LM2576T-15 - Motorola / Freescale Semiconductor

Device Operating

Temperature Range Package



EASY SWITCHER

3.0 A STEP–DOWN

VOLTAGE REGULATOR

ORDERING INFORMATION

LM2576T–XX

LM2576TV–XX TJ = –40° to +125°C

Straight Lead

Vertical Mount

D2T SUFFIX

PLASTIC PACKAGE

CASE 936A

(D2PAK)

Order this document by LM2576/D

T SUFFIX

PLASTIC PACKAGE

CASE 314D

TV SUFFIX

PLASTIC PACKAGE

CASE 314B

LM2576D2T–XX Surface Mount

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

Adjustable Output.

SEMICONDUCTOR

TECHNICAL DATA

DEVICE TYPE/NOMINAL OUTPUT VOLTAGE

LM2576–3.3

LM2576–5

LM2576–12

LM2576–15

LM2576–ADJ

3.3 V

5.0 V

12 V

15 V

1.23 V to 37 V

Pin 1. Vin

2. Output

3. Ground

4. Feedback

5. ON/OFF

Heatsink surface

connected to Pin 3.

Heatsink surface (shown as terminal 6 in case outline

drawing) is connected to Pin 3.

MOTOROLA ANALOG IC DEVICE DATA

  

!  



 

   

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 dif ferent

inductor manufacturers.

Since the LM2576 converter is a switch–mode power supply , its ef ficiency

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

This document contains information on a new product. Specifications and information herein

are subject to change without notice. Motorola, Inc. 1997 Rev 0

LM2576

2MOTOROLA ANALOG IC DEVICE DATA

Figure 1. Block Diagram and Typical Application

7.0 V – 40 V

Unregulated

DC Input

100

Gnd

+Vin

Cin

100

F3ON

/OFF5

Output

Feedback

1N5822 Cout

1000

Typical Application (Fixed Output Voltage Versions)

Representative Block Diagram and Typical Application

Unregulated

DC Input +Vin

Cout

Feedback

Cin

1.0 k Output

Gnd

ON/OFF

Reset

Latch

Thermal

Shutdown

52 kHz

Oscillator

1.235 V

Band–Gap

Reference

Freq

Shift

18 kHz

Comparator

Fixed Gain

Error Amplifier

Current

Limit

Driver

1.0 Amp

Switch

ON/OFF

3.1 V Internal

Regulator

Regulated

Output

Vout

Load

Output

Voltage V ersions

3.3 V

5.0 V

12 V

15 V

(

Ω

)

1.7 k

3.1 k

8.84 k

11.3 k

For adjustable version

R1 = open, R2 = 0

Ω

LM2576

5.0 V Regulated

Output 3.0 A Load

ABSOLUTE MAXIMUM RATINGS (Absolute Maximum Ratings indicate limits beyond

which damage to the device may occur.)

Rating Symbol Value Unit

Maximum Supply Voltage Vin 45 V

ON/OFF Pin Input Voltage – –0.3 V ≤ V ≤ +Vin V

Output V oltage to Ground (Steady–State) – –1.0 V

Power Dissipation

Case 314B and 314D (TO–220, 5–Lead) PDInternally Limited W

Thermal Resistance, Junction–to–Ambient RθJA 65 °C/W

Thermal Resistance, Junction–to–Case RθJC 5.0 °C/W

Case 936A (D2PAK) PDInternally Limited W

Thermal Resistance, Junction–to–Ambient RθJA 70 °C/W

Thermal Resistance, Junction–to–Case RθJC 5.0 °C/W

Storage Temperature Range Tstg –65 to +150 °C

Minimum ESD Rating (Human Body Model:

C = 100 pF, R = 1.5 kΩ)– 2.0 kV

Lead Temperature (Soldering, 10 seconds) – 260 °C

Maximum Junction Temperature TJ150 °C

NOTE: ESD data available upon request.

LM2576

MOTOROLA ANALOG IC DEVICE DATA

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 Symbol Value Unit

Operating Junction Temperature Range TJ–40 to +125 °C

Supply Voltage Vin 40 V

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

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

the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25°C, for min/max values TJ is the operating

junction temperature range that applies [Note 2], unless otherwise noted.)

