Motorola LM2576TV, LM2576T Datasheet

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The LM2576 series of regulators are monolithic integrated circuits ideally
suited for easy and convenient design of a step–down switching regulator
(buck converter). All circuits of this series are capable of driving a 3.0 A load
with excellent line and load regulation. These devices are available in fixed
output voltages of 3.3 V, 5.0 V, 12 V, 15 V, and an adjustable output version.

These regulators were designed to minimize the number of external
components to simplify the power supply design. Standard series of
inductors optimized for use with the LM2576 are offered by several different
inductor manufacturers.

Since the LM2576 converter is a switch–mode power supply , its efficiency
is significantly higher in comparison with popular three–terminal linear
regulators, especially with higher input voltages. In many cases, the power
dissipated is so low that no heatsink is required or its size could be reduced
dramatically .

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.

The LM2576 features include a guaranteed ±4% tolerance on output
voltage within specified input voltages and output load conditions, and ±10%
on the oscillator frequency (±2% over 0°C to 125°C). External shutdown is
included, featuring 80 µA (typical) standby current. The output switch
includes cycle–by–cycle current limiting, as well as thermal shutdown for full
protection under fault conditions.

Features

• 3.3 V, 5.0 V, 12 V, 15 V, and Adjustable Output Versions

• Adjustable Version Output Voltage Range, 1.23 to 37 V ±4% Maximum

Over Line and Load Conditions

• Guaranteed 3.0 A Output Current

• Wide Input Voltage Range

• 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

Applications

• Simple High–Efficiency Step–Down (Buck) Regulator

• Efficient Pre–Regulator for Linear Regulators

• On–Card Switching Regulators

• Positive to Negative Converter (Buck–Boost)

• Negative Step–Up Converters

• Power Supply for Battery Chargers

EASY SWITCHER

3.0 A STEP–DOWN

VOLTAGE REGULATOR

SEMICONDUCTOR

TECHNICAL DATA

T SUFFIX

PLASTIC PACKAGE

CASE 314D

Pin 1. V

2. Output

3. Ground

4. Feedback

5. ON

TV SUFFIX

PLASTIC PACKAGE

CASE 314B

Heatsink surface

connected to Pin 3.

D2T SUFFIX

PLASTIC PACKAGE

CASE 936A

(D2PAK)

Heatsink surface (shown as terminal 6 in case outline

DEVICE TYPE/NOMINAL OUTPUT VOLTAGE

LM2576–3.3
LM2576–5
LM2576–12
LM2576–15
LM2576–ADJ

ORDERING INFORMATION

Device

LM2576T–XX
LM2576TV–XX TJ = –40° to +125°C
LM2576D2T–XX

XX = Voltage Option, i.e. 3.3, 5, 12, 15 V; and ADJ for
Adjustable Output.

1

in

5

/OFF

1

5

1

5

drawing) is connected to Pin 3.

3.3 V

5.0 V
12 V
15 V

1.23 V to 37 V

Operating

Temperature Range

Package

Straight Lead

Vertical Mount

Surface Mount

This document contains information on a new product. Specifications and information herein
are subject to change without notice.

MOTOROLA ANALOG IC DEVICE DATA

 Motorola, Inc. 1997 Rev 0

1

LM2576

Figure 1. Block Diagram and T ypical Application

T ypical Application (Fixed Output Voltage Versions)

Feedback

7.0 V – 40 V

Unregulated

DC Input

100

+V

in

LM2576

1

C

in

µ

F

3ON

Gnd

4
Output

2

/OFF5

Representative Block Diagram and T ypical Application

L1

100

D1
1N5822

µ

H

C

out

1000

µ

F

5.0 V Regulated
Output 3.0 A Load

+V

Unregulated

DC Input

C

in

in

1

4

Feedback

R2

Fixed Gain
Error Amplifier

R1

1.0 k
18 kHz

1.235 V
Band–Gap
Reference

Freq
Shift

3.1 V Internal
Regulator

Comparator

52 kHz

Oscillator

Current

Latch

Reset

ON

Limit

/OFF

Driver

Thermal

Shutdown

ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond

which damage to the device may occur.)

Rating

Maximum Supply Voltage V
ON/OFF Pin Input Voltage – –0.3 V ≤ V ≤ +V
Output Voltage to Ground (Steady–State) – –1.0 V
Power Dissipation

Case 314B and 314D (TO–220, 5–Lead) P

Thermal Resistance, Junction–to–Ambient R
Thermal Resistance, Junction–to–Case R

Case 936A (D2PAK) P

Thermal Resistance, Junction–to–Ambient R

Thermal Resistance, Junction–to–Case R
Storage Temperature Range T
Minimum ESD Rating (Human Body Model:

C = 100 pF, R = 1.5 kΩ)
Lead Temperature (Soldering, 10 seconds) – 260 °C
Maximum Junction Temperature T

NOTE: ESD data available upon request.

Symbol Value Unit

in

D

θJA
θJC

D

θJA
θJC

stg

– 2.0 kV

J

45 V

in

Internally Limited W

65 °C/W

5.0 °C/W

Internally Limited W

70 °C/W

5.0 °C/W

–65 to +150 °C

150 °C

1.0 Amp
Switch

V

/OFF

ON
5

Output
2

Gnd
3

Output

Voltage Versions

3.3 V

5.0 V
12 V
15 V

For adjustable version
R1 = open, R2 = 0

L1

D1

1.7 k

3.1 k

8.84 k

11.3 k

Ω

C

out

R2

Ω

)

(

Regulated

Output

V

out

Load

2

MOTOROLA ANALOG IC DEVICE DATA

LM2576

OPERATING RATINGS (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.)

Rating

Operating Junction Temperature Range T
Supply Voltage V

SYSTEM PARAMETERS ([Note 1] Test Circuit Figure 15)

ELECTRICAL CHARACTERISTICS

the 12 V version, and Vin = 30 V for the 15 V version. I
junction temperature range that applies [Note 2], unless otherwise noted.)

Characteristics

LM2576–3.3 ([Note 1] Test Circuit Figure 15)

Output Voltage (Vin = 12 V, I
Output Voltage (6.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ I

TJ = 25°C 3.168 3.3 3.432
TJ = –40 to +125°C 3.135 – 3.465

Efficiency (Vin = 12 V, I

LM2576–5 ([Note 1] Test Circuit Figure 15)

Output Voltage (Vin = 12 V, I
Output Voltage (8.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ I

TJ = 25°C 4.8 5.0 5.2
TJ = –40 to +125°C 4.75 – 5.25

Efficiency (Vin = 12 V, I

LM2576–12 ([Note 1] Test Circuit Figure 15)

Output Voltage (Vin = 25 V, I
Output Voltage (15 V ≤ Vin ≤ 40 V, 0.5 A ≤ I

TJ = 25°C 11.52 12 12.48
TJ = –40 to +125°C 11.4 – 12.6

Efficiency (Vin = 15 V, I

LM2576–15 ([Note 1] Test Circuit Figure 15)

Output Voltage (Vin = 30 V, I
Output Voltage (18 V ≤ Vin ≤ 40 V, 0.5 A ≤ I

TJ = 25°C 14.4 15 15.6
TJ = –40 to +125°C 14.25 – 15.75

Efficiency (Vin = 18 V, I

LM2576 ADJUSTABLE VERSION ([Note 1] Test Circuit Figure 15)

Feedback Voltage (Vin = 12 V, I

Feedback Voltage (8.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ I

TJ = 25°C 1.193 1.23 1.267
TJ = –40 to +125°C 1.18 – 1.28

Efficiency (Vin = 12 V, I

NOTES: 1. External components such as the catch diode, inductor, input and output capacitors can af fect switching regulator system performance. When the

LM2576 is used as shown in the

2.Tested junction temperature range for the LM2576: T

Load

= 3.0 A) η – 75 – %

Load

Load

= 3.0 A) η – 77 – %

Load

Load

= 3.0 A) η – 88 – %

Load

Load

= 3.0 A) η – 88 – %

Load

Load

= 3.0 A, V

Load

(Unless otherwise specified, Vin = 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V for

= 0.5 A, TJ = 25°C) V

Load

= 0.5 A, TJ = 25°C) V

Load

= 0.5 A, TJ = 25°C) V

Load

= 0.5 A, TJ = 25°C) V

Load

= 0.5 A, V

Load

= 5.0 V) η – 77 – %

out

Figure 15 test circuit, system performance will be as shown in system parameters section

Symbol Value Unit

J

in

= 500 mA. For typical values TJ = 25°C, for min/max values TJ is the operating

Load

≤ 3.0 A) V

≤ 3.0 A) V

≤ 3.0 A) V

≤ 3.0 A) V

= 5.0 V, TJ = 25°C) V

out

≤ 3.0 A, V

low

–40 to +125 °C

= 5.0 V) V

out

= –40°C T

40 V

Symbol Min Typ Max Unit

3.234 3.3 3.366 V

4.9 5.0 5.1 V

11.76 12 12.24 V

14.7 15 15.3 V

1.217 1.23 1.243 V

.

high

out
out

out
out

out
out

out
out

out
out

= +125°C

V

V

V

V

V

MOTOROLA ANALOG IC DEVICE DATA

3

LM2576

DEVICE PARAMETERS

ELECTRICAL CHARACTERISTICS (Unless otherwise specified, V

the 12 V version, and Vin = 30 V for the 15 V version. I
junction temperature range that applies [Note 2], unless otherwise noted.)

