Device

Operating

Temperature Range

Package

LM2574

SEMICONDUCTOR

TECHNICAL DATA

EASY SWITCHER

0.5 A STEPDOWN

VOLTAGE REGULATOR

ORDERING INFORMATION

LM2574NXX TA = 40

to +125

DIP8

N SUFFIX

PLASTIC PACKAGE

CASE 626

PIN CONNECTIONS

Order this document by LM2574/D

DEVICE TYPE/NOMINAL OUTPUT VOLTAGE

LM25743.3
LM25745
LM257412
LM257415
LM2574ADJ

3.3 V
5.0 V

12 V
15 V

1.23 V to 37 V

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

* No internal connection, but should be soldered to

PC board for best heat transfer.

(Top View)

Sig Gnd

ON/OFF

Pwr Gnd

Output
*

Vin

Advance Information

Easy Switcher

0.5 A

Step-Down Voltage Regulator

The LM2574 series of regulators are monolithic integrated circuits ideally

suited for easy and convenient design of a stepdown switching regulator
(buck converter). All circuits of this series are capable of driving a 0.5 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 LM2574 are offered by several different
inductor manufacturers.

Since the LM2574 converter is a switchmode power supply, its efficiency

is significantly higher in comparison with popular threeterminal linear
regulators, especially with higher input voltages. In most cases, the power
dissipated by the LM2574 regulator is so low, that the copper traces on the
printed circuit board are normally the only heatsink needed and no additional
heatsinking is required.

The LM2574 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 60

A (typical) standby current. The output switch

includes cyclebycycle 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% max over

Line and Load Conditions

Guaranteed 0.5 A Output Current

Wide Input Voltage Range: 4.75 to 40 V

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 and HighEfficiency StepDown (Buck) Regulators

Efficient Preregulator for Linear Regulators

OnCard Switching Regulators

Positive to Negative Converters (BuckBoost)

Negative StepUp 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

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Figure 1. Block Diagram and Typical Application

7.0 40 V

Unregulated

DC Input

330

Pwr
Gnd

+Vin

Cin

ON/OFF

Output

Feedback

D1
1N5819

Cout

220

Typical Application (Fixed Output Voltage Versions)

Representative Block Diagram and Typical Application

Unregulated

DC Input

+Vin

Cout

Feedback

Cin

R1
1.0 k

Output

7
Pwr Gnd

ON/OFF

Reset

Latch

Thermal

Shutdown

52 kHz

Oscillator

1.235 V

BandGap

Reference

Freq

Shift

18 kHz

Comparator

Fixed Gain
Error Amplifier

Current

Limit

Driver

1.0 Amp

Switch

ON/OFF

3.1 V Internal

Regulator

Vout

Load

Output

Voltage Versions

3.3 V
5.0 V

12 V
15 V

(

)

1.7 k
3.1 k

8.84 k
11.3 k

For adjustable version
R1 = open, R2 = 0

LM2574

5.0 V Regulated
Output 0.5 A Load

Sig
Gnd

Sig Gnd

ABSOLUTE MAXIMUM RATINGS

(Absolute Maximum Ratings indicate limits beyond which

damage to the device may occur).

Rating

Symbol

Value

Unit

Maximum Supply Voltage

Vin

ON/OFF Pin Input Voltage

0.3 V

+Vin

Output Voltage to Ground (Steady State)

1.0

Power Dissipation

Internally Limited

Thermal Resistance, JunctiontoAmbient

100

C/W

Thermal Resistance, JunctiontoCase

5.0

C/W

Storage Temperature Range

Tstg

C to +150

Minimum ESD Rating

2.0

(Human Body Model: C = 100 pF, R = 1.5 k

)

Lead Temperature (Soldering, 10 seconds)

260

Maximum Junction Temperature

150

NOTE: ESD data available upon request.

LM2574

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

40 to +125

Supply Voltage

Vin

SYSTEM PARAMETERS

([Note 1] Test Circuit Figure 16)

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, Vin = 30 V for the 15 V version. ILoad = 100 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).

