacitor (C

)

A 100 µF aluminum electrolytic capacitor located near the
input and ground pins provides sufficient bypassing.

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LM2576 Series Buck Regulator Design Procedure

(Continued)

To further simplify the buck regulator design procedure, National Semiconductor is making available computer design software to
be used with the SIMPLE SWITCHER line of switching regulators. Switchers Made Simple (Version 3.3) is available on a (3

diskette for IBM compatible computers from a National Semiconductor sales office in your area.

Schottky

Fast Recovery

4A6A

20V

1N5820

1N5823

The following

diodes are all
rated to 100V

31DF1

HER302

The following

diodes are all
rated to 100V

50WF10

MUR410

HER602

MBR320P

SR302

30V

1N5821

50WQ03

MBR330

1N5824

31DQ03

SR303

40V

1N5822

MBR340

50WQ04

31DQ04

1N5825

SR304

50V

MBR350

50WQ05

31DQ05

SR305

60V

MBR360

50WR06

DQ06

50SQ060

SR306

FIGURE 8. Diode Selection Guide

Inductor

Schott

Pulse Eng.

Renco

Code

Value

(Note 12)

(Note 13)

(Note 14)

L47

47 µH

671 26980

PE-53112

RL2442

L68

68 µH

671 26990

PE-92114

RL2443

L100

100 µH

671 27000

PE-92108

RL2444

L150

150 µH

671 27010

PE-53113

RL1954

L220

220 µH

671 27020

PE-52626

RL1953

L330

330 µH

671 27030

PE-52627

RL1952

L470

470 µH

671 27040

PE-53114

RL1951

L680

680 µH

671 27050

PE-52629

RL1950

H150

150 µH

671 27060

PE-53115

RL2445

H220

220 µH

671 27070

PE-53116

RL2446

H330

330 µH

671 27080

PE-53117

RL2447

H470

470 µH

671 27090

PE-53118

RL1961

H680

680 µH

671 27100

PE-53119

RL1960

H1000

1000 µH

671 27110

PE-53120

RL1959

H1500

1500 µH

671 27120

PE-53121

RL1958

H2200

2200 µH

671 27130

PE-53122

RL2448

Note 12: Schott Corporation, (612) 475-1173, 1000 Parkers Lake Road, Wayzata, MN 55391.

Note 13: Pulse Engineering, (619) 674-8100, P.O. Box 12235, San Diego, CA 92112.

Note 14: Renco Electronics Incorporated, (516) 586-5566, 60 Jeffryn Blvd. East, Deer Park, NY 11729.

FIGURE 9. Inductor Selection by Manufacturer's Part Number

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Application Hints

INPUT CAPACITOR (C

)

To maintain stability, the regulator input pin must be by-
passed with at least a 100 µF electrolytic capacitor. The ca-
pacitor's leads must be kept short, and located near the
regulator.

If the operating temperature range includes temperatures
below -25°C, the input capacitor value may need to be
larger. With most electrolytic capacitors, the capacitance
value decreases and the ESR increases with lower tempera-
tures and age. Paralleling a ceramic or solid tantalum ca-
pacitor will increase the regulator stability at cold tempera-
tures. For maximum capacitor operating lifetime, the
capacitor's RMS ripple current rating should be greater than

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 regulator per-
formance and requirements.

The LM2576 (or any of the SIMPLE SWITCHER family) can
be used for both continuous and discontinuous modes of op-
eration.

The inductor value selection guides in

Figure 3 through Fig-

ure 7 were designed for buck regulator designs of the con-
tinuous inductor current type. When using inductor values
shown in the inductor selection guide, the peak-to-peak in-
ductor ripple current will be approximately 20% to 30% of the
maximum DC current. With relatively heavy load currents,
the circuit operates in the continuous mode (inductor current
always flowing), but under light load conditions, the circuit
will be forced to the discontinuous mode (inductor current
falls to zero for a period of time). This discontinuous mode of
operation is perfectly acceptable. For light loads (less than
approximately 300 mA) it may be desirable to operate the
regulator in the discontinuous mode, primarily because of
the lower inductor values required for the discontinuous
mode.

The selection guide chooses inductor values suitable for
continuous mode operation, but if the inductor value chosen
is prohibitively high, the designer should investigate the pos-
sibility of discontinuous operation. The computer design soft-
ware

Switchers Made Simple will provide all component

values for discontinuous (as well as continuous) mode of op-
eration.