Characteristics

ALL OUTPUT VOLTAGE VERSIONS

Feedback Bias Current (V

TJ = 25°C – 25 100
TJ = –40 to +125°C – – 200

Oscillator Frequency [Note 3] f

TJ = 25°C – 52 –
TJ = 0 to +125°C 47 – 58
TJ = –40 to +125°C 42 – 63

Saturation Voltage (I

TJ = 25°C – 1.5 1.8
TJ = –40 to +125°C – – 2.0

Max Duty Cycle (“on”) [Note 5] DC 94 98 – %
Current Limit (Peak Current [Notes 3 and 4]) I

TJ = 25°C 4.2 5.8 6.9
TJ = –40 to +125°C 3.5 – 7.5

Output Leakage Current [Notes 6 and 7], TJ = 25°C I

Output = 0 V – 0.8 2.0
Output = –1.0 V – 6.0 20

Quiescent Current [Note 6] I

TJ = 25°C – 5.0 9.0
TJ = –40 to +125°C – – 11

Standby Quiescent Current (ON/OFF Pin = 5.0 V (“off”)) I

TJ = 25°C – 80 200
TJ = –40 to +125°C – – 400

ON/OFF Pin Logic Input Level (Test Circuit Figure 15) V

V

= 0 V V

out

TJ = 25°C 2.2 1.4 –
TJ = –40 to +125°C 2.4 – –

V

= Nominal Output Voltage V

out

TJ = 25°C – 1.2 1.0
TJ = –40 to +125°C – – 0.8

ON/OFF Pin Input Current (Test Circuit Figure 15) µA

ON/OFF Pin = 5.0 V (“off”), TJ = 25°C I
ON/OFF Pin = 0 V (“on”), TJ = 25°C I

NOTES: 3. The oscillator frequency reduces to approximately 18 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 dissipation of the IC by lowering the
minimum duty cycle from 5% down to approximately 2%.

4.Output (Pin 2) sourcing current. No diode, inductor or capacitor connected to output pin.

5.Feedback (Pin 4) removed from output and connected to 0 V.

6.Feedback (Pin 4) removed from output and connected to +12 V for the Adjustable, 3.3 V, and 5.0 V versions, and +25 V for the 12 V and 15 V
versions, to force the output transistor “off”.

7.Vin = 40 V.

out

= 5.0 V [Adjustable Version Only]) I

out

= 3.0 A [Note 4]) V

= 500 mA. For typical values TJ = 25°C, for min/max values TJ is the operating

Load

= 12 V for the 3.3 V, 5.0 V, and Adjustable version, Vin = 25 V for

in

Symbol Min Typ Max Unit

b

osc

sat

CL

L

Q

stby

IH

IL

IH

IL

– 15 30
– 0 5.0

nA

kHz

V

A

mA

mA

µA

4

MOTOROLA ANALOG IC DEVICE DATA

LM2576

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)

1.0

Figure 2. Normalized Output Voltage

Vin = 20 V

0.8
I

= 500 mA

Load

0.6

Normalized at TJ = 25

0.4

0.2

0
–0.2
–0.4

–0.6

, OUTPUT VOLTAGE CHANGE (%)

out

–0.8

V

–1.0

2.0
I

Load

1.5

1.4

1.2

I

= 500 mA

Load

°

C

TJ = 25

, OUTPUT VOLTAGE CHANGE (%), STANDBY QUIESCENT CURRENT (
V

–0.2

out

–0.4
–0.6

1.0

0.8

0.6

0.4

0.2

3.3 V, 5.0 V and ADJ

0

Vin, INPUT VOLTAGE (V)

12 V and 15 V

°

C

TJ, JUNCTION TEMPERATURE (°C)

1251007550250–25–50 403530252015105.00

Figure 4. Dropout Voltage Figure 5. Current Limit

6.5

Figure 3. Line Regulation

= 3.0 A

6.0

Vin = 25 V

INPUT – OUTPUT DIFFERENTIAL (V)

, QUIESCENT CURRENT (mA)

Q

I

1.0

0.5

20
18
16
14
12
10

8.0

6.0

4.0

5.5

, OUTPUT CURRENT (A)

O

I

5.0

4.5

4.0

TJ, JUNCTION TEMPERATURE (°C)

I

= 500 mA

Load

L1 = 150 µH

R

0

TJ, JUNCTION TEMPERATURE (°C)

ind

= 0.1

Ω

1251007550250–25–50 1251007550250–25–50

Figure 6. Quiescent Current Figure 7. Standby Quiescent Current

200

A)

µ

stby

I

180
160
140
120
100

80
60
40
20

V

= 5.0 V

ON/OFF

Vin = 40 V

Vin = 12 V

0

TJ, JUNCTION TEMPERATURE (°C)

I

= 200 mA

Load

Vin, INPUT VOLTAGE (V)

I

Load

= 3.0 A

V

= 5.0 V

out

Measured at
Ground Pin

°

C

TJ = 25

403530252015105.00 1251007550250–25–50

MOTOROLA ANALOG IC DEVICE DATA

5

LM2576

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)

A)I

µ

, STANDBY QUIESCENT CURRENT (

stby

NORMALIZED FREQUENCY (%)

200
180
160
140

120
100

80
60

40
20

8.0

6.0

4.0

2.0

–2.0
–4.0
–6.0

–8.0

–10

Figure 8. Standby Quiescent Current

TJ = 25°C

0

Vin, INPUT VOLTAGE (V)

Figure 10. Oscillator Frequency Figure 11. Minimum Operating Voltage

Vin = 12 V
Normalized at

°

C

25

0

TJ, JUNCTION TEMPERATURE (°C)

1.6

Figure 9. Switch Saturation Voltage

1.4

1.2
–40°C

1.0

0.8

25°C

0.6
125°C

, SATURATION VOLTAGE (V)

0.4

sat

V

0.2

35 2.5

40302520151050 0 0.5 1.0 1.5 2.0 3.0

1251007550250–25–50 1251007550250–25–50

0

5.0

4.5

4.0

3.5

3.0

2.5

2.0

, INPUT VOLTAGE (V)

1.5

in

V

1.0

0.5

0

SWITCH CURRENT (A)

Adjustable Version Only

V

' 1.23 V

out

I

= 500 mA

Load

TJ, JUNCTION TEMPERATURE (°C)

100

Figure 12. Feedback Pin Current

80
60
40

20

0
–20
–40
–60

, FEEDBACK PIN CURRENT (nA)

b

I

–80

–100

TJ, JUNCTION TEMPERATURE (

6

Adjustable Version Only

°

C)

1251007550250–25–50

MOTOROLA ANALOG IC DEVICE DATA

LM2576

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)

50 V

Figure 13. Switching Waveforms Figure 14. Load Transient Response

A

0

4.0 A

B

2.0 A
0

4.0 A

C

2.0 A

D

0

Vout = 15 V
A: Output Pin Voltage, 10 V/DIV
B: Inductor Current, 2.0 A/DIV
C: Inductor Current, 2.0 A/DIV , AC–Coupled
D: Output Ripple Voltage, 50mV/dDIV, AC–Coupled

Horizontal Time Base: 5 µs/DIV

Output

Voltage

Change

– 100 mV

Load

Current

100 mV

0

3.0 A

2.0 A

1.0 A
0

100

µ

s/DIV5 µs/DIV

MOTOROLA ANALOG IC DEVICE DATA

7

7.0 V – 40 V
Unregulated

DC Input

LM2576

Figure 15. T ypical Test Circuit
Fixed Output Voltage Versions

Feedback

V

in

LM2576

Fixed Output

1

C

in

100

µ

F

Cin– 100 µF, 75 V, Aluminium Electrolytic
C

– 1000 µF, 25 V, Aluminium Electrolytic

out

D1 – Schottky , MBR360
L1 – 100 µH, Pulse Eng. PE–92108
R1 – 2.0 k, 0.1%
R2 – 6.12 k, 0.1%