Characteristics Symbol Min Typ Max Unit

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

Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25°C) Vout 3.234 3.3 3.366 V

Output Voltage (6.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A) Vout V

TJ = 25°C 3.168 3.3 3.432

TJ = –40 to +125°C 3.135 – 3.465

Efficiency (Vin = 12 V, ILoad = 3.0 A) η– 75 – %

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

Output Voltage (Vin = 12 V, ILoad = 0.5 A, TJ = 25°C) Vout 4.9 5.0 5.1 V

Output Voltage (8.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A) Vout V

TJ = 25°C 4.8 5.0 5.2

TJ = –40 to +125°C 4.75 – 5.25

Efficiency (Vin = 12 V, ILoad = 3.0 A) η– 77 – %

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

Output Voltage (Vin = 25 V, ILoad = 0.5 A, TJ = 25°C) Vout 11.76 12 12.24 V

Output Voltage (15 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A) Vout V

TJ = 25°C 11.52 12 12.48

TJ = –40 to +125°C 11.4 – 12.6

Efficiency (Vin = 15 V, ILoad = 3.0 A) η– 88 – %

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

Output Voltage (Vin = 30 V, ILoad = 0.5 A, TJ = 25°C) Vout 14.7 15 15.3 V

Output Voltage (18 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A) Vout V

TJ = 25°C 14.4 15 15.6

TJ = –40 to +125°C 14.25 – 15.75

Efficiency (Vin = 18 V, ILoad = 3.0 A) η– 88 – %

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

Feedback Voltage (Vin = 12 V, ILoad = 0.5 A, Vout = 5.0 V, TJ = 25°C) Vout 1.217 1.23 1.243 V

Feedback Voltage (8.0 V ≤ Vin ≤ 40 V, 0.5 A ≤ ILoad ≤ 3.0 A, Vout = 5.0 V) Vout V

TJ = 25°C 1.193 1.23 1.267

TJ = –40 to +125°C 1.18 – 1.28

Efficiency (Vin = 12 V, ILoad = 3.0 A, Vout = 5.0 V) η– 77 – %

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

LM2576 is used as shown in the

Figure 15

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

2.Tested junction temperature range for the LM2576: Tlow = –40°C Thigh = +125°C

LM2576

4MOTOROLA ANALOG IC DEVICE DATA

DEVICE PARAMETERS

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

the 12 V version, and Vin = 30 V for the 15 V version. ILoad = 500 mA. For typical values TJ = 25°C, for min/max values TJ is the operating

junction temperature range that applies [Note 2], unless otherwise noted.)

Characteristics Symbol Min Typ Max Unit

ALL OUTPUT VOLTAGE VERSIONS

Feedback Bias Current (Vout = 5.0 V [Adjustable V ersion Only]) IbnA

TJ = 25°C – 25 100

TJ = –40 to +125°C – – 200

Oscillator Frequency [Note 3] fosc kHz

TJ = 25°C – 52 –

TJ = 0 to +125°C 47 – 58

TJ = –40 to +125°C 42 – 63

Saturation Voltage (Iout = 3.0 A [Note 4]) Vsat V

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]) ICL A

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 ILmA

Output = 0 V – 0.8 2.0

Output = –1.0 V – 6.0 20

Quiescent Current [Note 6] IQmA

TJ = 25°C – 5.0 9.0

TJ = –40 to +125°C – – 11

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

TJ = 25°C – 80 200

TJ = –40 to +125°C – – 400

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

Vout = 0 V VIH

TJ = 25°C 2.2 1.4 –

TJ = –40 to +125°C 2.4 – –

Vout = Nominal Output Voltage VIL

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 IIH –15 30

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

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.

