LM2574HV - National Semiconductor

TL/H/11394

LM2574/LM2574HV Series SIMPLE SWITCHER 0.5A Step-Down Voltage Regulator

March 1995

LM2574/LM2574HV Series

SIMPLE SWITCHERTM 0.5A Step-Down Voltage Regulator

General Description

The LM2574 series of regulators are monolithic integrated

circuits that provide all the active functions for a step-down

(buck) switching regulator, capable of driving a 0.5A load

with excellent line and load regulation. These devices are

available in fixed output voltages of 3.3V, 5V, 12V, 15V, and

an adjustable output version.

Requiring a minimum number of external components,

these regulators are simple to use and include internal fre-

quency compensation and a fixed-frequency oscillator.

The LM2574 series offers a high-efficiency replacement for

popular three-terminal linear regulators. Because of its high

efficiency, the copper traces on the printed circuit board are

normally the only heat sinking needed.

A standard series of inductors optimized for use with the

LM2574 are available from several different manufacturers.

This feature greatly simplifies the design of switch-mode

power supplies.

Other features include a guaranteed g4% tolerance on out-

put voltage within specified input voltages and output load

conditions, and g10% on the oscillator frequency. External

shutdown is included, featuring 50 mA (typical) standby cur-

rent. The output switch includes cycle-by-cycle current limit-

ing, as well as thermal shutdown for full protection under

fault conditions.

Features

Y3.3V, 5V, 12V, 15V, and adjustable output versions

YAdjustable version output voltage range,

1.23V to 37V (57V for HV version) g4% max over line

and load conditions

YGuaranteed 0.5A output current

YWide input voltage range, 40V, up to 60V

for HV version

YRequires only 4 external components

Y52 kHz fixed frequency internal oscillator

YTTL shutdown capability, low power standby mode

YHigh efficiency

YUses readily available standard inductors

YThermal shutdown and current limit protection

Applications

YSimple high-efficiency step-down (buck) regulator

YEfficient pre-regulator for linear regulators

YOn-card switching regulators

YPositive to negative converter (Buck-Boost)

Typical Application (Fixed Output Voltage Versions)

TL/H/11394–1

Note: Pin numbers are for 8-pin DIP package.

Connection Diagrams

8-Lead DIP (N)

TL/H/11394–2

Top View

14-Lead Wide

Surface Mount (WM)

TL/H/11394–3

Top View

*No internal

connection, but

should be soldered

to PC board for

best heat transfer.

Order Number LM2574-3.3HVN, LM2574HVN-5.0,

LM2574HVN-12, LM2574HVN-15, LM2574HVN-ADJ,

LM2574N-3.3, LM2574N-5.0, LM2574N-12,

LM2574N-15 or LM2574N-ADJ

See NS Package Number N08A Order Number LM2574HVM-3.3, LM2574HVM-5.0,

LM2574HVM-12, LM2574HVM-15, LM2574HVM-ADJ,

LM2574M-3.3 LM2574M-5.0, LM2574M-12,

LM2574M-15 or LM2574M-ADJ

See NS Package Number M14B

Patent Pending

SIMPLE SWITCHERTM is a trademark of National Semiconductor Corporation.

C1995 National Semiconductor Corporation RRD-B30M75/Printed in U. S. A.

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Maximum Supply Voltage

LM2574 45V

LM2574HV 63V

ON/OFF Pin Input Voltage b0.3V sVsaVIN

Output Voltage to Ground

(Steady State) b1V

Power Dissipation Internally Limited

Storage Temperature Range b65§Ctoa

150§C

Minimum ESD Rating

(C e100 pF, R e1.5 kX)2kV

Lead Temperature

(Soldering, 10 seconds) 260§C

Maximum Junction Temperature 150§C

Operating Ratings

Temperature Range

LM2574/LM2574HV b40§CsTJsa125§C

Supply Voltage

LM2574 40V

LM2574HV 60V

LM2574-3.3, LM2574HV-3.3

Electrical Characteristics Specifications with standard type face are for TJe25§C, and those with boldface

type apply over full Operating Temperature Range.