4

Output

2

/OFFGnd

53ON

L1

µ

H

100

D1
MBR360

C

out

1000

V

out

µ

F

Load

7.0 V – 40 V
Unregulated

DC Input

C

in

100

V

µ

Adjustable Output Voltage Versions

Feedback

in

1

F

LM2576

Adjustable

4

Output

2

/OFFGnd

53ON

+

V

ref

ǒ

= 1.23 V, R1

ref

ǒ

1.0

V

out

V

ref

V

out

R2+R1

Where V
between 1.0 k and 5.0 k

PCB LAYOUT GUIDELINES

100

D1
MBR360

R2

)

R1

–1.0

L1

µ

Ǔ

Ǔ

H

C

out

1000

µ

R2

F

R1

V

out

5,000 V

Load

As in any switching regulator, the layout of the printed
circuit board is very important. Rapidly switching currents
associated with wiring inductance, stray capacitance and
parasitic inductance of the printed circuit board traces can
generate voltage transients which can generate
electromagnetic interferences (EMI) and affect the desired
operation. As indicated in the Figure 15, to minimize
inductance and ground loops, the length of the leads
indicated by heavy lines should be kept as short as possible.

For best results, single–point grounding (as indicated) or
ground plane construction should be used.

8

On the other hand, the PCB area connected to the Pin 2
(emitter of the internal switch) of the LM2576 should be kept
to a minimum in order to minimize coupling to sensitive
circuitry.

Another sensitive part of the circuit is the feedback. It is
important to keep the sensitive feedback wiring short. To
assure this, physically locate the programming resistors near
to the regulator, when using the adjustable version of the
LM2576 regulator.

MOTOROLA ANALOG IC DEVICE DATA

LM2576

PIN FUNCTION DESCRIPTION

Pin Symbol Description (Refer to Figure 1)

1 V

2 Output This is the emitter of the internal switch. The saturation voltage V

3 Gnd Circuit ground pin. See the information about the printed circuit board layout.
4 Feedback This pin senses regulated output voltage to complete the feedback loop. The signal is divided by the

5 ON/OFF It allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total

in

This pin is the positive input supply for the LM2576 step–down switching regulator. In order to minimize
voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass
capacitor must be present (Cin in

It should be kept in mind that the PCB area connected to this pin should be kept to a minimum in order to
minimize coupling to sensitive circuitry.

internal resistor divider network R2, R1 and applied to the non–inverting input of the internal error amplifier.
In the Adjustable version of the LM2576 switching regulator this pin is the direct input of the error amplifier
and the resistor network R2, R1 is connected externally to allow programming of the output voltage.

input supply current to approximately 80 µA. The threshold voltage is typically 1.4 V. Applying a voltage
above this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than 1.4 V or
if this pin is left open, the regulator will be in the “on” condition.

Figure 1).

of this output switch is typically 1.5 V.

sat

DESIGN PROCEDURE

Buck Converter Basics

The LM2576 is a “Buck” or Step–Down Converter which is
the most elementary forward–mode converter. Its basic
schematic can be seen in

Figure 16.

The operation of this regulator topology has two distinct
time periods. The first one occurs when the series switch is
on, the input voltage is connected to the input of the inductor.

The output of the inductor is the output voltage, and the
rectifier (or catch diode) is reverse biased. During this period,
since there is a constant voltage source connected across
the inductor, the inductor current begins to linearly ramp
upwards, as described by the following equation:

I

L(on)

+

ǒ

Vin–V

L

out

Ǔ

t

on

During this “on” period, energy is stored within the core
material in the form of magnetic flux. If the inductor is properly
designed, there is sufficient energy stored to carry the
requirements of the load during the “off” period.

Figure 16. Basic Buck Converter

Power

Switch

in

DV

L

C

out

R

Load

This period ends when the power switch is once again
turned on. Regulation of the converter is accomplished by
varying the duty cycle of the power switch. It is possible to
describe the duty cycle as follows:

t

on

d

+

, where T is the period of switching.

T

For the buck converter with ideal components, the duty
cycle can also be described as:

V

out

d

+

V

in

Figure 17 shows the buck converter, idealized waveforms
of the catch diode voltage and the inductor current.

Figure 17. Buck Converter Idealized Waveforms

V

on(SW)

Power
Switch

Off

Diode VoltageInductor Current

VD(FWD)

Power
Switch

On

Power
Switch

Off

Power

Switch

On

Time

The next period is the “off” period of the power switch.
When the power switch turns off, the voltage across the
inductor reverses its polarity and is clamped at one diode
voltage drop below ground by the catch diode. The current
now flows through the catch diode thus maintaining the load
current loop. This removes the stored energy from the
inductor. The inductor current during this time is:

I

L(off)

+

ǒ

V

out

–V

L

D

Ǔ

t

off

MOTOROLA ANALOG IC DEVICE DATA

I

min

Diode Diode

Power

Switch

I

pk

I

(AV)

Load

Power
Switch

Time

9

LM2576

Procedure

(Fixed Output Voltage Version) In order to simplify the switching regulator design, a step–by–step

design procedure and some examples are provided.

Procedure Example

Given Parameters:

V

= Regulated Output Voltage (3.3 V, 5.0 V, 12 V or 15 V)

out

V
I

1. Controller IC Selection

According to the required input voltage, output voltage and
current, select the appropriate type of the controller IC output
voltage version.

2. Input Capacitor Selection (Cin)

To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
input pin +Vin and ground pin Gnd. This capacitor should be
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.

3. Catch Diode Selection (D1)

A.Since the diode maximum peak current exceeds the

B.The reverse voltage rating of the diode should be at least

4. Inductor Selection (L1)

A.According to the required working conditions, select the

B.From the appropriate inductor selection guide, identify the

C.Select an appropriate inductor from the several different

= Maximum Input Voltage

in(max)

Load(max)

regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design the diode should have a
current rating equal to the maximum current limit of the
LM2576 to be able to withstand a continuous output short

1.25 times the maximum input voltage.

correct inductor value using the selection guide from
Figures 18 to 22

inductance region intersected by the Maximum Input
Voltage line and the Maximum Load Current line. Each
region is identified by an inductance value and an inductor
code.

manufacturers part numbers listed in Table 2.
The designer must realize that the inductor current rating
must be higher than the maximum peak current flowing
through the inductor. This maximum peak current can be
calculated as follows

where ton is the “on” time of the power switch and

For additional information about the inductor, see the
inductor section in the “Application Hints” section of
this data sheet.

= Maximum Load Current

.

:

I

+

p(max)

I

Load(max)

t

on

)

V

out

+

V

in

ǒ

x

Vin–V

1.0

f

osc

2L

out

Ǔ

t

on

Given Parameters:

V

= 5.0 V

out

V
I

1. Controller IC Selection

According to the required input voltage, output voltage, current
polarity and current value, use the LM2576–5 controller IC

2. Input Capacitor Selection (Cin)

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

3. Catch Diode Selection (D1)

A.For this example the current rating of the diode is 3.0 A.

B.Use a 20 V 1N5820 Schottky diode, or any of the suggested

4. Inductor Selection (L1)

A.Use the inductor selection guide shown in Figures 19.

B.From the selection guide, the inductance area intersected

C.Inductor value required is 100 µH. From Table 2

= 15 V

in(max)

Load(max)

fast recovery diodes shown in Table 1.

by the 15 V line and 3.0 A line is L100.

an inductor from any of the listed manufacturers.

= 3.0 A

,

choose

10

MOTOROLA ANALOG IC DEVICE DATA

LM2576

Procedure (Fixed Output V oltage Version) (continued)In order to simplify the switching regulator design, a step–by–step

design procedure and some examples are provided.

Procedure Example

5. Output Capacitor Selection (C

A.Since the LM2576 is a forward–mode switching regulator

with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair
determined by the output capacitor and inductor values. For
stable operation and an acceptable ripple voltage,
(approximately 1% of the output voltage) a value between
680 µF and 2000 µF is recommended.

B.Due to the fact that the higher voltage electrolytic capacitors

generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating at least 8.0 V is appropriate, and a 10 V or
16 V rating is recommended.