Characteristic

Symbol

Min

Typ

Max

Unit

LM25743.3 ([Note 1] Test Circuit

Figure 16)

Output Voltage (Vin = 12 V, ILoad = 100 mA, TJ = 25

Vout

3.234

3.3

3.366

Output Voltage (4.75 V

Vin

40 V, 0.1 A

ILoad

0.5 A)

Vout

TJ = 25

3.168

3.3

3.432

TJ = 40 to +125

3.135

3.465

Efficiency (Vin = 12 V, ILoad = 0.5 A)

LM25745 ([Note 1] Test Circuit Figure 16)

Output Voltage (Vin = 12 V, ILoad = 100 mA, TJ = 25

Vout

4.9

5.0

5.1

Output Voltage (7.0 V

Vin

40 V, 0.1 A

ILoad

0.5 A)

Vout

J = 25

4.8

5.0

5.2

J = 40 to +125

4.75

5.25

Efficiency (Vin = 12 V, ILoad = 0.5 A)

LM257412 ([Note 1] Test Circuit Figure 16)

Output Voltage (Vin = 25 V, ILoad = 100 mA, TJ = 25

Vout

11.76

12.24

Output Voltage (15 V

Vin

40 V, 0.1 A

ILoad

0.5 A)

Vout

J = 25

11.52

12.48

J = 40 to +125

11.4

12.6

Efficiency (Vin = 15 V, ILoad = 0.5 A)

LM257415 ([Note 1] Test Circuit Figure 16)

Output Voltage (Vin = 30 V, ILoad = 100 mA, TJ = 25

Vout

14.7

15.3

Output Voltage (18 V < Vin < 40 V, 0.1 A < ILoad < 0.5 A)

Vout

TJ = 25

14.4

15.6

TJ = 40 to +125

14.25

15.75

Efficiency (Vin = 18 V, ILoad = 0.5 A)

LM2574 ADJUSTABLE VERSION ([Note 1] Test Circuit

Figure 16)

Feedback Voltage Vin = 12 V, ILoad = 100 mA, Vout = 5.0 V, TJ = 25

VFB

1.217

1.23

1.243

Feedback Voltage 7.0 V

Vin

40 V, 0.1 A

ILoad

0.5 A, Vout = 5.0 V

VFBT

TJ = 25

1.193

1.23

1.267

TJ = 40 to +125

1.18

1.28

Efficiency (Vin = 12 V, ILoad = 0.5 A, Vout = 5.0 V)

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

performance. When the LM2574 is used as shown in the Figure 16 test circuit, the system performance will be as
shown in the system parameters section of the Electrical Characteristics.

2. Tested junction temperature range for the LM2574: Tlow = 40

C Thigh = +125

LM2574

MOTOROLA ANALOG IC DEVICE DATA

SYSTEM PARAMETERS

([Note 1] Test Circuit Figure 16)

ELECTRICAL CHARACTERISTICS

(continued) (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, Vin = 30 V for the 15 V version. ILoad = 100 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).

Characteristic

Unit

Max

Typ

Min

Symbol

ALL OUTPUT VOLTAGE VERSIONS

Feedback Bias Current Vout = 5.0 V (Adjustable Version Only)

TJ = 25

100

TJ = 40 to +125

200

Oscillator Frequency (Note 3)

kHz

TJ = 25

TJ = 0 to +125

TJ = 40 to +125

Saturation Voltage (Iout = 0.5 A, [Note 4])

Vsat

TJ = 25

1.0

1.2

TJ = 40 to +125

1.4

Max Duty Cycle ("on") [Note 5]

Current Limit Peak Current (Notes 3 and 4)

ICL

TJ = 25

0.7

1.0

1.6

TJ = 40 to +125

0.65

1.8

Output Leakage Current (Notes 6 and 7), TJ = 25

Output = 0 V

0.6

2.0

Output = 1.0 V

Quiescent Current (Note 6)

TJ = 25

5.0

9.0

TJ = 40 to +125

Standby Quiescent Current (ON/OFF Pin = 5.0 V ("off"))

Istby

TJ = 25

200

TJ = 40 to +125

400

ON/OFF Pin Logic Input Level

Vout = 0 V

VIH

TJ = 25

2.2

1.4

TJ = 40 to +125

2.4

Nominal Output Voltage

VIL

TJ = 25

1.2

1.0

TJ = 40 to +125

0.8

ON/OFF Pin Input Current

ON/OFF Pin = 5.0 V ("off"), TJ = 25

IIH

ON/OFF Pin = 0 V ("on"), TJ = 25

IIL

5.0

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

performance. When the LM2574 is used as shown in the Figure 16 test circuit, the system performance will be as
shown in the system parameters section of the Electrical Characteristics.