Symbol Parameter Conditions

LM2574-3.3

(Limits)

Units

LM2574HV-3.3

Typ Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

VOUT Output Voltage VIN e12V, ILOAD e100 mA 3.3 V

3.234 V(Min)

3.366 V(Max)

VOUT Output Voltage 4.75V sVIN s40V, 0.1A sILOAD s0.5A 3.3 V

LM2574 3.168/3.135 V(Min)

3.432/3.465 V(Max)

VOUT Output Voltage 4.75V sVIN s60V, 0.1A sILOAD s0.5A 3.3

LM2574HV 3.168/3.135 V(Min)

3.450/3.482 V(Max)

hEfficiency VIN e12V, ILOAD e0.5A 72 %

LM2574-5.0, LM2574HV-5.0

Electrical Characteristics Specifications with standard type face are for TJe25§C, and those with boldface

type apply over full Operating Temperature Range.

Symbol Parameter Conditions

LM2574-5.0

(Limits)

Units

LM2574HV-5.0

Typ Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

VOUT Output Voltage VIN e12V, ILOAD e100 mA 5 V

4.900 V(Min)

5.100 V(Max)

VOUT Output Voltage 7V sVIN s40V, 0.1A sILOAD s0.5A 5 V

LM2574 4.800/4.750 V(Min)

5.200/5.250 V(Max)

VOUT Output Voltage 7V sVIN s60V, 0.1A sILOAD s0.5A 5

LM2574HV 4.800/4.750 V(Min)

5.225/5.275 V(Max)

hEfficiency VIN e12V, ILOAD e0.5A 77 %

LM2574-12, LM2574HV-12

Electrical Characteristics Specifications with standard type face are for TJe25§C, and those with boldface

type apply over full Operating Temperature Range.

Symbol Parameter Conditions

LM2574-12

(Limits)

Units

LM2574HV-12

Typ Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

VOUT Output Voltage VIN e25V, ILOAD e100 mA 10 V

11.76 V(Min)

12.24 V(Max)

VOUT Output Voltage 15V sVIN s40V, 0.1A sILOAD s0.5A 12 V

LM2574 11.52/11.40 V(Min)

12.48/12.60 V(Max)

VOUT Output Voltage 15V sVIN s60V, 0.1A sILOAD s0.5A 12

LM2574HV 11.52/11.40 V(Min)

12.54/12.66 V(Max)

hEfficiency VIN e15V, ILOAD e0.5A 88 %

LM2574-15, LM2574HV-15

Electrical Characteristics Specifications with standard type face are for TJe25§C, and those with boldface

type apply over full Operating Temperature Range.

Symbol Parameter Conditions

LM2574-15

(Limits)

Units

LM2574HV-15

Typ Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

VOUT Output Voltage VIN e30V, ILOAD e100 mA 15 V

14.70 V(Min)

15.30 V(Max)

VOUT Output Voltage 18V sVIN s40V, 0.1A sILOAD s0.5A 15 V

LM2574 14.40/14.25 V(Min)

15.60/15.75 V(Max)

VOUT Output Voltage 18V sVIN s60V, 0.1A sILOAD s0.5A 15

LM2574HV 14.40/14.25 V(Min)

15.68/15.83 V(Max)

hEfficiency VIN e18V, ILOAD e0.5A 88 %

LM2574-ADJ, LM2574HV-ADJ

Electrical Characteristics Specifications with standard type face are for TJe25§C, and those with boldface

type apply over full Operating Temperature Range. Unless otherwise specified, VIN e12V, ILOAD e100 mA.