Procedure (Adjustable Output Version: LM2576–ADJ)

Procedure Example

Given Parameters:

V

= Regulated Output Voltage

out

V
I

1. Programming Output Voltage

To select the right programming resistor R1 and R2 value (see
Figure 2) use the following formula:

Resistor R1 can be between 1.0 k and 5.0 kΩ. (For best
temperature coefficient and stability with time, use 1% metal
film resistors).

= Maximum DC Input Voltage

in(max)

Load(max)

= Maximum Load Current

+

V

ref

R2+R1

ǒ

V

out

1.0

)

)

out

R2

Ǔ

where V

R1

V

out

ǒ

–1.0

V

ref

ref

Ǔ

= 1.23 V

5. Output Capacitor Selection (C

A.C

= 680 µF to 2000 µF standard aluminium electrolytic.

out

B.Capacitor voltage rating = 20 V.

Given Parameters:

V

= 8.0 V

out

V
I

1. Programming Output Voltage (selecting R1 and R2)
Select R1 and R2:

= 25 V

in(max)

Load(max)

R2 = 9.91 kΩ, choose a 9.88 k metal film resistor.

= 2.5 A

V

+

out

R2+R1

1.23ǒ1.0

V

out

ǒ

V

ref

)

*

1.0Ǔ+

)

out

R2

Ǔ

Select R1 = 1.8 kΩ

R1

1.8 k

8.0 V

ǒ

1.23 V

*

1.0

Ǔ

2. Input Capacitor Selection (Cin)

To prevent large voltage transients from appearing at the input
and for stable operation of the converter, an aluminium or
tantalum electrolytic bypass capacitor is needed between the
input pin +Vin and ground pin Gnd This capacitor should be
located close to the IC using short leads. This capacitor should
have a low ESR (Equivalent Series Resistance) value.

For additional information see input capacitor section in the
“Application Hints” section of this data sheet.

3. Catch Diode Selection (D1)

A.Since the diode maximum peak current exceeds the

regulator maximum load current the catch diode current
rating must be at least 1.2 times greater than the maximum
load current. For a robust design, the diode should have a
current rating equal to the maximum current limit of the
LM2576 to be able to withstand a continuous output short.

B.The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

2. Input Capacitor Selection (Cin)

A 100 µF, 150 V aluminium electrolytic capacitor located near
the input and ground pin provides sufficient bypassing.

3. Catch Diode Selection (D1)
A.For this example, a 3.0 A current rating is adequate.

B.Use a 30 V 1N5821 Schottky diode or any

suggested fast recovery diode in the Table 1.

MOTOROLA ANALOG IC DEVICE DATA

11

Procedure (Adjustable Output Version:

Procedure Example

4. Inductor Selection (L1)

A.Use the following formula to calculate the inductor Volt x

microsecond [V x µs] constant:

V

+

I

p(max)

C

out

ǒ

Vin–V

+

I

Load(max)

ton+

w

13,300

ExT

B.Match the calculated E x T value with the corresponding

number on the vertical axis of the Inductor Value Selection
Guide shown in Figure 22. This E x 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
number of turns, and the duty cycle.

C.Next step is to identify the inductance region intersected by

the E x T value and the maximum load current value on the
horizontal axis shown in Figure 25.

D.From the inductor code, identify the inductor value. Then

select an appropriate inductor from Table 2.
The inductor chosen must be rated for a switching
frequency of 52 kHz and for a current rating of 1.15 x
The inductor current rating can also be determined by
calculating the inductor peak current

where ton is the “on” time of the power switch and

For additional information about the inductor, see the
inductor section in the “External Components” section of
this data sheet.

5. Output Capacitor Selection (C

A.Since the LM2576 is a forward–mode switching regulator

with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair
determined by the output capacitor and inductor values.

For stable operation, the capacitor must satisfy the
following requirement:

B.Capacitor values between 10 µF and 2000 µF will satisfy

the loop requirements for stable operation. To achieve an
acceptable output ripple voltage and transient response, the
output capacitor may need to be several times larger than
the above formula yields.

C.Due to the fact that the higher voltage electrolytic capacitors

generally have lower ESR (Equivalent Series Resistance)
numbers, the output capacitor’s voltage rating should be at
least 1.5 times greater than the output voltage. For a 5.0 V
regulator, a rating of at least 8.0 V is appropriate, and a 10 V
or 16 V rating is recommended.

out

Ǔ

V

V

V

out

V

out

in

V

out

in

)

x

out

in(max)

xL[µH]

LM2576–ADJ) (continued)

6

10

x

ǒ

Vin–V

)

1.0

f

osc

F[Hz]

:

[V xms]

out

2L

[µF]

Ǔ

t

on

LM2576

I

.

Load

4. Inductor Selection (L1)
A.Calculate E x T [V x µs] constant:

ExT

+(25 – 8.0)x

B.E x T = 80 [V x µs]

C.I

Load(max)

Inductance Region = H150

D.Proper inductor value = 150 µH

Choose the inductor from Table 2.

5. Output Capacitor Selection (C
A.

To achieve an acceptable ripple voltage, select
C

out

= 2.5 A

C

w

13,300 x

out

= 680 µF electrolytic capacitor.

8.0
25

out

25

8 x 150

x

1000

52

)

+

+

80 [V xms]

332.5 µF

12

MOTOROLA ANALOG IC DEVICE DATA

LM2576

LM2576 Series Buck Regulator Design Procedures (continued)

Indicator Value Selection Guide (For Continuous Mode Operation)

MAXIMUM INPUT VOLTAGE (V)

MAXIMUM INPUT VOLTAGE (V)

60
40
20

15
10

8.0

7.0

6.0

5.0

60
40

35
30

25
20

18

16
15

14

Figure 18. LM2576–3.3

L680

L470

L330

L220

L150

L100

L68

0.4 0.6 1.0 2.0
IL, MAXIMUM LOAD CURRENT (A)

Figure 20. LM2576–12 Figure 21. LM2576–15

H1500

H1000

H680

H470

H330

L680

L470

L330

IL, MAXIMUM LOAD CURRENT (A)

L220

L150

1.0 1.5 2.0 2.5

H220

L100

L47

3.02.51.50.80.50.3 0.3

H150

L68

3.00.80.60.50.40.3

MAXIMUM INPUT VOLTAGE (V)MAXIMUM INPUT VOLTAGE (V)

60
40

20
15

12
10

9.0

8.0

7.0

60
40

35
30

25
22

20
19

18

17

L680

H1500

L680

Figure 19. LM2576–5

H470H1000 H680 H220H330 H150

L470

L330

L220

L150

1.2

IL, MAXIMUM LOAD CURRENT (A)

H1000

H680

H470

H330

L470

L330

IL, MAXIMUM LOAD CURRENT (A)

L220

L150

L100

H220

L100

L68

L47

3.02.51.50.80.50.4 0.6 1.0 2.0

H150

L68

3.00.80.60.50.40.3 1.0 1.5 2.0 2.5

300
250

200

µ

150
100

90
80
70
60
50
45
40

ET, VOLTAGE TIME (V s)

35
30

25
20

MOTOROLA ANALOG IC DEVICE DATA

Figure 22. LM2576–ADJ

H2000

H1500

H1000

H680

H470

H330

H220

L680

L470

L330

L220

L150

L100

L68

0.4 0.6 0.8 1.0 2.5
IL, MAXIMUM LOAD CURRENT (A)

H150

L47

3.02.01.50.50.3

13

Through

HER302

30WF10
100 V)

100 V)

100 V)

100 V)

V

R

20 V 1N5820

Hole

MBR320P

SR302

LM2576

T able 1. Diode Selection Guide

Schottky Fast Recovery

3.0 A 4.0 – 6.0 A 3.0 A 4.0 – 6.0 A
Surface

Mount

SK32 1N5823

Through

Hole

SR502
SB520

Surface

Mount

Through

Hole

Surface

Mount

Through

Hole

Surface

Mount

30 V 1N5821

MBR330

SR303

31DQ03

40 V 1N5822

MBR340

SR304

31DQ04

50 V MBR350

31DQ05

SR305

60 V MBR360

DQ06

SR306

NOTE: Diofes listed inbold are available from Motorola.