2. Tested junction temperature range for the LM2574: Tlow = 40

C Thigh = +125

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 power 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 the 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.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

I stby

, ST

ANDBY

QUIESCENT

CURRENT

(

I Q

, QUIESCENT

CURRENT

(mA)

V out

,
OUTPUT

VOL

T
AGE CHANGE (%)

TJ, JUNCTION TEMPERATURE (

I O

, OUTPUT

CURRENT

(A)

TJ, JUNCTION TEMPERATURE (

Vin, INPUT VOLTAGE (V)

INPUT

OUTPUT

DIFFERENTIAL

(V)

TJ, JUNCTION TEMPERATURE (

V out

,
OUTPUT

VOL

T
AGE CHANGE (%)

Figure 2. Normalized Output Voltage

TJ, JUNCTION TEMPERATURE (

Figure 3. Line Regulation

Vin = 20 V

ILoad = 100 mA

Normalized at TJ = 25

Figure 4. Dropout Voltage

Figure 5. Current Limit

Figure 6. Quiescent Current

Figure 7. Standby Quiescent Current

ILoad = 100 mA

TJ = 25

3.3 V, 5.0 V and ADJ

12 V and 15 V

Vin = 25 V

ILoad = 100 mA

ILoad = 500 A

Vin = 12 V

Vin = 40 V

L = 300

ILoad = 500 mA

ILoad = 100 mA

Vout = 5.0 V

Measured at
Ground Pin
TJ = 25

VON/OFF = 5.0 V

TYPICAL PERFORMANCE CHARACTERISTICS

(Circuit of Figure 16)

1.0

0.8

0.6

0.4

0.2

0.4

0.6

0.8

1.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.4

0.6

2.0

1.5

1.0

0.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

8.0

6.0

4.0

200

180

160

140

120

100

125

100

5.0

125

100

125

100

5.0

125

100

LM2574

MOTOROLA ANALOG IC DEVICE DATA

, INPUT

VOL

T
AGE (V)

sat

, SA

TURA

TION VOL

T
AGE

(V)

I FB

, FEEDBACK PIN CURRENT

(nA)

s/DIV

TJ, JUNCTION TEMPERATURE (

SWITCH CURRENT (A)

s/DIV

TJ, JUNCTION TEMPERATURE (

NORMALIZED FREQUENCY

(%)

Figure 8. Oscillator Frequency

TJ, JUNCTION TEMPERATURE (

Figure 9. Switch Saturation Voltage

Figure 10. Minimum Operating Voltage

Figure 11. Feedback Pin Current

Figure 12. Continuous Mode Switching Waveforms

Vout = 5.0 V, 500 mA Load Current, L = 330

Figure 13. Discontinuous Mode Switching Waveforms

Vout = 5.0 V, 100 mA Load Current, L = 100

Vin = 1.23 V

ILoad = 100 mA

Adjustable Version Only

Vin = 12 V

Normalized at 25

Adjustable Version Only

A: Output Pin Voltage, 10 V/DIV.
B: Inductor Current, 0.2 A/DIV.
C: Output Ripple Voltage, 20 mV/DIV, ACCoupled

TYPICAL PERFORMANCE CHARACTERISTICS

(Circuit of Figure 16) (continued)

8.0

6.0

4.0

2.0

4.0

6.0

8.0

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

100

125

100

0.1

0.2

0.3

0.4

0.5

125

100

125

100

20 V

10 V

0.6 A

0.4 A

0.2 A

20 mV

20 V

10 V

0.6 A

0.4 A

0.2 A

20 mV

125

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Figure 16. Test Circuit and Layout Guidelines

D1
1N5819

330

Output

Feedback

Cout

220

Cin

LM2574

Fixed Output

ON/OFF

Pwr
Gnd

Vin

Load

Vout

D1
1N5819

330

Output

Feedback

Cout

220

Cin

LM2574

Adjustable

Vin

Load

Vout

5.0 V

Fixed Output Voltage Versions

Adjustable Output Voltage Versions

Vout

ref

1.0

)

R2
R1

Vout

ref

1.0

Where Vref = 1.23 V, R1

between 1.0 k

and 5.0 k

R2
6.12 k

R1
2.0 k

7.0 40 V

Unregulated

DC Input

Sig
Gnd

ON/OFF

Pwr
Gnd

Sig
Gnd

7.0 V 40 V
Unregulated

DC Input

Cin 22

F, 60 V, Aluminium Electrolytic

Cout 220

F, 25 V, Aluminium Electrolytic

Schottky, 1N5819

330

H, (For 5.0 Vin, 3.3 Vout, use 100

2.0 k, 0.1%

6.12 k, 0.1%

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
g e n e r a t e v o l t a g e t r a n s i e n t s w h i c h c a n g e n e r a t e
electromagnetic interferences (EMI) and affect the desired
operation. As indicated in the Figure 16, 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, singlepoint grounding (as indicated) or

ground plane construction should be used.