Symbol Parameter Conditions

LM2574-ADJ

(Limits)

Units

LM2574HV-ADJ

Typ Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit

Figure 2

VFB Feedback Voltage VIN e12V, ILOAD e100 mA 1.230 V

1.217 V(Min)

1.243 V(Max)

VFB Feedback Voltage 7V sVIN s40V, 0.1A sILOAD s0.5A 1.230 V

LM2574 VOUT Programmed for 5V. Circuit of

Figure 2

1.193/1.180 V(Min)

1.267/1.280 V(Max)

VFB Feedback Voltage 7V sVIN s60V, 0.1A sILOAD s0.5A 1.230

LM2574HV VOUT Programmed for 5V. Circuit of

Figure 2

1.193/1.180 V(Min)

1.273/1.286 V(Max)

hEfficiency VIN e12V, VOUT e5V, ILOAD e0.5A 77 %

All Output Voltage Versions

Electrical Characteristics Specifications with standard type face are for TJe25§C, and those with boldface

type apply over full Operating Temperature Range. Unless otherwise specified, VIN e12V for the 3.3V, 5V, and

Adjustable version, VIN e25V for the 12V version, and VIN e30V for the 15V version. ILOAD e100 mA.

Symbol Parameter Conditions

LM2574-XX

(Limits)

Units

LM2574HV-XX

Typ Limit

(Note 2)

DEVICE PARAMETERS

IbFeedback Bias Current Adjustable Version Only, VOUT e5V 50 100/500 nA

fOOscillator Frequency (see Note 10) 52 kHz

47/42 kHz(Min)

58/63 kHz(Max)

VSAT Saturation Voltage IOUT e0.5A (Note 4) 0.9 V

1.2/1.4 V(max)

DC Max Duty Cycle (ON) (Note 5) 98 %

93 %(Min)

ICL Current Limit Peak Current, (Notes 4, 10) 1.0 A

0.7/0.65 A(Min)

1.6/1.8 A(Max)

ILOutput Leakage Current (Notes 6, 7) Output e0V 2 mA(Max)

Output eb

1V 7.5 mA

Output eb

1V 30 mA(Max)

IQQuiescent Current (Note 6) 5 mA

10 mA(Max)

ISTBY Standby Quiescent ON/OFF Pine5V (OFF) 50 mA

Current 200 mA(Max)

iJA Thermal Resistance N Package, Junction to Ambient (Note 8) 92

iJA N Package, Junction to Ambient (Note 9) 72 §C/W

iJA M Package, Junction to Ambient (Note 8) 102

iJA M Package, Junction to Ambient (Note 9) 78

ON/OFF CONTROL Test Circuit

Figure 2

VIH ON/OFF Pin Logic VOUT e0V 1.4 2.2/2.4 V(Min)

VIL Input Level VOUT eNominal Output Voltage 1.2 1.0/0.8 V(Max)

IHON/OFF Pin Input ON/OFF Pin e5V (OFF) 12 mA

Current 30 mA(Max)

IIL ON/OFF Pin e0V (ON) 0 mA

10 mA(Max)

Electrical Characteristics (Continued)

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: All limits guaranteed at room temperature (Standard type face) and at temperature extremes (bold type face). All room temperature limits are 100%

production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used

to calculate Average Outgoing Quality Level.

Note 3: External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2574

is used as shown in the

Figure 2

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

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

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

Note 6: Feedback pin removed from output and connected to a12V for the Adjustable, 3.3V, and 5V versions, and a25V for the 12V and 15V versions, to force

the output transistor OFF.

Note 7: VIN e40V (60V for high voltage version).

Note 8: Junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. Additional copper area will

lower thermal resistance further. See application hints in this data sheet and the thermal model in

Switchers Made Simple

software.

Note 9: Junction to ambient thermal resistance with approximately 4 square inches of 1 oz. (0.0014 in. thick) printed circuit board copper surrounding the leads.

Additional copper area will lower thermal resistance further. (See Note 8.)

Note 10: 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%.

Typical Performance Characteristics (Circuit of

Figure 2

)

Normalized Output Voltage Line Regulation Dropout Voltage

Current Limit Supply Current Quiescent Current

Standby

TL/H/11394–17

Typical Performance Characteristics (Circuit of

Figure 2

) (Continued)

Oscillator Frequency Voltage

Switch Saturation

Efficiency

Minimum Operating Voltage vs Duty Cycle

Supply Current

vs Duty Cycle

Feedback Voltage

TL/H/11394–4

Pin Current

Feedback

Thermal Resistance

Junction to Ambient

TL/H/11394–5

Typical Performance Characteristics (Circuit of

Figure 2

) (Continued)