SK33

30WQ03

SK34

30WQ04

MBRS340T3

MBRD340

SK35

30WQ05

MBRS360T3

MBRD360

1N5824

SR503
SB530

1N5825

SR504
SB540

SB550 50WQ05

50SQ080 MBRD660CT

50WQ03

MBRD640CT

50WQ04

MUR320

31DF1

(all diodes

rated

to at least

MURS320T3

MURD320

(all diodes

rated

to at least

MUR420

HER602

(all diodes

rated

to at least

T able 2. Inductor Selection by Manufacturer’ s Part Number

Inductor

Code

L47 47 µH 77 212 671 26980 PE–53112 RL2442
L68 68 µH 77 262 671 26990 PE–92114 RL2443

L100 100 µH 77 312 671 27000 PE–92108 RL2444

Inductor

Value

Tech 39 Schott Corp. Pulse Eng. Renco

MURD620CT

50WF10

(all diodes

rated

to at least

L150 150 µH 77 360 671 27010 PE–53113 RL1954
L220 220 µH 77 408 671 27020 PE–52626 RL1953
L330 330 µH 77 456 671 27030 PE–52627 RL1952
L470 470 µH * 671 27040 PE–53114 RL1951

L680 680 µH 77 506 671 27050 PE–52629 RL1950
H150 150 µH 77 362 671 27060 PE–531 15 RL2445
H220 220 µH 77 412 671 27070 PE–531 16 RL2446
H330 330 µH 77 462 671 27080 PE–531 17 RL2447
H470 470 µH * 671 27090 PE–53118 RL1961
H680 680 µH 77 508 671 27100 PE–531 19 RL1960

H1000 1000 µH 77 556 671 27110 PE–53120 RL1959
H1500 1500 µH * 671 27120 PE–53121 RL1958
H2200 2200 µH * 671 27130 PE–53122 RL2448

NOTE: * Contact Manufacturer

14

MOTOROLA ANALOG IC DEVICE DATA

LM2576

T able 3. Example of Several Inductor Manufacturers Phone/Fax Numbers

Pulse Engineering, Inc.

Pulse Engineering, Inc. Europe

Renco Electronics, Inc.

Tech 39

Schott Corporation

EXTERNAL COMPONENTS

Phone
Fax

Phone
Fax

Phone
Fax

Phone
Fax

Phone
Fax

+ 1–619–674–8100
+ 1–619–674–8262

+ 353–9324–107
+ 353–9324–459

+ 1–516–645–5828
+ 1–516–586–5562

+ 33–1–4115–1681
+ 33–1–4709–5051

+ 1–612–475–1173
+ 1–612–475–1786

Input Capacitor (Cin)

The Input Capacitor Should Have a Low ESR

For stable operation of the switch mode converter a low
ESR (Equivalent Series Resistance) aluminium or solid
tantalum bypass capacitor is needed between the input pin
and the ground pin, to prevent large voltage transients from
appearing at the input. It must be located near the regulator
and use short leads. With most electrolytic capacitors, the
capacitance value decreases and the ESR increases with
lower temperatures. For reliable operation in temperatures
below –25°C larger values of the input capacitor may be
needed. Also paralleling a ceramic or solid tantalum
capacitor will increase the regulator stability at cold
temperatures.

RMS Current Rating of C

in

The important parameter of the input capacitor is the RMS
current rating. Capacitors that are physically large and have
large surface area will typically have higher RMS current
ratings. For a given capacitor value, a higher voltage
electrolytic 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
current rating. The consequence of operating an electrolytic
capacitor beyond the RMS current rating is a shortened
operating life. In order to assure maximum capacitor
operating lifetime, the capacitor’s RMS ripple current rating
should be:

I

> 1.2 x d x I

rms

Load

where d is the duty cycle, for a buck regulator

V

t

t

+

on

T

+

|V

out

and d

Output Capacitor (C

d

|V

out

|)V

out

+

on

|

)

out

+

T

V

in

for a buck*boost regulator.

in

For low output ripple voltage and good stability, low ESR
output capacitors are recommended. An output capacitor has
two main functions: it filters the output and provides regulator
loop stability. The ESR of the output capacitor and the
peak–to–peak value of the inductor ripple current are the
main factors contributing to the output ripple voltage value.
Standard aluminium electrolytics could be adequate for some
applications but for quality design, low ESR types are
recommended.

An aluminium electrolytic capacitor’s ESR value is related
to many factors such as the capacitance value, the voltage
rating, the physical size and the type of construction. In most
cases, the higher voltage electrolytic capacitors have lower
ESR value. Often capacitors with much higher voltage
ratings may be needed to provide low ESR values that, are
required for low output ripple voltage.

The Output Capacitor Requires an ESR Value
That Has an Upper and Lower Limit

As mentioned above, a low ESR value is needed for low
output ripple voltage, typically 1% to 2% of the output voltage.
But if the selected capacitor’s ESR is extremely low (below

0.05 Ω), there is a possibility of an unstable feedback loop,
resulting in oscillation at the output. This situation can occur
when a tantalum capacitor, that can have a very low ESR, is
used as the only output capacitor.

At Low Temperatures, Put in Parallel Aluminium
Electrolytic Capacitors with Tantalum Capacitors

Electrolytic capacitors are not recommended for
temperatures below –25°C. The ESR rises dramatically at
cold temperatures and typically rises 3 times at –25°C and as
much as 10 times at –40°C. Solid tantalum capacitors have
much better ESR spec at cold temperatures and are
recommended for temperatures below –25°C. They can be
also used in parallel with aluminium electrolytics. The value
of the tantalum capacitor should be about 10% or 20% of the
total capacitance. The output capacitor should have at least
50% higher RMS ripple current rating at 52 kHz than the
peak–to–peak inductor ripple current.

Catch Diode

Locate the Catch Diode Close to the LM2576

The LM2576 is a step–down buck converter; it requires a
fast diode to provide a return path for the inductor current
when the switch turns off. This diode must be located close to
the LM2576 using short leads and short printed circuit traces
to avoid EMI problems.

Use a Schottky or a Soft Switching
Ultra–Fast Recovery Diode

Since the rectifier diodes are very significant sources of
losses within switching power supplies, choosing the rectifier
that best fits into the converter design is an important
process. Schottky diodes provide the best performance

MOTOROLA ANALOG IC DEVICE DATA

15

LM2576

because of their fast switching speed and low forward
voltage drop.

They provide the best efficiency especially in low output
voltage applications (5.0 V and lower). Another choice could
be Fast–Recovery, or Ultra–Fast Recovery diodes. It has to
be noted, that some types of these diodes with an abrupt
turnoff characteristic may cause instability or EMI troubles.

A fast–recovery diode with soft recovery characteristics
can better fulfill some quality , low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2576
regulator. Standard 50/60 Hz rectifier diodes, such as the
1N4001 series or 1N5400 series are NOT suitable.

Inductor

The magnetic components are the cornerstone of all
switching power supply designs. The style of the core and
the winding technique used in the magnetic component’s
design has a great influence on the reliability of the overall
power supply.

Using an improper or poorly designed inductor can cause
high voltage spikes generated by the rate of transitions in
current within the switching power supply , and the possibility
of core saturation can arise during an abnormal operational
mode. Voltage spikes can cause the semiconductors to enter
avalanche breakdown and the part can instantly fail if enough
energy is applied. It can also cause significant RFI (Radio
Frequency Interference) and EMI (Electro–Magnetic
Interference) problems.

Continuous and Discontinuous Mode of Operation

The LM2576 step–down converter can operate in both the
continuous and the discontinuous modes of operation. The
regulator works in the continuous mode when loads are
relatively heavy, the current flows through the inductor
continuously and never falls to zero. Under light load
conditions, the circuit will be forced to the discontinuous
mode when inductor current falls to zero for certain period of
time (see Figure 23 and Figure 24). Each mode has
distinctively different operating characteristics, which can
affect the regulator performance and requirements. In many
cases the preferred mode of operation is the continuous
mode. It offers greater output power, lower peak currents in
the switch, inductor and diode, and can have a lower output
ripple voltage. On the other hand it does require larger
inductor values to keep the inductor current flowing
continuously, especially at low output load currents and/or
high input voltages.

To simplify the inductor selection process, an inductor
selection guide for the LM2576 regulator was added to this
data sheet (Figures 18
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 percentage is allowed to change as

through 22). This guide assumes that

different design load currents are selected. For light loads
(less than approximately 300 mA) it may be desirable to
operate the regulator in the discontinuous mode, because
the inductor value and size can be kept relatively low.
Consequently, the percentage of inductor peak–to–peak
current increases. This discontinuous mode of operation is
perfectly acceptable for this type of switching converter. Any
buck regulator will be forced to enter discontinuous mode if
the load current is light enough.