On the other hand, the PCB area connected to the Pin 7

(emitter of the internal switch) of the LM2574 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
LM2574 regulator.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

PIN FUNCTION DESCRIPTION

Pin

Symbol

Description (Refer to Figure 1)

Vin

This pin is the positive input supply for the LM2574 stepdown 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).

Output

This is the emitter of the internal switch. The saturation voltage Vsat of this output switch is typically 1.0 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.

Sig Gnd

Circuit signal ground pin. See the information about the printed circuit board layout.

Pwr Gnd

Circuit power ground pin. See the information about the printed circuit board layout.

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 noninverting input of the internal error amplifier.
In the Adjustable version of the LM2574 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.

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

A. The input threshold voltage is typically 1.5 V. Applying a

voltage above this value (up to +Vin) shuts the regulator off. If the voltage applied to this pin is lower than
1.5 V or if this pin is left open, the regulator will be in the "on" condition.

DESIGN PROCEDURE

Buck Converter Basics

The LM2574 is a "Buck" or StepDown Converter which is

the most elementary forwardmode converter. Its basic
schematic can be seen in

Figure 17.

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:

L(on)

Vout 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 17. Basic Buck Converter

Vin

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. 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:

L(off)

Vout VD toff

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

, where T is the period of switching.

For the buck converter with ideal components, the duty

cycle can also be described as:

Vout

Figure 18 shows the buck converter idealized waveforms

of the catch diode voltage and the inductor current.

Figure 18. Buck Converter Idealized Waveforms

Power

Switch

Power

Switch

Off

Power

Switch

Off

Power

Switch

Power

Switch

Von(SW)

VD(FWD)

Time

ILoad(AV)

Imin

Ipk

Diode

Power

Switch

Diode V

oltage

Inductor Current

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Procedure

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

design procedure and example is 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) = 0.4 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 LM25745 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 22

F, 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
LM2574 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 1.0 A.

B. Use a 20 V 1N5817 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 19 to 23.

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 "EXTERNAL COMPONENTS" section of this
data sheet.

(

max

)

Load

(

max

)

out

1.0

osc

4. Inductor Selection (L1)

A. Use the inductor selection guide shown in Figure 20.

B. From the selection guide, the inductance area intersected

by the 15 V line and 0.4 A line is 330.

C. Inductor value required is 330

H. From Table 2, choose an

inductor from any of the listed manufacturers.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Procedure

(Fixed Output Voltage Version) (continued) In order to simplify the switching regulator design, a stepbystep

design procedure and example is provided.

Procedure

Example

5. Output Capacitor Selection (Cout)

A. Since the LM2574 is a forwardmode switching regulator

with voltage mode control, its open loop 2pole1zero
frequency characteristic has the dominant polepair
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
100

F and 470

F 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 = 100

F to 470

F standard aluminium electrolytic.

B. Capacitor voltage rating = 20 V.

Procedure

(Adjustable Output Version: LM2574ADJ)

Procedure

Example

Given Parameters:

Vout = Regulated Output Voltage
Vin(max) = Maximum DC Input Voltage
ILoad(max) = Maximum Load Current

Given Parameters:

Vout = 24 V
Vin(max) = 40 V
ILoad(max) = 0.4 A

1. Programming Output Voltage

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

where

Vref = 1.23 V

Resistor R1 can be between 1.0 k

and 5.0 k

. (For best

temperature coefficient and stability with time, use 1% metal
film resistors).

Vout

ref

1.0

)

R2
R1

Vout

ref

1.0

1. Programming Output Voltage

(selecting R1 and R2)

Select R1 and R2 :

Vout = 1.23

Select R1 = 1.0 k

R2 = 18.51 k

, choose a 18.7 k

metal film resistor.

1.0

)

R2
R1

Vout

ref

1.0

1.0 k

10 V

1.23 V

1.0

2. Input Capacitor Selection (Cin)

A 22

F 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
LM2574 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 1.0 A current rating is adequate.

B. Use a 50 V MBR150 Schottky diode or any suggested fast

recovery diodes in Table 1.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Procedure

(Adjustable Output Version: LM2574ADJ)

Procedure

Example

4. Inductor Selection (L1)

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

microsecond [V x

s] 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 23. 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 27.