Continuous Mode Switching Waveforms

VOUT e5V, 500 mA Load Current, L e330 mH

TL/H/11394–6

A: Output Pin Voltage, 10V/div

B: Inductor Current, 0.2 A/div

C: Output Ripple Voltage, 20 mV/div,

AC-Coupled

Horizontal Time Base: 5ms/div

Discontinuous Mode Switching Waveforms

VOUT e5V, 100 mA Load Current, L e100 mH

TL/H/11394–7

A: Output Pin Voltage, 10V/div

B: Inductor Current, 0.2 A/div

C: Output Ripple Voltage, 20 mV/div,

AC-Coupled

Horizontal Time Base: 5 ms/div

500 mA Load Transient Response for Continuous

Mode Operation, L e330 mH, COUT e300 mF

TL/H/11394–8

A: Output Voltage, 50 mV/div.

AC Coupled

B: 100 mA to 500 mA Load Pulse

Horizontal Time Base: 200 ms/div

250 mA Load Transient Response for Discontinuous

Mode Operation. L e68 mH, COUT e470 mF

TL/H/11394–9

A: Output Voltage, 50 mV/div.

AC Coupled

B: 50 mA to 250 mA Load Pulse

Horizontal Time Base: 200 ms/div

Block Diagram

R1 e1k

3.3V, R2 e1.7k

5V, R2 e3.1k

12V, R2 e8.84k

15V, R2 e11.3k

For Adj. Version

R1 eOpen, R2 e0X

Note: Pin numbers are for the 8-pin DIP package. TL/H/11394–10

FIGURE 1

Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

TL/H/11394–11

CINÐ22mF, 75V

Aluminum Electrolytic

COUTÐ 220 mF, 25V

Aluminum Electrolytic

D1Ð Schottky, 11DQ06

L1Ð 330 mH, 52627

(for 5V in, 3.3V out, use

100 mH, RL-1284-100)

R1Ð 2k, 0.1%

R2Ð 6.12k, 0.1%

Adjustable Output Voltage Version

TL/H/11394–12

VOUT eVREF #1aR2

R1J

R2eR1#VOUT

VREF

b1J

where VREF e1.23V,

R1 between 1k & 5k.

FIGURE 2

As in any switching regulator, layout is very important. Rap-

idly switching currents associated with wiring inductance

generate voltage transients which can cause problems. For

minimal inductance and ground loops, the length of the

leads indicated by heavy lines should be kept as short as

possible. Single-point grounding (as indicated) or ground

plane construction should be used for best results. When

using the Adjustable version, physically locate the program-

ming resistors near the regulator, to keep the sensitive feed-

back wiring short.

Inductor Pulse Eng. Renco NPI

Value (Note 1) (Note 2) (Note 3)

68 mH*RL-1284-68 NP5915

100 mH*RL-1284-100 NP5916

150 mH 52625 RL-1284-150 NP5917

220 mH 52626 RL-1284-220 NP5918/5919

330 mH 52627 RL-1284-330 NP5920/5921

470 mH 52628 RL-1284-470 NP5922

680 mH 52629 RL-1283-680 NP5923

1000 mH 52631 RL-1283-1000 *

1500 mH*RL-1283-1500 *

2200 mH*RL-1283-2200 *

FIGURE 3. Inductor Selection by

Manufacturer’s Part Number

U.S. Source

Note 1: Pulse Engineering, (619) 674-8100

P.O. Box 12236, San Diego, CA 92112

Note 2: Renco Electronics Inc., (516) 586-5566

60 Jeffryn Blvd. East, Deer Park, NY 11729

*Contact Manufacturer

European Source

Note 3: NPI/APC a44 (0) 634 290588

47 Riverside, Medway City Estate

Strood, Rochester, Kent ME2 4DP. UK

*Contact Manufacturer

LM2574 Series Buck Regulator Design Procedure

PROCEDURE (Fixed Output Voltage Versions) EXAMPLE (Fixed Output Voltage Versions)

Given: Given:

VOUT eRegulated Output Voltage (3.3V, 5V, 12V, or 15V) VOUT e5V

VIN(Max) eMaximum Input Voltage VIN(Max) e15V

ILOAD(Max) eMaximum Load Current ILOAD(Max) e0.4A

1. Inductor Selection (L1) 1. Inductor Selection (L1)

A. Select the correct Inductor value selection guide from A. Use the selection guide shown in

Figure 5

Figures 4, 5, 6

. (Output voltages of 3.3V, 5V, 12V or B. From the selection guide, the inductance area

15V respectively). For other output voltages, see the intersected by the 15V line and 0.4A line is 330.

design procedure for the adjustable version. C. Inductor value required is 330 mH. From the table in

B. From the inductor value selection guide, identify the

Figure 3

, choose Pulse Engineering PE-52627,

inductance region intersected by VIN(Max) and Renco RL-1284-330, or NPI NP5920/5921.

ILOAD(Max).

C. Select an appropriate inductor from the table shown in

Figure 3

. Part numbers are listed for three inductor

manufacturers. The inductor chosen must be rated for

operation at the LM2574 switching frequency (52 kHz) and

for a current rating of 1.5 cILOAD. For additional inductor

information, see the inductor section in the Application

Hints section of this data sheet.

2. Output Capacitor Selection (COUT) 2. Output Capacitor Selection (COUT)

A. The value of the output capacitor together with the A. COUT e100 mFto470mF standard aluminum

inductor defines the dominate pole-pair of the switching electrolytic.

regulator loop. For stable operation and an acceptable B. Capacitor voltage rating e20V.

output ripple voltage, (approximately 1% of the output

voltage) a value between 100 mF and 470 mFis

recommended.

B. The capacitor’s voltage rating should be at least 1.5

times greater than the output voltage. For a 5V regulator,

a rating of at least 8V is appropriate, and a 10V or 15V

rating is recommended.

Higher voltage electrolytic capacitors generally have lower

ESR numbers, and for this reasion it may be necessary to

select a capacitor rated for a higher voltage than would

normally be needed.

3. Catch Diode Selection (D1) 3. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.5 A. For this example, a 1A current rating is adequate.

times greater than the maximum load current. Also, if the B. Use a 20V 1N5817 or SR102 Schottky diode, or any of

power supply design must withstand a continuous output the suggested fast-recovery diodes shown in

Figure 9

short, the diode should have a current rating equal to the

maximum current limit of the LM2574. The most stressful

condition for this diode is an overload or shorted output

condition.

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

least 1.25 times the maximum input voltage.

4. Input Capacitor (CIN) 4. Input Capacitor (CIN)

An aluminum or tantalum electrolytic bypass capacitor A 22 mF aluminum electrolytic capacitor located near the

located close to the regulator is needed for stable input and ground pins provides sufficient bypassing.

operation.

LM2574 Series Buck Regulator Design Procedure (Continued)

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

TL/H/11394–26

FIGURE 4. LM2574HV-3.3 Inductor Selection Guide TL/H/11394–13

FIGURE 5. LM2574HV-5.0 Inductor Selection Guide

TL/H/11394–14

FIGURE 6. LM2574HV-12 Inductor Selection Guide

TL/H/11394–15

FIGURE 7. LM2574HV-15 Inductor Selection Guide

TL/H/11394–16

FIGURE 8. LM2574HV-ADJ Inductor Selection Guide

LM2574 Series Buck Regulator Design Procedure (Continued)

PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)

Given: Given:

VOUT eRegulated Output Voltage VOUT e24V

VIN(Max) eMaximum Input Voltage VIN(Max) e40V

ILOAD(Max) eMaximum Load Current ILOAD(Max) e0.4A

FeSwitching Frequency

(Fixed at 52 kHz)

Fe52 kHz

1. Programming Output Voltage

(Selecting R1 and R2, as

1. Programming Output Voltage

(Selecting R1 and R2)

shown in Figure 2)

Use the following formula to select the appropriate VOUT e1.23 #1aR2

R1JSelect R1 e1k

resistor values.