Figure 23. Continuous Mode Switching Current

Waveforms

2.0 A

Inductor

Current

Waveform

0 A

2.0 A

Power

Switch

Current

Waveform

0 A

HORIZONTAL TIME BASE: 5.0 µs/DIV

Selecting the Right Inductor Style

Some important considerations when selecting a core
type are core material, cost, the output power of the power
supply, the physical volume the inductor must fit within, and
the amount of EMI (Electro–Magnetic Interference) shielding
that the core must provide. The inductor selection guide
covers different styles of inductors, such as pot core, E–core,
toroid and bobbin core, as well as different core materials
such as ferrites and powdered iron from different
manufacturers.

For high quality design regulators the toroid core seems to
be the best choice. Since the magnetic flux is contained
within the core, it generates less EMI, reducing noise
problems in sensitive circuits. The least expensive is the
bobbin core type, which consists of wire wound on a ferrite
rod core. This type of inductor generates more EMI due to the
fact that its core is open, and the magnetic flux is not
contained within the core.

When multiple switching regulators are located on the
same printed circuit board, open core magnetics can cause
interference between two or more of the regulator circuits,
especially at high currents due to mutual coupling. A toroid,
pot core or E–core (closed magnetic structure) should be
used in such applications.

VERTRICAL RESOLUTION 1.0 A/DIV

16

MOTOROLA ANALOG IC DEVICE DATA

LM2576

Do Not Operate an Inductor Beyond its
Maximum Rated Current

Exceeding an inductor’s maximum current rating may
cause the inductor to overheat because of the copper wire
losses, or the core may saturate. Core saturation occurs
when the flux density is too high and consequently the cross
sectional area of the core can no longer support additional
lines of magnetic flux.

This causes the permeability of the core to drop, the
inductance value decreases rapidly and the inductor begins
to look mainly resistive. It has only the DC resistance of the
winding. This can cause the switch current to rise very rapidly
and force the LM2576 internal switch into cycle–by–cycle
current limit, thus reducing the DC output load current. This
can also result in overheating of the inductor and/or the
LM2576. Different inductor types have different saturation
characteristics, and this should be kept in mind when
selecting an inductor.

GENERAL RECOMMENDATIONS

Output V oltage Ripple and Transients

Source of the Output Ripple

Since the LM2576 is a switch mode power supply
regulator, its output voltage, if left unfiltered, will contain a
sawtooth ripple voltage at the switching frequency. The
output ripple voltage value ranges from 0.5% to 3% of the
output voltage. It is caused mainly by the inductor sawtooth
ripple current multiplied by the ESR of the output capacitor.

Short Voltage Spikes and How to Reduce Them

The regulator output voltage may also contain short
voltage spikes at the peaks of the sawtooth waveform (see
Figure 25). These voltage spikes are present because of the
fast switching action of the output switch, and the parasitic
inductance of the output filter capacitor. There are some
other important factors such as wiring inductance, stray
capacitance, as well as the scope probe used to evaluate
these transients, all these contribute to the amplitude of these
spikes. To minimize these voltage spikes, low inductance
capacitors should be used, and their lead lengths must be
kept short. The importance of quality printed circuit board
layout design should also be highlighted.

Figure 25. Output Ripple Voltage Waveforms

Voltage spikes

caused by

Filtered

Output

Voltage

Unfiltered

Output

Voltage

HORIZONTAL TIME BASE: 5.0 µs/DIV

Minimizing the Output Ripple

In order to minimize the output ripple voltage it is possible
to enlarge the inductance value of the inductor L1 and/or to
use a larger value output capacitor. There is also another way

switching action

of the output

switch and the

parasitic

inductance of the

output capacitor

20 mV/DIV

VERTRICAL

RESOLUTION

Figure 24. Discontinuous Mode Switching Current

Waveforms

0.4 A

Inductor

Current

Waveform

Waveform

Power
Switch

Current

0 A

0.4 A

0 A

HORIZONTAL TIME BASE: 5.0 µs/DIV

to smooth the output by means of an additional LC filter (20 µH,
100 µF), that can be added to the output (see Figure 34) to
further reduce the amount of output ripple and transients.
With such a filter it is possible to reduce the output ripple
voltage transients 10 times or more. Figure 25

shows the
difference between filtered and unfiltered output waveforms
of the regulator shown in Figure 34.

The lower waveform is from the normal unfiltered output of
the converter, while the upper waveform shows the output
ripple voltage filtered by an additional LC filter.

Heatsinking and Thermal Considerations

The Through–Hole Package TO–220

The LM2576 is available in two packages, a 5–pin
TO–220(T, TV) and a 5–pin surface mount D2PAK(D2T).
Although the TO–220(T) package needs a heatsink under
most conditions, there are some applications that require no
heatsink to keep the LM2576 junction temperature within the
allowed operating range. Higher ambient temperatures
require some heat sinking, either to the printed circuit (PC)
board or an external heatsink.

The Surface Mount Package D2PAK and its Heatsinking

The other type of package, the surface mount D2PAK, is
designed to be soldered to the copper on the PC board. The
copper and the board are the heatsink for this package and
the other heat producing components, such as the catch
diode and inductor. The PC board copper area that the
package is soldered to should be at least 0.4 in2 (or
260 mm2) and ideally should have 2 or more square inches
(1300 mm2) of 0.0028 inch copper. Additional increases of
copper area beyond approximately 6.0 in2 (4000 mm2) will
not improve heat dissipation significantly. If further thermal
improvements are needed, double sided or multilayer PC
boards with large copper areas should be considered. In
order to achieve the best thermal performance, it is highly
recommended to use wide copper traces as well as large
areas of copper in the printed circuit board layout. The only
exception to this is the OUTPUT (switch) pin, which should
not have large areas of copper (see page 8 ’PCB Layout
Guideline’).

VERTICAL RESOLUTION 200 mA/DIV

MOTOROLA ANALOG IC DEVICE DATA

17

LM2576

Thermal Analysis and Design

The following procedure must be performed to determine

whether or not a heatsink will be required. First determine:

1. P

maximum regulator power dissipation in the

D(max)

application.

2. T

) maximum ambient temperature in the

A(max

application.

3. T

J(max)

maximum allowed junction temperature
(125°C for the LM2576). For a conservative
design, the maximum junction temperature
should not exceed 1 10°C to assure safe
operation. For every additional +10°C
temperature rise that the junction must
withstand, the estimated operating lifetime of
the component is halved.

4. R

5. R

θJC
θJA

package thermal resistance junction–case.
package thermal resistance junction–ambient.

(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or R

θJC

and R

θJA

values).

The following formula is to calculate the approximate total

power dissipated by the LM2576:

PD = (Vin x IQ) + d x I

Load

x V

sat

where d is the duty cycle and for buck converter

V

t

I

(quiescent current) and V

Q

on

d

+

O

+

T

,

V

in

can be found in the

sat

LM2576 data sheet,
Vinis minimum input voltage applied,
VOis the regulator output voltage,
I

is the load current.

Load

The dynamic switching losses during turn–on and turn–off

can be neglected if proper type catch diode is used.

Packages Not on a Heatsink (Free–Standing)

For a free–standing application when no heatsink is used,
the junction temperature can be determined by the following
expression:

where (R

TJ = (R

)(PD) represents the junction temperature rise

θJA

) (PD) + T

θJA

A

caused by the dissipated power and TA is the maximum
ambient temperature.

Packages on a Heatsink

If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, than a heatsink is required. The junction
temperature will be calculated as follows:

where R

TJ = PD (R

is the thermal resistance junction–case,

θJC

R

is the thermal resistance case–heatsink,

θCS

R

is the thermal resistance heatsink–ambient.

θSA

θJA

+ R

θCS

+ R

θSA

) + T

A

If the actual operating temperature is greater than the
selected safe operating junction temperature, then a larger
heatsink is required.

Some Aspects That can Influence Thermal Design

It should be noted that the package thermal resistance and
the junction temperature rise numbers are all approximate,
and there are many factors that will affect these numbers,
such as PC board size, shape, thickness, physical position,
location, board temperature, as well as whether the
surrounding air is moving or still.

Other factors are trace width, total printed circuit copper
area, copper thickness, single– or double–sided, multilayer
board, the amount of solder on the board or even colour of
the traces.

The size, quantity and spacing of other components on
the board can also influence its effectiveness to dissipate
the heat.

Figure 26. Inverting Buck–Boost Develops –12 V

12 to 40 V
Unregulated
DC Input

C

in

100

µ

F

+V

in

1

LM2576–12

Feedback

4
Output

2

53ON

/OFFGnd

L1

µ

H

68

D1
1N5822

C

out

µ

F

2200

–12 V @ 0.7 A

Regulated

Output

ADDITIONAL APPLICATIONS

Inverting Regulator

An inverting buck–boost regulator using the LM2576–12 is
shown in Figure 26. This circuit converts a positive input
voltage to a negative output voltage with a common ground
by bootstrapping the regulators ground to the negative output
voltage. By grounding the feedback pin, the regulator senses
the inverted output voltage and regulates it.