LM2576

MOTOROLA ANALOG IC DEVICE DATA

IQ, QUIESCENT CURRENT (mA)

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)

Vout, OUTPUT VOLTAGE CHANGE (%)

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

TJ, JUNCTION TEMPERATURE (

IO, OUTPUT CURRENT (A)

TJ, JUNCTION TEMPERATURE (

Vin, INPUT VOLTAGE (V)

INPUT – OUTPUT DIFFERENTIAL (V)

TJ, JUNCTION TEMPERATURE (

Figure 2. Normalized Output Voltage

TJ, JUNCTION TEMPERATURE (

Figure 3. Line Regulation

Figure 4. Dropout Voltage Figure 5. Current Limit

Figure 6. Quiescent Current Figure 7. Standby Quiescent Current

ILoad = 200 mA

ILoad = 3.0 A

Vin = 12 V

Vin = 40 V

L1 = 150

Rind = 0.1

Ω

ILoad = 500 mA

ILoad = 3.0 A

Vout = 5.0 V

Measured at

Ground Pin

TJ = 25

VON/OFF = 5.0 V

1.0

0.6

0.2

–0.2

–0.4

–1.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

–0.2

–0.4

–0.6

2.0

1.5

1.0

0.5

6.5

6.0

5.5

5.0

4.5

4.0

8.0

6.0

4.0

200

180

160

140

120

100

1251007550250–25–50 403530252015105.00

1251007550250–25–50 1251007550250–25–50

403530252015105.00 1251007550250–25–50

–0.8

–0.6

0.4

0.8 Vin = 20 V

ILoad = 500 mA

Normalized at TJ = 25

ILoad = 500 mA

TJ = 25

3.3 V, 5.0 V and ADJ

12 V and 15 V

Vin = 25 V

Istby

LM2576

6MOTOROLA ANALOG IC DEVICE DATA

Vsat, SATURATION VOLT AGE (V)

2.0

2.5

3.0

4.0

Ib, FEEDBACK PIN CURRENT (nA)

, STANDBY QUIESCENT CURRENT (

A)Istby

, INPUT VOLT AGE (V)

TJ, JUNCTION TEMPERATURE (

SWITCH CURRENT (A)

NORMALIZED FREQUENCY (%)

TJ, JUNCTION TEMPERATURE (

Figure 8. Standby Quiescent Current

Vin, INPUT VOLTAGE (V)

Figure 9. Switch Saturation Voltage

Figure 10. Oscillator Frequency Figure 11. Minimum Operating Voltage

Figure 12. Feedback Pin Current

Vin = 12 V

Normalized at

TJ = 25

Adjustable Version Only

200

180

140

120

100

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

8.0

6.0

4.0

2.0

–2.0

–4.0

–6.0

–8.0

–10

5.0

4.5

3.5

1.5

1.0

0.5

40302520151050 0 0.5 1.0 1.5 2.0 3.0

1251007550250–25–50 1251007550250–25–50

TJ, JUNCTION TEMPERATURE (

Adjustable Version Only

100

–20

–40

–60

–80

–100 1251007550250–25–50

160

35 2.5

–40

125

Vout

1.23 V

ILoad = 500 mA

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)

Vin

LM2576

MOTOROLA ANALOG IC DEVICE DATA

2.0 A

100

s/DIV5

s/DIV

Figure 13. Switching Waveforms Figure 14. Load Transient Response

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

50 V

4.0 A

2.0 A

100 mV

Output

Voltage

Change 0

3.0 A

2.0 A

1.0 A

4.0 A

– 100 mV

Load

Current

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 15)

LM2576

8MOTOROLA ANALOG IC DEVICE DATA

Figure 15. Typical Test Circuit

MBR360

100

Output

Feedback

Cout

1000

Cin

100

LM2576

Fixed Output

53ON

/OFFGnd

Vin

Load

Vout

MBR360

100

Output

Feedback

Cout

1000

Cin

100

LM2576

Adjustable

53ON

/OFFGnd

Vin

Load

Vout

5,000 V

Fixed Output Voltage Versions

Adjustable Output Voltage Versions

Vout

Vref

1.0

)

Vout

Vref –1.0

Where Vref = 1.23 V, R1

between 1.0 k and 5.0 k

Cin – 100 µF, 75 V, Aluminium Electrolytic

Cout – 1000 µF, 25 V, Aluminium Electrolytic

D1 – Schottky, MBR360

L1 – 100 µH, Pulse Eng. PE–92108

R1 – 2.0 k, 0.1%

R2 – 6.12 k, 0.1%

7.0 V – 40 V

Unregulated

DC Input

7.0 V – 40 V

Unregulated

DC Input

PCB LAYOUT GUIDELINES

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.