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.

out

1.0

osc

(

max

)

Load

(

max

)

out

E x T

Vout)

Vout

106

F[Hz]

V x

4. Inductor Selection (L1)

C. ILoad(max) = 0.4 A

Inductance Region = 1000

D. Proper inductor value = 1000

Choose the inductor from

Table 2.

E x T

(40

24) x 24

1000

105 V x

E x T

185 V x

Calculate E x T V x

s constant :

5. Output Capacitor Selection (Cout)

A. Since the LM2574 is a forwardmode switching regulator with

voltage mode control, its open loop 2pole1zero frequency
characteristic has the dominant polepair determined by the
output capacitor and inductor values.

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

B. Capacitor values between 10

F and 2000

F 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 16V rating is recommended.

Cout

13, 300

(

max

)

Vout x L

5. Output Capacitor Selection (Cout)

To achieve an acceptable ripple voltage, select
Cout = 100

F electrolytic capacitor.

Cout

13, 300 x

24 x 1000

22.2

LM2574

MOTOROLA ANALOG IC DEVICE DATA

, VOL

T
AGE

TIME (V s)

, MAXIMUM INPUT

VOL

T
AGE (V)

, MAXIMUM INPUT

VOL

T
AGE (V)

, MAXIMUM INPUT

VOL

T
AGE (V)

, MAXIMUM INPUT

VOL

T
AGE (V)

IL, MAXIMUM LOAD CURRENT (A)

Figure 19. LM25743.3

IL, MAXIMUM LOAD CURRENT (A)

Figure 20. LM25745

680

Figure 21. LM257412

Figure 22. LM257415

Figure 23. LM2574ADJ

150

470

220

100

330

1000

330

680

470

150

220

2200

470

1500

1000

330

680

220

2200

470

1500

1000

680

2200

470

1500

1000

330

680

220

150

100

LM2574 Series Buck Regulator Design Procedures

(continued)

Indicator Value Selection Guide (For Continuous Mode Operation)

60
20

12
10

9.0
8.0
7.0

6.0

5.0

9.0

8.0

7.0

40
30
25

18
17

250
200
150

100

60
50
40

20
15

0.5

0.4

0.3

0.2

0.15

0.1

0.5

0.4

0.3

0.2

0.15

0.1

0.5

0.4

0.3

0.2

0.15

0.1

0.5

0.4

0.3

0.2

0.15

0.1

0.5

0.4

0.3

0.2

0.15

0.1

330

220

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Table 1. Diode Selection Guide gives an overview about throughhole diodes

for an effective design. Device listed in bold are available from Motorola

1.0 Amp Diodes

Schottky

Fast Recovery

20 V

1N5817

MBR120P

30 V

1N5818

MBR130P

MUR110

40 V

1N5819

MBR140P

MUR110

(rated to 100 V)

50 V

MBR150

60 V

MBR160

Table 2. Inductor Selection Guide

Inductor

Value

Pulse Engineering

Tech 39

Renco

NPI

55 258 SN

RL128468

NP5915

100

55 308 SN

RL1284100

NP5916

150

52625

55 356 SN

RL1284150

NP5917

220

52626

55 406 SN

RL1284220

NP5918/5919

330

52627

55 454 SN

RL1284330

NP5920/5921

470

52628

RL1284470

NP5922

680

52629

55 504 SN

RL1284680

NP5923

1000

52631

55 554 SN

RL12841000

1500

RL12841500

2200

RL12842200

* : Contact Manufacturer

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

Pulse Engineering Inc.

Phone
Fax

16196748100

16196748262

Pulse Engineering Inc. Europe

Phone
Fax

3539324107

3539324459

Renco Electronics Inc.

Phone
Fax

15166455828

15165865562

Tech 39

Phone
Fax

33141151681

33147095051

NPI/APC

Phone
Fax

+ 44634290588

LM2574

MOTOROLA ANALOG IC DEVICE DATA

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 consequences 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 I

Load

where d is the duty cycle, for a continuous mode buck
regulator

ton

Vout

and d

ton

|Vout|

)

for a buckboost 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
peaktopeak 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

0.03

), 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
peaktopeak inductor ripple current.

Catch Diode
Locate the Catch Diode Close to the LM2574

The LM2574 is a stepdown 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 LM2574 using short leads and short printed circuit traces
to avoid EMI problems.

Use a Schottky or a Soft Switching
UltraFast Recovery Diode

Since the rectifier diodes are very significant source 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
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 FastRecovery, or UltraFast 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 fastrecovery diode with soft recovery characteristics

can better fulfill some quality, low noise design requirements.
Table 1 provides a list of suitable diodes for the LM2574
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
have 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 (ElectroMagnetic
Interference) problems.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

Continuous and Discontinuous Mode of Operation.