In this example the LM2576–12 is used to generate a
–12 V output. The maximum input voltage in this case
cannot exceed +28 V because the maximum voltage
appearing across the regulator is the absolute sum of the
input and output voltages and this must be limited to a
maximum of 40 V.

This circuit configuration is able to deliver approximately

0.7 A to the output when the input voltage is 12 V or higher. At
lighter loads the minimum input voltage required drops to
approximately 4.7 V, because the buck–boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.

18

MOTOROLA ANALOG IC DEVICE DATA

LM2576

Since the switch currents in this buck–boost configuration
are higher than in the standard buck converter topology, the
available output current is lower.

This type of buck–boost inverting regulator can also
require a larger amount of start–up input current, even for
light loads. This may overload an input power source with a
current limit less than 5.0 A.

Such an amount of input start–up current is needed for at
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.

Because of the relatively high start–up currents required
by this inverting regulator topology, the use of a delayed
start–up or an undervoltage lockout circuit is recommended.

Using a delayed start–up arrangement, the input capacitor
can charge up to a higher voltage before the switch–mode
regulator begins to operate.

The high input current needed for start–up is now partially
supplied by the input capacitor Cin.

It has been already mentioned above, that in some
situations, the delayed start–up or the undervoltage lockout
features could be very useful. A delayed start–up circuit
applied to a buck–boost converter is shown in Figure 27,
Figure 33 in the “Undervoltage Lockout” section describes an
undervoltage lockout feature for the same converter
topology.

Design Recommendations:

The inverting regulator operates in a different manner than
the buck converter and so a different design procedure has to
be used to select the inductor L1 or the output capacitor C

out

The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of µF).

The recommended range of inductor values for the
inverting converter design is between 68 µH and 220 µH. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.

The following formula is used to obtain the peak inductor
current:

I

peak

where ton+

I

[

Vin)

Load(Vin

|VO|

|VO|

)

|VO|)

V

in

1.0

x

f

osc

)

, and f

Vinxt

2L

1

+

osc

on

52 kHz.

Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.

Figure 27. Inverting Buck–Boost Regulator

with Delayed start–up

12 V to 25 V
Unregulated
DC Input

C

in

µ

F

100

0.1

/50 V

C1

µ

+V

in

LM2576–12

1

F

R1

47 k

/OFF Gnd

R2
47 k

35ON

Feedback

4
Output

2

D1
1N5822

68

L1

µ

Figure 28. Inverting Buck–Boost Regulator Shutdown

Circuit Using an Optocoupler

+V

in

C

in

Shutdown

5.0 V

.

0

Input

Off

On

NOTE: This picture does not show the complete circuit.

R3

470

100

R1

µ

F

47 k

MOC8101

+V

in

1

LM2576–XX

With the inverting configuration, the use of the ON/OFF pin
requires some level shifting techniques. This is caused by the
fact, that the ground pin of the converter IC is no longer at
ground. Now, the ON

/OFF pin threshold voltage (1.3 V
approximately) has to be related to the negative output
voltage level. There are many different possible shut down
methods, two of them are shown in Figures 28

H

C

out

µ

F

2200
/16 V

–12 V @ 700 m A

Regulated

Output

35 GndON/OFF

R2
47 k

–V

and 29.

out

MOTOROLA ANALOG IC DEVICE DATA

19

LM2576

Figure 29. Inverting Buck–Boost Regulator Shutdown

Circuit Using a PNP Transistor

+V

+V

in

NOTE: This picture does not show the complete circuit.

Off

0

On

C

in

100

Negative Boost Regulator

This example is a variation of the buck–boost topology and
it is called negative boost regulator. This regulator
experiences relatively high switch current, especially at low
input voltages. The internal switch current limiting results in
lower output load current capability .

The circuit in Figure 30 shows the negative boost
configuration. The input voltage in this application ranges
from –5.0 V to –12 V and provides a regulated –12 V output.
If the input voltage is greater than –12 V, the output will rise
above –12 V accordingly , but will not damage the regulator.

Figure 30. Negative Boost Regulator

V

in
1

C

in

100

µ

F

V

in

–5.0 V to –12 V

Design Recommendations:

The same design rules as for the previous inverting
buck–boost converter can be applied. The output capacitor
C

must be chosen larger than would be required for a what

out

standard buck converter. Low input voltages or high output

Shutdown
Input

R2

5.6 k

µ

F

LM2576–12

100

Q1
2N3906

µ

H

+V

53

in
1

ON

/OFFGnd

LM2576–XX

35 GndON/OFF

R1
12 k

4
Feedback
Output
2

1N5820

Typical Load Current
400 mA for Vin = –5.2 V
750 mA for Vin = –7.0 V

–V

out

V

C

out

µ

2200

Low Esr

= –12 V

out

F

currents require a large value output capacitor (in the range
of thousands of µF). The recommended range of inductor
values for the negative boost regulator is the same as for
inverting converter design.

Another important point is that these negative boost
converters cannot provide current limiting load protection in
the event of a short in the output so some other means, such
as a fuse, may be necessary to provide the load protection.

Delayed Start–up

There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
start–up current. In such cases, if the input power source is
limited, this delayed start–up feature becomes very useful.

To provide a time delay between the time when the input
voltage is applied and the time when the output voltage
comes up, the circuit in Figure 31 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
on the ON

/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON

/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
C1 discharge current, but its use is not mandatory .

When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the ON

/OFF
pin, the regulator could be switched periodically on and off
with the line (or double) frequency .

Figure 31. Delayed start–up Circuitry

C1

0.1

+V

µ

R1

47 k

in

1

F

LM2576–XX

35 GndON/OFF

R2
47 k

+V

in

C

in

100

µ

F

NOTE: This picture does not show the complete circuit.

Undervoltage Lockout

Some applications require the regulator to remain off until
the input voltage reaches a certain threshold level. Figure 32
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck–boost converter is
shown in Figure 33. Resistor R3 pulls the ON

/OFF pin high

and keeps the regulator off until the input voltage reaches a

20

MOTOROLA ANALOG IC DEVICE DATA

LM2576

predetermined threshold level with respect to the ground
Pin 3, which is determined by the following expression:

VZ1)

ǒ

Vth[

Figure 32. Undervoltage Lockout Circuit for

Buck Converter

+V

in

R2

10 k

1N5242B

10 k

R3

47 k

Z1

Q1
2N3904

R1

1.0

)

C
100

+V

in

R2
R1

in

1

µ

Ǔ

V

BE

LM2576–XX

F

Vth ≈ 13 V

(Q1)

35 GndON/OFF

Under normal continuous inductor current operating
conditions, the worst case occurs when Vin is minimal.

Figure 33. Undervoltage Lockout Circuit for

Buck–Boost Converter

+V

in

R2

15 k

Z1

1N5242B

R1

15 k

R3

47 k

Q1
2N3904

NOTE: This picture does not show the complete circuit.

+V

C

in

100

in

1

µ

F

LM2576–XX

35 GndON/OFF

Vth ≈ 13 V

V

out

NOTE: This picture does not show the complete circuit.

The following formula is used to obtain the peak inductor

current:

I

peak

where ton+

I

[

Vin)

Load(Vin

|VO|

|VO|

)

|VO|)

V

in

1.0

x

f

osc

)

, and f

Vinxt

2L

1

+

osc

on

52 kHz.

Figure 34. 1.2 to 35 V Adjustable 3.0 A Power Supply with Low Output Ripple

40 V Max
Unregulated
DC Input

C

in

100

µ

F

+V

in

1

LM2574–Adj

53ON

Feedback

4

Output

2

/OFFGnd

L1

150

D1
1N5822

Adjustable Output, Low–Ripple Power Supply

A 3.0 A output current capability power supply that
features an adjustable output voltage is shown in Figure 34.

This regulator delivers 3.0 A into 1.2 V to 35 V output. The
input voltage ranges from roughly 3.0 V to 40 V. In order to
achieve a 10 or more times reduction of output ripple, an
additional L–C filter is included in this circuit.

µ

H

R2
50 k

C

out

µ

F

2200

R1

1.21 k

20

100

L2

µ

H

C1

µ

F

Output

Voltage

1.2 to 35 V @ 3.0 A

MOTOROLA ANALOG IC DEVICE DATA

Optional Output

Ripple Filter

21

LM2576

THE LM2576–5 STEP–DOWN VOLTAGE REGULAT OR WITH 5.0 V @ 3.0 A OUTPUT POWER CAPABILITY.

TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUT

Figure 35. Schematic Diagram of the LM2576–5 Step–Down Converter

Feedback

Unregulated

DC Input

+Vin = 7.0 to 40 V

+V

in
1

LM2576–5

4

Output
2

53ON

/OFFGnd

150

L1

µ

H

Regulated Output
V

= 5.0 V @ 3.0 A

out1

C1

µ

F

100

/50 V

Gnd

in

C1 – 100 µF, 50 V, Aluminium Electrolytic
C2 – 1000 µF, 16 V, Aluminium Electrolytic

D1 – 3.0 A, 40 V, Schottky Rectifier, 1N5822

L1 – 150 µH, RL2444, Renco Electronics

Figure 36. Printed Circuit Board Layout

Component Side

LM2576

U1

D1

C1

+

+

C2

V

ou
t

ON/OFF

00060_00

C

D1
1N5822

out

1000

/16 V

µ

F

Gnd

out

Figure 37. Printed Circuit Board Layout

Copper Side

22

+V

Gnd

/OFF

ON

in

in

L1

Gnd

out

NOTE: Not to scale. NOTE: Not to scale.

MOTOROLA ANALOG IC DEVICE DATA

LM2576

THE LM2576–ADJ STEP–DOWN VOLTAGE REGULATOR WITH 8.0 V @ 1.0 A OUTPUT POWER

CAPABILITY. TYPICAL APPLICATION WITH THROUGH–HOLE PC BOARD LAYOUT

Figure 38. Schematic Diagram of the 8.0 V @ 3.0 A Step–Down Converter Using the LM2576–ADJ

4 Feedback

Unregulated
DC Input

+Vin = 10 V to 40 V

100

/50 V

C1 – 100 µF, 50 V, Aluminium Electrolytic
C2 – 1000 µF, 16 V, Aluminium Electrolytic

D1 – 3.0 A, 40 V, Schottky Rectifier, 1N5822

L1 – 150 µH, RL2444, Renco Electronics
R1 – 1.8 kΩ, 0.25 W
R2 – 10 kΩ, 0.25 W

C1

µ

+V

in

LM2576–ADJ

1

53ON/OFFGnd

F

ON/OFF

Figure 39. Printed Circuit Board Layout

Component Side

LM2576

Output

2

L1

150

D1
1N5822

µ

H

R2
10 k

C2

µ

F

1000

/16 V

R1

1.8 k

V

+

V

out

= 1.23 V

ref

ref

V
R1 is between 1.0 k and 5.0 k

Figure 40. Printed Circuit Board Layout

Copper Side

Regulated
Output Filtered

V

= 8.0 V @ 3.0 A

out2

)ǒ1.0

R2

)

R1

Ǔ

00059_00

U1

D1 R1

R2

ON

V

Gnd

/OFF

out

out

+V

Gnd

C1

+

in

L1

in

NOTE: Not to scale. NOTE: Not to scale.

+

C2

References

• National Semiconductor LM2576 Data Sheet and Application Note

• National Semiconductor LM2595 Data Sheet and Application Note

• Marty Brown “Practical Switching Power Supply Design”, Academic Press, Inc., San Diego 1990

• Ray Ridley “High Frequency Magnetics Design”, Ridley Engineering, Inc. 1995

MOTOROLA ANALOG IC DEVICE DATA

23

–Q–

U

K

D

5 PL

0.356 (0.014) T

B

12345

M

G

Q

A

S

M

LM2576

OUTLINE DIMENSIONS

T SUFFIX

PLASTIC PACKAGE

CASE 314D–03

ISSUE D

SEATING

–T–

PLANE

C

E

L

J
H

NOTES:

1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.

2. CONTROLLING DIMENSION: INCH.

3. DIMENSION D DOES NOT INCLUDE
INTERCONNECT BAR (DAMBAR) PROTRUSION.
DIMENSION D INCLUDING PROTRUSION SHALL
NOT EXCEED 10.92 (0.043) MAXIMUM.

DIM MIN MAX MIN MAX

A 0.572 0.613 14.529 15.570
B 0.390 0.415 9.906 10.541
C 0.170 0.180 4.318 4.572
D 0.025 0.038 0.635 0.965
E 0.048 0.055 1.219 1.397
G 0.067 BSC 1.702 BSC
H 0.087 0.112 2.210 2.845

J 0.015 0.025 0.381 0.635

K 1.020 1.065 25.908 27.051

L 0.320 0.365 8.128 9.271
Q 0.140 0.153 3.556 3.886
U 0.105 0.117 2.667 2.972
S 0.543 0.582 13.792 14.783

MILLIMETERSINCHES

Q

U

F

K

5X D

0.10 (0.254) PMT

M

B

–P–

TV SUFFIX

PLASTIC PACKAGE

CASE 314B–05

ISSUE J

NOTES:

OPTIONAL
CHAMFER

E

A

C

L

S

5X J

G

0.24 (0.610) T

M

H

V

W

N

SEATING

–T–

PLANE

1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.

2. CONTROLLING DIMENSION: INCH.

3. DIMENSION D DOES NOT INCLUDE
INTERCONNECT BAR (DAMBAR) PROTRUSION.
DIMENSION D INCLUDING PROTRUSION SHALL
NOT EXCEED 0.043 (1.092) MAXIMUM.

DIM MIN MAX MIN MAX

A 0.572 0.613 14.529 15.570
B 0.390 0.415 9.906 10.541
C 0.170 0.180 4.318 4.572
D 0.025 0.038 0.635 0.965
E 0.048 0.055 1.219 1.397
F 0.850 0.935 21.590 23.749
G 0.067 BSC 1.702 BSC
H 0.166 BSC 4.216 BSC
J 0.015 0.025 0.381 0.635
K 0.900 1.100 22.860 27.940
L 0.320 0.365 8.128 9.271
N 0.320 BSC 8.128 BSC
Q 0.140 0.153 3.556 3.886
S ––– 0.620 ––– 15.748
U 0.468 0.505 11.888 12.827
V ––– 0.735 ––– 18.669

W 0.090 0.110 2.286 2.794

MILLIMETERSINCHES

24

MOTOROLA ANALOG IC DEVICE DATA

K

B

D

0.010 (0.254) T

M

C

A

123

45

G

S

H

OPTIONAL
CHAMFER

LM2576

OUTLINE DIMENSIONS

D2T SUFFIX

PLASTIC PACKAGE

CASE 936A–02

(D2PAK)
ISSUE A

–T–

E

M

N

R

TERMINAL 6

V

L

P

U

NOTES:

1 DIMENSIONING AND TOLERANCING PER ANSI

Y14.5M, 1982.
2 CONTROLLING DIMENSION: INCH.
3 TAB CONTOUR OPTIONAL WITHIN DIMENSIONS

A AND K.
4 DIMENSIONS U AND V ESTABLISH A MINIMUM

MOUNTING SURFACE FOR TERMINAL 6.
5 DIMENSIONS A AND B DO NOT INCLUDE MOLD

FLASH OR GATE PROTRUSIONS. MOLD FLASH

AND GATE PROTRUSIONS NOT TO EXCEED

0.025 (0.635) MAXIMUM.

INCHES

DIMAMIN MAX MIN MAX

0.386 0.403 9.804 10.236

B 0.356 0.368 9.042 9.347
C 0.170 0.180 4.318 4.572
D 0.026 0.036 0.660 0.914
E 0.045 0.055 1.143 1.397
G 0.067 BSC 1.702 BSC
H 0.539 0.579 13.691 14.707
K 0.050 REF 1.270 REF
L 0.000 0.010 0.000 0.254
M 0.088 0.102 2.235 2.591
N 0.018 0.026 0.457 0.660
P 0.058 0.078 1.473 1.981

__

R 5 REF
S 0.116 REF 2.946 REF
U 0.200 MIN 5.080 MIN
V 0.250 MIN 6.350 MIN

MILLIMETERS

5 REF

MOTOROLA ANALOG IC DEVICE DATA

25

LM2576

Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty , representation or guarantee regarding
the suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, and
specifically disclaims any and all liability, including without limitation consequential or incidental damages. “T ypical” parameters which may be provided in Motorola
data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”
must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights of
others. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other
applications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injury
or death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorola
and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees
arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that
Motorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an Equal
Opportunity/Affirmative Action Employer.

26

MOTOROLA ANALOG IC DEVICE DATA

LM2576

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MOTOROLA ANALOG IC DEVICE DATA

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LM2576/D

27