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.

LM2576

MOTOROLA ANALOG IC DEVICE DATA

PIN FUNCTION DESCRIPTION

Pin Symbol Description (Refer to Figure 1)

1 Vin 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

Figure 1).

2 Output This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is typically 1.5 V.

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.

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

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.

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

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.

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:

IL(on)

+ǒ

Vin –V

out

ton

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

DVin RLoad

Cout

Power

Switch

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:

IL(off)

+ǒ

Vout –V

off

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:

ton

T, where T is the period of switching.

For the buck converter with ideal components, the duty

cycle can also be described as:

Vout

Vin

Figure 17 shows the buck converter, idealized waveforms

of the catch diode voltage and the inductor current.

Power

Switch

Figure 17. Buck Converter Idealized Waveforms

Power

Switch

Off

Power

Switch

Off

Power

Switch

Power

Switch

Von(SW)

VD(FWD)

Time

ILoad(AV)

Imin

Ipk

Diode Diode Power

Switch

Diode VoltageInductor Current

LM2576

10 MOTOROLA ANALOG IC DEVICE DATA

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:

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

Vin(max) = Maximum Input Voltage

ILoad(max) = Maximum Load Current

Given Parameters:

Vout = 5.0 V

Vin(max) = 15 V

ILoad(max) = 3.0 A

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.

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)

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.

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.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.

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

fast recovery diodes shown in Table 1.

4. Inductor Selection (L1)

A.According to the required working conditions, select the

correct inductor value using the selection guide from

Figures 18 to 22

B.From the appropriate inductor selection guide, identify the

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.

C.Select an appropriate inductor from the several different

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.

Ip(max)

ILoad(max)

)ǒ

Vin–Vout

ton

Vout

Vin x1.0

fosc

4. Inductor Selection (L1)

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

B.From the selection guide, the inductance area intersected

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

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

choose

an inductor from any of the listed manufacturers.

LM2576

MOTOROLA ANALOG IC DEVICE DATA

Procedure

(Fixed Output Voltage 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 (Cout)

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.

5. Output Capacitor Selection (Cout)

A.Cout = 680 µF to 2000 µF standard aluminium electrolytic.

B.Capacitor voltage rating = 20 V.

Procedure (Adjustable Output Version:

LM2576–ADJ)

Procedure Example

Given Parameters:

Vout = Regulated Output V oltage

Vin(max) = Maximum DC Input Voltage

ILoad(max) = Maximum Load Current

Given Parameters:

Vout = 8.0 V

Vin(max) = 25 V

ILoad(max) = 2.5 A

1. Programming Output V oltage

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).

Vout

Vref

1.0

)

Vout

Vref –1.0

where V ref = 1.23 V

1. Programming Output V oltage (selecting R1 and R2)

Select R1 and R2:

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

Vout

Vref

1.0

Ǔ+

1.8 k

8.0 V

1.23 V

1.0

Vout

1.23

1.0

)

Select R1 = 1.8 kΩ

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.

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.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.

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.

LM2576

12 MOTOROLA ANALOG IC DEVICE DATA

Procedure (Adjustable Output Version:

LM2576–ADJ) (continued)

Procedure Example

4. Inductor Selection (L1)

A.Use the following formula to calculate the inductor V olt x

microsecond [V x µs] constant:

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

ILoad.

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.

ExT

in –V

out

Vout

Vin x106

F[Hz] [V x

Ip(max)

ILoad(max)

)ǒ

Vin –V

out

ton

Vout

Vin x1.0

fosc

4. Inductor Selection (L1)

A.Calculate E x T [V x µs] constant:

B.E x T = 80 [V x µs]

C.ILoad(max) = 2.5 A

Inductance Region = H150

D.Proper inductor value = 150 µH

Choose the inductor from Table 2.

ExT

(

25 – 8.0)x8.0

25 x1000

80 [V x

5. Output Capacitor Selection (Cout)

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.