Inductors are available in different styles such as pot core,
toriod, E-frame, bobbin core, etc., as well as different core
materials, such as ferrites and powdered iron. The least ex-
pensive, the bobbin core type, consists of wire wrapped on a
ferrite rod core. 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 (EMI). This EMI can cause problems
in sensitive circuits, or can give incorrect scope readings be-
cause of induced voltages in the scope probe.

The inductors listed in the selection chart include ferrite pot
core construction for AIE, powdered iron toroid for Pulse En-
gineering, and ferrite bobbin core for Renco.

An inductor should not be operated beyond its maximum
rated current because it may saturate. When an inductor be-
gins to saturate, the inductance decreases rapidly and the
inductor begins to look mainly resistive (the DC resistance of
the winding). This will cause the switch current to rise very
rapidly. Different inductor types have different saturation
characteristics, and this should be kept in mind when select-
ing an inductor.

The inductor manufacturer's data sheets include current and
energy limits to avoid inductor saturation.

INDUCTOR RIPPLE CURRENT

When the switcher is operating in the continuous mode, the
inductor current waveform ranges from a triangular to a saw-
tooth type of waveform (depending on the input voltage). For
a given input voltage and output voltage, the peak-to-peak
amplitude of this inductor current waveform remains con-
stant. As the load current rises or falls, the entire sawtooth
current waveform also rises or falls. The average DC value
of this waveform is equal to the DC load current (in the buck
regulator configuration).

If the load current drops to a low enough level, the bottom of
the sawtooth current waveform will reach zero, and the
switcher will change to a discontinuous mode of operation.
This is a perfectly acceptable mode of operation. Any buck
switching regulator (no matter how large the inductor value
is) will be forced to run discontinuous if the load current is
light enough.

OUTPUT CAPACITOR

An output capacitor is required to filter the output voltage and
is needed for loop stability. The capacitor should be located
near the LM2576 using short pc board traces. Standard alu-
minum electrolytics are usually adequate, but low ESR types
are recommended for low output ripple voltage and good
stability. The ESR of a capacitor depends on many factors,
some which are: the value, the voltage rating, physical size
and the type of construction. In general, low value or low
voltage (less than 12V) electrolytic capacitors usually have
higher ESR numbers.

The amount of output ripple voltage is primarily a function of
the ESR (Equivalent Series Resistance) of the output ca-
pacitor and the amplitude of the inductor ripple current
(

IND

). See the section on inductor ripple current in Applica-

tion Hints.

The lower capacitor values (220 µF1000 µF) will allow typi-
cally 50 mV to 150 mV of output ripple voltage, while
larger-value capacitors will reduce the ripple to approxi-
mately 20 mV to 50 mV.

Output Ripple Voltage = (

IND

) (ESR of C

OUT

)

To further reduce the output ripple voltage, several standard
electrolytic capacitors may be paralleled, or a higher-grade
capacitor may be used. Such capacitors are often called
"high-frequency," "low-inductance," or "low-ESR." These will
reduce the output ripple to 10 mV or 20 mV. However, when
operating in the continuous mode, reducing the ESR below
0.03

can cause instability in the regulator.

www.national.com

Application Hints

(Continued)

Tantalum capacitors can have a very low ESR, and should
be carefully evaluated if it is the only output capacitor. Be-
cause of their good low temperature characteristics, a tanta-
lum can be used in parallel with aluminum electrolytics, with
the tantalum making up 10% or 20% of the total capacitance.

The capacitor's ripple current rating at 52 kHz should be at
least 50% higher than the peak-to-peak inductor ripple cur-
rent.

CATCH DIODE

Buck regulators require a diode to provide a return path for
the inductor current when the switch is off. This diode should
be located close to the LM2576 using short leads and short
printed circuit traces.

Because of their fast switching speed and low forward volt-
age drop, Schottky diodes provide the best efficiency, espe-
cially in low output voltage switching regulators (less than
5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery
diodes are also suitable, but some types with an abrupt
turn-off characteristic may cause instability and EMI prob-
lems. A fast-recovery diode with soft recovery characteristics
is a better choice. Standard 60 Hz diodes (e.g., 1N4001 or
1N5400, etc.) are also not suitable. See

Figure 8 for Schot-

tky and "soft" fast-recovery diode selection guide.