The LM2574 stepdown 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 24 and Figure 25). 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 LM2574 regulator was added to this
data sheet (Figures 19 through 23). This guide assumes that
the regulator is operating in the continuous mode, and
selects an inductor that will allow a peaktopeak 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 0.2 A) 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 peaktopeak
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.

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 (ElectroMagnetic Interference) shielding
that the core must provide. There are many different styles of
inductors available, such as pot core, Ecore, 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 Ecore (closed magnetic structure) should be
used in such applications.

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 LM2574 internal switch into cyclebycycle
current limit, thus reducing the dc output load current. This
can also result in overheating of the inductor and/or the
LM2574. Different inductor types have different saturation
characteristics, and this should be kept in mind when
selecting an inductor.

HORIZONTAL TIME BASE: 5.0

s/DIV

VER

TRICAL

RESOLUTION 200 mADV

Figure 24. Continuous Mode Switching

Current Waveforms

0.5 A

0 A

0.5 A

0 A

Power

Switch

Current

Waveform

Inductor

Current

Waveform

VER

TICAL

RESOLUTION 100 mADV

HORIZONTAL TIME BASE: 5.0

s/DIV

Figure 25. Continuous Mode Switching

Current Waveforms

0.1 A

0 A

0.1 A

0 A

Power

Switch

Current

Waveform

Inductor

Current

Waveform

LM2574

MOTOROLA ANALOG IC DEVICE DATA

GENERAL RECOMMENDATIONS

Output Voltage Ripple and Transients
Source of the Output Ripple

Since the LM2574 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 26). 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.

HORIZONTAL TIME BASE: 5.0

s/DIV

VER

TRICAL

RESOLUTION 20 mV/DIV

Voltage spikes caused by switching action of the output
switch and the parasitic inductance of the output capacitor

Figure 26. Output Ripple Voltage Waveforms

Unfiltered

Output

Voltage

Filtered

Output

Voltage

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

100

F), that can be added to the output (see Figure 35) 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 26 shows the
difference between filtered and unfiltered output waveforms
of the regulator shown in Figure 34.

The upper waveform is from the normal unfiltered output of

the converter, while the lower waveform shows the output
ripple voltage filtered by an additional LC filter.

Heatsinking and Thermal Considerations

The LM2574 is available in a 8pin DIP package. When

used in the typical application the copper lead frame conducts

the majority of the heat from the die, through the leads, to the
printed circuit copper. The copper and the board are the
heatsink for this package and the other heat producing
components, such as the catch diode and inductor.

For the best thermal performance, wide copper traces

should be used and all ground and unused pins should be
soldered to generous amounts of printed circuit board
copper, such as a ground plane. Large areas of copper
provide the best transfer of heat to the surrounding air. One
exception to this is the output (switch) pin, which should not
have large areas of copper in order to minimize coupling to
sensitive circuitry.

Additional improvement in heat dissipation can be

achieved even by using of double sided or multilayer boards
which can provide even better heat path to the ambient.
Using a socket for the 8pin DIP package is not
recommended because socket represents an additional
thermal resistance, and as a result the junction temperature
will be higher.

Since the current rating of the LM2574 is only 0.5 A, the

total package power dissipation for this switcher is quite low,
ranging from approximately 0.1 W up to 0.75 W under varying
conditions. In a carefully engineered printed circuit board, the
throughhole DIP package can easily dissipate up to 0.75 W,
even at ambient temperatures of 60

C, and still keep the

maximum junction temperature below 125

Thermal Analysis and Design

The following procedure must be performed to determine

the operating junction temperature. 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 LM2574). For a conservative

design, the maximum junction temperature
should not exceed 110

C to assure safe

operation. For every additional +10

temperature rise that the junction must
withstand, the estimated operating lifetime of
the component is halved.

4. R

package thermal resistance junctioncase.

5. R

package thermal resistance junctionambient.

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

JC and R

JA values).