Cout

13,300 Vin(max)

Vout xL[µH] [µF]

5. Output Capacitor Selection (Cout)

To achieve an acceptable ripple voltage, select

Cout = 680 µF electrolytic capacitor.

Cout

13,300 x 25

8 x 150

332.5 µF

LM2576

MOTOROLA ANALOG IC DEVICE DATA

ET, VOLTAGE TIME (V s)

100

3.02.51.50.80.50.4 0.6 1.0 2.0

5.0

8.0

7.0

6.0

MAXIMUM INPUT VOL TAGE (V)MAXIMUM INPUT VOL TAGE (V)

IL, MAXIMUM LOAD CURRENT (A)

MAXIMUM INPUT VOL TAGE (V)

IL, MAXIMUM LOAD CURRENT (A)

MAXIMUM INPUT VOL TAGE (V)

Figure 18. LM2576–3.3

IL, MAXIMUM LOAD CURRENT (A)

Figure 19. LM2576–5

L680

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

Figure 22. LM2576–ADJ

LM2576 Series Buck Regulator Design Procedures (continued)

Indicator Value Selection Guide (For Continuous Mode Operation)

9.0

8.0

7.0

300

3.02.51.50.80.50.3 0.3

3.00.80.60.50.40.3

3.02.01.50.50.3

L330

L470

L150

L220

0.4 0.6 1.0 2.0

L100 L68

L47

H470H1000 H680 H220H330 H150

L680

L330

L470

L150

L220

L100 L68 L47

1.2

H470

H1000 H680 H220

H330 H150

L680 L330

L470

L150

L220 L100 L68

H1500

1.0 1.5 2.0 2.5

H470

H1000 H680 H220

H330 H150

L680 L330 L150

L220 L100 L68

H1500

3.00.80.60.50.40.3 1.0 1.5 2.0 2.5

L470

H680

H220

H330 H150

L680 L330 L150

L220 L100 L68

H1500

L470

H2000

L47

H470

H1000

150

200

250

0.4 0.6 0.8 1.0 2.5

LM2576

14 MOTOROLA ANALOG IC DEVICE DATA

Table 1. Diode Selection Guide

Schottky Fast Recovery

3.0 A 4.0 – 6.0 A 3.0 A 4.0 – 6.0 A

VRThrough

Hole Surface

Mount Through

Hole Surface

Mount Through

Hole Surface

Mount Through

Hole Surface

Mount

20 V 1N5820

MBR320P

SR302

SK32 1N5823

SR502

SB520

30 V 1N5821

MBR330

SR303

31DQ03

SK33

30WQ03 1N5824

SR503

SB530

50WQ03

MUR320

31DF1

HER302

MURS320T3

MURD320

30WF10

MUR420

HER602 MURD620CT

50WF10

40 V 1N5822

MBR340

SR304

31DQ04

SK34

30WQ04

MBRS340T3

MBRD340

1N5825

SR504

SB540

MBRD640CT

50WQ04

HER302

(all diodes

rated

to at least

100 V)

30WF10

(all diodes

rated

to at least

100 V)

(all diodes

rated

to at least

100 V)

(all diodes

rated

to at least

100 V)

50 V MBR350

31DQ05

SR305

SK35

30WQ05 SB550 50WQ05

100

60 V MBR360

DQ06

SR306

MBRS360T3

MBRD360 50SQ080 MBRD660CT

NOTE: Diofes listed inbold are available from Motorola.