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply will contain a
sawtooth ripple voltage at the switcher frequency, typically
about 1% of the output voltage, and may also contain short
voltage spikes at the peaks of the sawtooth waveform.

The output ripple voltage is due mainly to the inductor saw-
tooth ripple current multiplied by the ESR of the output ca-
pacitor. (See the inductor selection in the application hints.)

The voltage spikes are present because of the the fast
switching action of the output switch, and the parasitic induc-
tance of the output filter capacitor. To minimize these voltage
spikes, special low inductance capacitors can be used, and
their lead lengths must be kept 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.

An additional small LC filter (20 µH & 100 µF) can be added
to the output (as shown in

Figure 15) to further reduce the

amount of output ripple and transients. A 10 x reduction in
output ripple voltage and transients is possible with this filter.

FEEDBACK CONNECTION

The LM2576 (fixed voltage versions) feedback pin must be
wired to the output voltage point of the switching power sup-
ply. When using the adjustable version, physically locate
both output voltage programming resistors near the LM2576
to avoid picking up unwanted noise. Avoid using resistors
greater than 100 k

because of the increased chance of

noise pickup.

ON /OFF INPUT

For normal operation, the ON /OFF pin should be grounded
or driven with a low-level TTL voltage (typically below 1.6V).
To put the regulator into standby mode, drive this pin with a
high-level TTL or CMOS signal. The ON /OFF pin can be
safely pulled up to +V

without a resistor in series with it.

The ON /OFF pin should not be left open.

GROUNDING

To maintain output voltage stability, the power ground con-
nections must be low-impedance (see

Figure 2). For the

5-lead TO-220 and TO-263 style package, both the tab and
pin 3 are ground and either connection may be used, as they
are both part of the same copper lead frame.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, only a small heat sink is required to keep the
LM2576 junction temperature within the allowed operating
range. For each application, to determine whether or not a
heat sink will be required, the following must be identified:

Maximum ambient temperature (in the application).

Maximum regulator power dissipation (in application).

Maximum allowed junction temperature (125°C for the
LM2576). For a safe, conservative design, a tempera-
ture approximately 15°C cooler than the maximum tem-
peratures should be selected.

LM2576 package thermal resistances

and

Total power dissipated by the LM2576 can be estimated as
follows:

= (V

)(I

) + (V

)(I

LOAD

)(V

SAT

)

where I

(quiescent current) and V

SAT

can be found in the

Characteristic Curves shown previously, V

is the applied

minimum input voltage, V

is the regulated output voltage,

and I

LOAD

is the load current. The dynamic losses during

turn-on and turn-off are negligible if a Schottky type catch di-
ode is used.

When no heat sink is used, the junction temperature rise can
be determined by the following:

= (P

) (

)

To arrive at the actual operating junction temperature, add
the junction temperature rise to the maximum ambient tem-
perature.

+ T

If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, then a heat sink is required.

When using a heat sink, the junction temperature rise can be
determined by the following:

= (P

) (

interface

Heat sink

)

The operating junction temperature will be:

= T

As above, if the actual operating junction temperature is
greater than the selected safe operating junction tempera-
ture, then a larger heat sink is required (one that has a lower
thermal resistance).

Included on the Switcher Made Simple design software is a
more precise (non-linear) thermal model that can be used to
determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulators junction temperature below the maximum operat-
ing temperature.

Additional Applications

INVERTING REGULATOR

Figure 10 shows a LM2576-12 in a buck-boost configuration
to generate a negative 12V output from a positive input volt-
age. This circuit bootstraps the regulator's ground pin to the

www.national.com

Additional Applications

(Continued)

negative output voltage, then by grounding the feedback pin,
the regulator senses the inverted output voltage and regu-
lates it to -12V.

For an input voltage of 12V or more, the maximum available
output current in this configuration is approximately 700 mA.
At lighter loads, the minimum input voltage required drops to
approximately 4.7V.

The switch currents in this buck-boost configuration are
higher than in the standard buck-mode design, thus lowering
the available output current. Also, the start-up input current
of the buck-boost converter is higher than the standard
buck-mode regulator, and this may overload an input power
source with a current limit less than 5A. Using a delayed
turn-on or an undervoltage lockout circuit (described in the
next section) would allow the input voltage to rise to a high
enough level before the switcher would be allowed to turn
on.