The following formula is to calculate the approximate total

power dissipated by the LM2574:

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

where d is the duty cycle and for buck converter

ton

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

LM2574 data sheet,

Vin is minimum input voltage applied,
VO is the regulator output voltage,
ILoad is the load current.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

D1
MBR150

Output

Feedback

8.0 to 25 V

Unregulated

DC Input

Cin

ON/OFF

Pwr
Gnd

+Vin

12 V @ 100 mA

Regulated

Output

Cout

680

LM257412

Sig
Gnd

Figure 27. Inverting BuckBoost Develops 12 V

The dynamic switching losses during turnon and turnoff

can be neglected if a proper type catch diode is used. The
junction temperature can be determined by the following
expression:

TJ = (R

JA)(PD) + TA

where (R

JA)(PD) represents the junction temperature rise

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

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. At higher power levels the
thermal resistance decreases due to the increased air
current activity.

Other factors are trace width, total printed circuit copper

area, copper thickness, single or doublesided, multilayer
board, the amount of solder on the board or even color of the
traces.

The size, quantity and spacing of other components on the

board can also influence its effectiveness to dissipate the
heat. Some of them, like the catch diode or the inductor will
generate some additional heat.

ADDITIONAL APPLICATIONS

Inverting Regulator

An inverting buckboost regulator using the LM257412 is

shown in Figure 27. 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 LM257412 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

0.1 A to the output when the input voltage is 8.0 V or higher.
At lighter loads the minimum input voltage required drops to
approximately 4.7 V, because the buckboost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.

Since the switch currents in this buckboost configuration

are higher than in the standard buck converter topology, the
available output current is lower.

This type of buckboost inverting regulator can also

require a larger amount of startup input current, even for light
loads. This may overload an input power source with a
current limit less than 0.6 A.

Because of the relatively high startup currents required by

this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.

While using a delayed startup arrangement, the input

capacitor can charge up to a higher voltage before the
switchmode regulator begins to operate.

The high input current needed for startup is now partially

supplied by the input capacitor Cin.

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

F).

The recommended range of inductor values for the

inverting converter design is between 68

H and 220

H. To

select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.

D1
MBR150

Output

Feedback

12 to 25 V

Unregulated

DC Input

Cin

/50 V

ON/OFF

Pwr
Gnd

+Vin

12 V @ 100 mA

Regulated

Output

Cout

680

/16 V

LM257412

0.1

47 k

R2
47 k

Sig
Gnd

Figure 28. Inverting BuckBoost Regulator with

Delayed Startup

The following formula is used to obtain the peak inductor

current:

peak

[

Load

)

x ton

where ton

)

1.0

fosc

, and fosc = 52 kHz.

Under normal continuous inductor current operating

conditions, the worst case occurs when Vin is minimal.

It has been already mentioned above, that in some

situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit

LM2574

MOTOROLA ANALOG IC DEVICE DATA

applied to a buckboost converter is shown in Figure 28.
Figure 34 in the "Undervoltage Lockout" section describes an
undervoltage lockout feature for the same converter
topology.

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 shutdown
methods, two of them are shown in Figures 29 and 30.

LM2574XX

and

Gnds
Pins

ON/OFF

+Vin

R2
47 k

Cin

NOTE: This picture does not show the complete circuit.

47 k

470

Shutdown
Input

MOC8101

Vout

Off

5.0 V

+Vin

Figure 29. Inverting BuckBoost Regulator Shutdown

Circuit Using an Optocoupler

NOTE: This picture does not show the complete circuit.

5.6 k

Q1
2N3906

LM2574XX

and

Gnds
Pins

ON/OFF

R1
12 k

Vout

+Vin

Shutdown
Input

Off

+Vin

Cin

Figure 30. Inverting BuckBoost Regulator Shutdown

Circuit Using a PNP Transistor

Negative Boost Regulator

This example is a variation of the buckboost 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 31 shows the negative boost

configuration. The input voltage in this application ranges
from 5.0 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.

1N5817

330

Output

Feedback

Vout = 12 V

Load Current
60 mA for Vin = 5.2 V

120 mA for Vin = 7.0 V

Vin

Cout

1000

Cin

LM257412

ON/OFF

Pwr
Gnd

+Vin

Sig
Gnd

5.0 to 12 V

Figure 31. Negative Boost Regulator

Design Recommendations:

The same design rules as for the previous inverting

buckboost converter can be applied. The output capacitor
Cout must be chosen larger than what would be required for a
standard buck converter. Low input voltages or high output
currents require a large value output capacitor (in the range
of thousands of

F). 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 any 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 Startup

There are some applications, like the inverting regulator

already mentioned above, which require a higher amount of
startup current. In such cases, if the input power source is
limited, this delayed startup 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 32 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.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

47 k

LM2574XX

and

Gnds
Pins

ON/OFF

R2
47 k

+Vin

C1
0.1

Cin

NOTE: This picture does not show the complete circuit.