Table 2. Inductor Selection by Manufacturer’s Part Number

Inductor

Code Inductor

Value Tech 39 Schott Corp. Pulse Eng. Renco

L47 47 µH 77 212 671 26980 PE–53112 RL2442

L68 68 µH77 262 671 26990 PE–92114 RL2443

L100 100 µH77 312 671 27000 PE–92108 RL2444

L150 150 µH77 360 671 27010 PE–53113 RL1954

L220 220 µH77 408 671 27020 PE–52626 RL1953

L330 330 µH77 456 671 27030 PE–52627 RL1952

L470 470 µH * 671 27040 PE–53114 RL1951

L680 680 µH77 506 671 27050 PE–52629 RL1950

H150 150 µH77 362 671 27060 PE–53115 RL2445

H220 220 µH77 412 671 27070 PE–53116 RL2446

H330 330 µH77 462 671 27080 PE–53117 RL2447

H470 470 µH * 671 27090 PE–53118 RL1961

H680 680 µH77 508 671 27100 PE–53119 RL1960

H1000 1000 µH77 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

LM2576

MOTOROLA ANALOG IC DEVICE DATA

Table 3. Example of Several Inductor Manufacturers Phone/Fax Numbers

Pulse Engineering, Inc. Phone

Fax + 1–619–674–8100

+ 1–619–674–8262

Pulse Engineering, Inc. Europe Phone

Fax + 353–9324–107

+ 353–9324–459

Renco Electronics, Inc. Phone

Fax + 1–516–645–5828

+ 1–516–586–5562

Tech 39 Phone

Fax + 33–1–4115–1681

+ 33–1–4709–5051

Schott Corporation Phone

Fax + 1–612–475–1 173

+ 1–612–475–1786

EXTERNAL COMPONENTS

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 Cin

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: Irms > 1.2 x d x ILoad

where d is the duty cycle, for a buck regulator

ton

Vout

Vin

and d

ton

|Vout|

)

Vin for a buck

boost regulator.

Output Capacitor (Cout)

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

LM2576

16 MOTOROLA ANALOG IC DEVICE DATA

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. V oltage 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

through

22). This guide assumes that

the regulator is operating in the continuous mode, and

selects an inductor that will allow a peak–to–peak inductor

ripple current to be a certain percentage of the maximum

design load current. This percentage is allowed to change as

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.

HORIZONTAL TIME BASE: 5.0

s/DIV

Figure 23. Continuous Mode Switching Current

Waveforms

VERTRICAL RESOLUTION 1.0 A/DIV

2.0 A

0 A

2.0 A

0 A

Inductor

Current

Waveform

Power

Switch

Current

Waveform

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.

LM2576

MOTOROLA ANALOG IC DEVICE DATA

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.

0.4 A

0 A

0.4 A

0 A

Inductor

Current

Waveform

Power

Switch

Current

Waveform

Figure 24. Discontinuous Mode Switching Current

Waveforms

VERTICAL RESOLUTION 200 mA/DIV

HORIZONTAL TIME BASE: 5.0

s/DIV

GENERAL RECOMMENDATIONS

Output Voltage 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.

Unfiltered

Output

Voltage

Filtered

Output

Voltage

HORIZONTAL TIME BASE: 5.0

s/DIV

Figure 25. Output Ripple Voltage Waveforms

VERTRICAL

V oltage spikes

caused by

switching action

of the output

switch and the

parasitic

inductance of the

output capacitor

RESOLUTION

20 mV/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

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’).

LM2576

18 MOTOROLA ANALOG IC DEVICE DATA

Thermal Analysis and Design

The following procedure must be performed to determine

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

1. PD(max) maximum regulator power dissipation in the

application.

2. TA(max) maximum ambient temperature in the

application.

3. TJ(max) maximum allowed junction temperature

(125°C for the LM2576). For a conservative

design, the maximum junction temperature

should not exceed 110°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. RθJC package thermal resistance junction–case.

5. RθJA package thermal resistance junction–ambient.

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

sheet or RθJC and RθJA values).

The following formula is to calculate the approximate total

power dissipated by the LM2576:

PD = (Vin x IQ) + d x ILoad x Vsat

where d is the duty cycle and for buck converter

ton

Vin,

IQ(quiescent current) and Vsat can be found in the

LM2576 data sheet,

Vin is minimum input voltage applied,

VOis the regulator output voltage,

ILoadis the load current.

The dynamic switching losses during turn–on and turn–of f

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: TJ = (RθJA) (PD) + TA

where (RθJA)(PD) represents the junction temperature rise

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:

TJ = PD (RθJA + RθCS + RθSA) + TA

where RθJC is the thermal resistance junction–case,

RθCS is the thermal resistance case–heatsink,

RθSA is the thermal resistance heatsink–ambient.

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

1N5822

Output

Feedback

12 to 40 V

Unregulated

DC Input

Cin

100

53ON

/OFFGnd

+Vin

–12 V @ 0.7 A

Regulated

Output

Cout

2200

LM2576–12

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.