Because of the structural differences between the buck and
the buck-boost regulator topologies, the buck regulator de-
sign procedure section can not be used to to select the in-
ductor or the output capacitor. The recommended range of
inductor values for the buck-boost design is between 68 µH
and 220 µH, and the output capacitor values must be larger
than what is normally required for buck designs. Low input
voltages or high output currents require a large value output
capacitor (in the thousands of micro Farads).

The peak inductor current, which is the same as the peak
switch current, can be calculated from the following formula:

Where f

osc

= 52 kHz. Under normal continuous inductor cur-

rent operating conditions, the minimum V

represents the

worst case. Select an inductor that is rated for the peak cur-
rent anticipated.

Also, the maximum voltage appearing across the regulator is
the absolute sum of the input and output voltage. For a -12V
output, the maximum input voltage for the LM2576 is +28V,
or +48V for the LM2576HV.

The

Switchers Made Simple (version 3.0) design software

can be used to determine the feasibility of regulator designs
using different topologies, different input-output parameters,
different components, etc.

NEGATIVE BOOST REGULATOR

Another variation on the buck-boost topology is the negative
boost configuration. The circuit in

Figure 11 accepts an input

voltage ranging from -5V to -12V and provides a regulated
-12V output. Input voltages greater than -12V will cause the
output to rise above -12V, but will not damage the regulator.

Because of the boosting function of this type of regulator, the
switch current is relatively high, especially at low input volt-
ages. Output load current limitations are a result of the maxi-
mum current rating of the switch. Also, boost regulators can
not provide current limiting load protection in the event of a
shorted load, so some other means (such as a fuse) may be
necessary.

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off
until the input voltage reaches a certain threshold. An under-
voltage lockout circuit which accomplishes this task is shown
in

Figure 12, while Figure 13 shows the same circuit applied

to a buck-boost configuration. These circuits keep the regu-
lator off until the input voltage reaches a predetermined
level.

+ 2V

(Q1)

DS011476-14

FIGURE 10. Inverting Buck-Boost Develops -12V

DS011476-15

Typical Load Current
400 mA for V

= -5.2V

750 mA for V

= -7V

Note: Heat sink may be required.

FIGURE 11. Negative Boost

DS011476-16

Note: Complete circuit not shown.

FIGURE 12. Undervoltage Lockout for Buck Circuit

www.national.com

Additional Applications

(Continued)

DELAYED STARTUP

The ON /OFF pin can be used to provide a delayed startup
feature as shown in

Figure 14. With an input voltage of 20V

and for the part values shown, the circuit provides approxi-
mately 10 ms of delay time before the circuit begins switch-

ing. Increasing the RC time constant can provide longer de-
lay times. But excessively large RC time constants can
cause problems with input voltages that are high in 60 Hz or
120 Hz ripple, by coupling the ripple into the ON /OFF pin.

ADJUSTABLE OUTPUT, LOW-RIPPLE POWER SUPPLY

A 3A power supply that features an adjustable output voltage
is shown in

Figure 15. An additional L-C filter that reduces

the output ripple by a factor of 10 or more is included in this
circuit.

Definition of Terms

BUCK REGULATOR

A switching regulator topology in which a higher voltage is
converted to a lower voltage. Also known as a step-down
switching regulator.

BUCK-BOOST REGULATOR

A switching regulator topology in which a positive voltage is
converted to a negative voltage without a transformer.

DUTY CYCLE (D)

Ratio of the output switch's on-time to the oscillator period.

CATCH DIODE OR CURRENT STEERING DIODE

The diode which provides a return path for the load current
when the LM2576 switch is OFF.

EFFICIENCY (

)

The proportion of input power actually delivered to the load.

CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)

The purely resistive component of a real capacitor's imped-
ance (see

Figure 16). It causes power loss resulting in ca-

pacitor heating, which directly affects the capacitor's operat-
ing lifetime. When used as a switching regulator output filter,
higher ESR values result in higher output ripple voltages.

DS011476-17

Note: Complete circuit not shown (see

Figure 10).

FIGURE 13. Undervoltage Lockout

for Buck-Boost Circuit

DS011476-18

Note: Complete circuit not shown.

FIGURE 14. Delayed Startup

DS011476-19

FIGURE 15. 1.2V to 55V Adjustable 3A Power Supply with Low Output Ripple

DS011476-20

FIGURE 16. Simple Model of a Real Capacitor

www.national.com

Definition of Terms

(Continued)

Most standard aluminum electrolytic capacitors in the
100

µF1000

µF

range

have

0.5

0.1

ESR.