Figure 32. Delayed Startup Circuitry

Undervoltage Lockout

Some applications require the regulator to remain off until

the input voltage reaches a certain threshold level. Figure 33
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buckboost converter is
shown in Figure 34. Resistor R3 pulls the ON/OFF pin high
and keeps the regulator off until the input voltage reaches a
predetermined threshold level, which is determined by the
following expression:

[

)

1.0

)

R2
R1

(Q1)

10 k

1N5242B

10 k

Q1
2N3904

47 k

Cin

LM2574XX

and

Gnds
Pins

ON/OFF

+Vin

NOTE: This picture does not show the complete circuit.

Figure 33. Undervoltage Lockout Circuit for

Buck Converter

15 k

1N5242

15 k

Q1
2N3904

68 k

Cin

LM2574XX

and

Gnds
Pins

ON/OFF

+Vin

Vout

NOTE: This picture does not show the complete circuit (see Figure 27).

Figure 34. Undervoltage Lockout Circuit for

BuckBoost Converter

Adjustable Output, LowRipple Power Supply

A 0.5 A output current capability power supply that

features an adjustable output voltage is shown in Figure 35.

This regulator delivers 0.5 A into 1.2 to 35 V output. The

input voltage ranges from roughly 3.0 to 40 V. In order to
achieve a 10 or more times reduction of output ripple, an
additional LC filter is included in this circuit.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

D1
1N5819

150

Output

Feedback

R2
50 k

R1
1.1 k

Output

Voltage

1.2 to 35 V @ 0.5 A

Optional Output

Ripple Filter

40 V Max
Unregulated
DC Input

Cout

1000

100

Cin

LM2574ADJ

ON/OFF

Pwr
Gnd

+Vin

Sig
Gnd

Figure 35. 1.2 to 35 V Adjustable 500 mA Power Supply with Low Output Ripple

The LM25745 StepDown Voltage Regulator with 5.0 V @ 0.5 A Output Power Capability.

Typical Application With ThroughHole PC Board Layout

D1
1N5819

330

Output

Feedback

Unregulated

DC Input

+Vin = 7.0 to 40 V

C2
220

LM25745

ON/OFF

Pwr
Gnd

+Vin

Regulated Output
+Vout = 5.0 V @ 0.5 A

Gnd

F, 63 V, Aluminium Electrolytic

220

F, 16 V, Aluminium Electrolytic

1.0 A, 40 V, Schottky Rectifier, 1N5819

330

H, RL1284330, Renco Electronics

Sig
Gnd

Figure 36. Schematic Diagram of the LM25745 StepDown Converter

LM25745.0

Vout

Gnd

+Vin

Figure 37. PC Board Layout Component Side

NOTE: Not to scale.

Figure 38. PC Board Layout Copper Side

NOTE: Not to scale.

LM2574

MOTOROLA ANALOG IC DEVICE DATA

The LM2574ADJ StepDown Voltage Regulator with 5.0 V @ 0.5 A Output Power Capability Typical

Application With ThroughHole PC Board Layout

D1
1N5819

330

Output

Feedback

R2
6.12 k

R1
2.0 k

Regulated
Output Filtered

Vout = 5.0 V @ 0.5 A

Output

Ripple Filter

Unregulated
DC Input

C2
220

100

LM2574ADJ

ON/OFF

Pwr
Gnd

+Vin

Sig
Gnd

+Vin = 7.0 to 40 V

Gnd

F, 63 V, Aluminium Electrolytic

220

F, 16 V, Aluminium Electrolytic

100

F, 16 V Aluminium Electrolytic

1.0 A, 40 V, Schottky Rectifier, 1N5819

330

H, RL1284330, Renco Electronics

H, SFT52501, TDK

2.0 k

, 0.1%, 0.25 W

6.12 k

, 0.1%, 0.25 W

Figure 39. Schematic Diagram of the 5.0 V @ 0.5 A StepDown Converter Using the LM2574ADJ

(An additional LC filter is included to achieve low output ripple voltage)

LM2574

Vout

Gnd

+Vin

Gnd

R1 R2

Figure 40. PC Board Layout Component Side

NOTE: Not to scale.

Figure 41. PC Board Layout Copper Side

NOTE: Not to scale.

References

National Semiconductor LM2574 Data Sheet and Application Note

National Semiconductor LM2594 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

LM2574

MOTOROLA ANALOG IC DEVICE DATA

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. "Typical" 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
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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.

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LM2574/D

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LM2574-3.3

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: LM2574-3.3