LM2576

MOTOROLA ANALOG IC DEVICE DATA

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 Cout.

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:

where ton

|VO|

Vin

)

|VO|x1.0

fosc, and fosc

52 kHz.

Ipeak

[

ILoad (Vin

)

|VO|)

Vin

)

Vin xt

2L1

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

1N5822

Output

Feedback

12 V to 25 V

Unregulated

DC Input

Cin

100

/50 V

35ON

/OFF Gnd

+Vin

–12 V @ 700 m A

Regulated

Output

Cout

2200

/16 V

LM2576–12

0.1

47 k R2

47 k

Figure 28. Inverting Buck–Boost Regulator Shutdown

Circuit Using an Optocoupler

LM2576–XX

35 GndON/OFF

+Vin

47 k

Cin

100

NOTE: This picture does not show the complete circuit.

47 k

470

Shutdown

Input

MOC8101

–Vout

Off

5.0 V

+Vin

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

and

29.

LM2576

20 MOTOROLA ANALOG IC DEVICE DATA

Figure 29. Inverting Buck–Boost Regulator Shutdown

Circuit Using a PNP Transistor

NOTE: This picture does not show the complete circuit.

5.6 k

2N3906

LM2576–XX

35 GndON/OFF

12 k –Vout

+Vin

Shutdown

Input

Off

+Vin

Cin

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

1N5820

100

Output

Feedback

Vout = –12 V

T ypical Load Current

400 mA for Vin = –5.2 V

750 mA for Vin = –7.0 V

–5.0 V to –12 V

Cout

2200

Low Esr

Cin

100

LM2576–12

53 ON/OFFGnd

Vin

Design Recommendations:

The same design rules as for the previous inverting

buck–boost converter can be applied. The output capacitor

Cout must be chosen larger than would be required for a what

standard buck converter. 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 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

47 k

LM2576–XX

35 GndON/OFF

47 k

+Vin

0.1

Cin

100

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

LM2576

MOTOROLA ANALOG IC DEVICE DATA

predetermined threshold level with respect to the ground

Pin 3, which is determined by the following expression:

Vth

[

VZ1

)ǒ

1.0

)

VBE (Q1)

Figure 32. Undervoltage Lockout Circuit for

Buck Converter

10 k

1N5242B

10 k

2N3904

47 k

Vth

≈

13 V

Cin

100

LM2576–XX

35 GndON/OFF

+Vin

NOTE: This picture does not show the complete circuit.

The following formula is used to obtain the peak inductor

current:

where ton

|VO|

Vin

)

|VO|x1.0

fosc, and fosc

52 kHz.

Ipeak

[

ILoad (Vin

)

|VO|)

Vin

)

Vin xt

2L1

Under normal continuous inductor current operating

conditions, the worst case occurs when Vin is minimal.

Figure 33. Undervoltage Lockout Circuit for

Buck–Boost Converter

15 k

1N5242B

15 k

2N3904

47 k

Vth

≈

13 V

Cin

100

LM2576–XX

35 GndON/OFF

+Vin

Vout

NOTE: This picture does not show the complete circuit.

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.

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

1N5822

150

Output

Feedback

50 k

1.21 k

HOutput

Voltage

1.2 to 35 V @ 3.0 A

Optional Output

Ripple Filter

40 V Max

Unregulated

DC Input

Cout

2200

FC1

100

Cin

100

LM2574–Adj

53ON

/OFFGnd

+Vin

LM2576

22 MOTOROLA ANALOG IC DEVICE DATA

THE LM2576–5 STEP–DOWN VOLTAGE REGULATOR 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

1N5822

150

Output

Feedback

Unregulated

DC Input

+Vin = 7.0 to 40 V

Cout

1000

/16 V

100

/50 V

LM2576–5

53ON

/OFFGnd

+Vin

ON/OFF

Regulated Output

Vout1 = 5.0 V @ 3.0 A

Gndin Gndout

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 Figure 37. Printed Circuit Board Layout