Higher-grade capacitors ("low-ESR", "high-frequency", or
"low-inductance") in the 100 µF1000 µF range generally
have ESR of less than 0.15

EQUIVALENT SERIES INDUCTANCE (ESL)

The pure inductance component of a capacitor (see

Figure

16). The amount of inductance is determined to a large ex-
tent on the capacitor's construction. In a buck regulator, this
unwanted inductance causes voltage spikes to appear on
the output.

OUTPUT RIPPLE VOLTAGE

The AC component of the switching regulator's output volt-
age. It is usually dominated by the output capacitor's ESR
multiplied by the inductor's ripple current (

IND

). The

peak-to-peak value of this sawtooth ripple current can be de-
termined by reading the Inductor Ripple Current section of
the Application hints.

CAPACITOR RIPPLE CURRENT

RMS value of the maximum allowable alternating current at
which a capacitor can be operated continuously at a speci-
fied temperature.

STANDBY QUIESCENT CURRENT (I

STBY

)

Supply current required by the LM2576 when in the standby
mode (ON /OFF pin is driven to TTL-high voltage, thus turn-
ing the output switch OFF).

INDUCTOR RIPPLE CURRENT (

IND

)

The peak-to-peak value of the inductor current waveform,
typically a sawtooth waveform when the regulator is operat-
ing in the continuous mode (vs. discontinuous mode).

CONTINUOUS/DISCONTINUOUS MODE OPERATION

Relates to the inductor current. In the continuous mode, the
inductor current is always flowing and never drops to zero,
vs. the discontinuous mode, where the inductor current
drops to zero for a period of time in the normal switching
cycle.

INDUCTOR SATURATION

The condition which exists when an inductor cannot hold any
more magnetic flux. When an inductor saturates, the induc-
tor appears less inductive and the resistive component domi-
nates. Inductor current is then limited only by the DC resis-
tance of the wire and the available source current.

OPERATING VOLT MICROSECOND CONSTANT (E

·T

)

The product (in VoIt

·µs) of the voltage applied to the inductor

and the time the voltage is applied. This E

·T

constant is a

measure of the energy handling capability of an inductor and
is dependent upon the type of core, the core area, the num-
ber of turns, and the duty cycle.

Connection Diagrams

(Note 15)

Note 15: (XX indicates output voltage option. See ordering information table
for complete part number.)

Straight Leads

5-Lead TO-220 (T)

Top View

DS011476-21

LM2576T-XX or LM2576HVT-XX

NS Package Number T05A

TO-263 (S)

5-Lead Surface-Mount Package

Top View

DS011476-25

LM2576S-XX or LM2576HVS-XX

NS Package Number TS5B

LM2576SX-XX or LM2576HVSX-XX

NS Package Number TS5B, Tape and Reel

Bent, Staggered Leads

5-Lead TO-220 (T)

Top View

DS011476-22

LM2576T-XX Flow LB03

or LM2576HVT-XX Flow LB03

NS Package Number T05D

www.national.com

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) 1 80-530 85 86

Email: europe.support@nsc.com

Deutsch Tel: +49 (0) 1 80-530 85 85
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Tel: +49 (0) 1 80-532 78 32

Français Tel: +49 (0) 1 80-532 93 58
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Tel: +49 (0) 1 80-534 16 80

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Tel: 65-2544466
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Email: sea.support@nsc.com

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Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507

www.national.com

5-Lead TO-263 (S)

Order Number LM2576S-3.3, LM2576S-5.0,

LM2576S-12,LM2576S-15, LM2576S-ADJ,

LM2576HVS-3.3, LM2576HVS-5.0, LM2576HVS-12,

LM2576HVS-15, or LM2576HVS-ADJ

NS Package Number TS5B

5-Lead TO-263 in Tape & Reel (SX)

Order Number LM2576SX-3.3, LM2576SX-5.0,

LM2576SX-12, LM2576SX-15, LM2576SX-ADJ,

LM2576HVSX-3.3, LM2576HVSX-5.0, LM2576HVSX-12,

LM2576HVSX-15, or LM2576HVSX-ADJ

NS Package Number TS5B

LM2576/LM2576HV

Series

SIMPLE

SWITCHER

Step-Down

oltage

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.

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