Copper Side

LM2576

C1 C2

Vou

Gndout

Gndin

+Vin

ON/OFF

00060_00

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

LM2576

MOTOROLA ANALOG IC DEVICE DATA

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

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

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

Vref = 1.23 V

R1 is between 1.0 k and 5.0 k

1N5822

150

Output

2R2

10 k

1.8 k

Regulated

Output Filtered

Vout2 = 8.0 V @ 3.0 A

Unregulated

DC Input

1000

/16 V

100

/50 V

LM2576–ADJ

53ON/OFFGnd

+Vin

+Vin = 10 V to 40 V

4 Feedback

Vout

Vref

)ǒ

1.0

)

ON/OFF

Figure 39. Printed Circuit Board Layout

Component Side Figure 40. Printed Circuit Board Layout

Copper Side

LM2576

D1 R1

+C2

ON/OFF

Vout

Gndout

Gndin

+Vin

00059_00

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

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

LM2576

24 MOTOROLA ANALOG IC DEVICE DATA

T SUFFIX

PLASTIC PACKAGE

CASE 314D–03

ISSUE D

TV SUFFIX

PLASTIC PACKAGE

CASE 314B–05

ISSUE J

OUTLINE DIMENSIONS

–Q–

12345

5 PL

0.356 (0.014) T

SEATING

PLANE

–T–

DIM MIN MAX MIN MAX

MILLIMETERSINCHES

A0.572 0.613 14.529 15.570

B0.390 0.415 9.906 10.541

C0.170 0.180 4.318 4.572

D0.025 0.038 0.635 0.965

E0.048 0.055 1.219 1.397

G0.067 BSC 1.702 BSC

H0.087 0.112 2.210 2.845

J0.015 0.025 0.381 0.635

K1.020 1.065 25.908 27.051

L0.320 0.365 8.128 9.271

Q0.140 0.153 3.556 3.886

U0.105 0.117 2.667 2.972

S0.543 0.582 13.792 14.783

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.

–P–

0.10 (0.254) P M

5X J

0.24 (0.610) T

OPTIONAL

CHAMFER

SLW

–T–

SEATING

PLANE

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 0.043 (1.092) MAXIMUM.

DIM MIN MAX MIN MAX

MILLIMETERSINCHES

A0.572 0.613 14.529 15.570

B0.390 0.415 9.906 10.541

C0.170 0.180 4.318 4.572

D0.025 0.038 0.635 0.965

E0.048 0.055 1.219 1.397

F0.850 0.935 21.590 23.749

G0.067 BSC 1.702 BSC

H0.166 BSC 4.216 BSC

J0.015 0.025 0.381 0.635

K0.900 1.100 22.860 27.940

L0.320 0.365 8.128 9.271

N0.320 BSC 8.128 BSC

Q0.140 0.153 3.556 3.886

S––– 0.620 ––– 15.748

U0.468 0.505 11.888 12.827

V––– 0.735 ––– 18.669

W0.090 0.110 2.286 2.794

5X D

LM2576

MOTOROLA ANALOG IC DEVICE DATA

D2T SUFFIX

PLASTIC PACKAGE

CASE 936A–02

(D2PAK)

ISSUE A

OUTLINE DIMENSIONS

5 REF

123

TERMINAL 6

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.

DIM

AMIN MAX MIN MAX

MILLIMETERS

0.386 0.403 9.804 10.236

INCHES

B0.356 0.368 9.042 9.347

C0.170 0.180 4.318 4.572

D0.026 0.036 0.660 0.914

E0.045 0.055 1.143 1.397

G0.067 BSC 1.702 BSC

H0.539 0.579 13.691 14.707

K0.050 REF 1.270 REF

L0.000 0.010 0.000 0.254

M0.088 0.102 2.235 2.591

N0.018 0.026 0.457 0.660

P0.058 0.078 1.473 1.981

R5 REF

S0.116 REF 2.946 REF

U0.200 MIN 5.080 MIN

V0.250 MIN 6.350 MIN

0.010 (0.254) T

–T–

OPTIONAL

CHAMFER

LM2576

26 MOTOROLA ANALOG IC DEVICE DATA

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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”

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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/Af firmative Action Employer .

LM2576

MOTOROLA ANALOG IC DEVICE DATA

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