VOUT eVREF #1aR2

R1Jwhere VREF e1.23V R2eR1#VOUT

VREF

b1Je1k #24V

1.23V b1J

R1can be between 1k and 5k.

(For best temperature

coefficient and stability with time, use 1% metal film

R2e1k (19.51 b1) e18.51k, closest 1% value is 18.7k

resistors)

R2eR1#VOUT

VREF

b1J

2. Inductor Selection (L1) 2. Inductor Selection (L1)

A. Calculate the inductor Volt #microsecond constant, A. Calculate E #T(V#ms)

E#T(V#ms), from the following formula:

E#Te(40 b24) #24

40 #1000

52 e185 V #ms

E#Te(VIN bVOUT)VOUT

VIN

#1000

(in kHz)

(V #ms)

B. E#Te185 V #ms

B. Use the E #T value from the previous formula and C. ILOAD(Max) e0.4A

match it with the E #T number on the vertical axis of the

D. Inductance Region e1000

Inductor Value Selection Guide shown in

Figure 8

E. Inductor Value e1000 mH

Choose from Pulse

C. On the horizontal axis, select the maximum load

Engineering Part

PE-52631, or Renco

current.

Part

RL-1283-1000.

D. Identify the inductance region intersected by the E #T

value and the maximum load current value, and note the

inductor value for that region.

E. Select an appropriate inductor from the table shown in

Figure 3

. Part numbers are listed for three inductor

manufacturers. The inductor chosen must be rated for

operation at the LM2574 switching frequency (52 kHz)

and for a current rating of 1.5 cILOAD. For additional

inductor information, see the inductor section in the

application hints section of this data sheet.

3. Output Capacitor Selection (COUT) 3. Output Capacitor Selection (COUT)

A. The value of the output capacitor together with the A. COUT l13,300 40

24 #1000 e22.2 mF

inductor defines the dominate pole-pair of the switching

However, for acceptable output ripple voltage select

regulator loop. For stable operation, the capacitor must

COUT t100 mF

satisfy the following requirement:

COUT e100 mF electrolytic capacitor

COUT t13,300 VIN(Max)

VOUT #L(mH) (mF)

The above formula yields capacitor values between 5 mF

and 1000 mF that will satisfy the loop requirements for

stable operation. But to achieve an acceptable output

ripple voltage, (approximately 1% of the output voltage)

and transient response, the output capacitor may need to

be several times larger than the above formula yields.

B. The capacitor’s voltage rating should be at last 1.5

times greater than the output voltage. For a 24V regulator,

a rating of at least 35V is recommended.

Higher voltage electrolytic capacitors generally have

lower ESR numbers, and for this reasion it may be

necessary to select a capacitor rate for a higher voltage

than would normally be needed.

LM2574 Series Buck Regulator Design Procedure (Continued)

PROCEDURE (Adjustable Output Voltage Versions) EXAMPLE (Adjustable Output Voltage Versions)

4. Catch Diode Selection (D1) 4. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.5 A. For this example, a 1A current rating is adequate.

times greater than the maximum load current. Also, if the B. Use a 50V MBR150 or 11DQ05 Schottky diode, or any of the

power supply design must withstand a continuous output suggested fast-recovery diodes in

Figure 9

short, the diode should have a current rating equal to the

maximum current limit of the LM2574. The most stressful

condition for this diode is an overload or shorted output

condition. Suitable diodes are shown in the selection

guide of

Figure 9

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

least 1.25 times the maximum input voltage.

5. Input Capacitor (CIN) 5. Input Capacitor (CIN)

An aluminum or tantalum electrolytic bypass capacitor A 22 mF aluminum electrolytic capacitor located near the input

located close to the regulator is needed for stable and ground pins provides sufficient bypassing.

operation.

1 Amp Diodes

Schottky Fast Recovery

1N5817

20V SR102

MBR120P

1N5818

SR103

30V 11DQ03

The

MBR130P

following

10JQ030

diodes

1N5819 are all

SR104 rated to

40V 11DQ04 100V

11JQ04

MBR140P 11DF1

MBR150 10JF1

50V SR105 MUR110

11DQ05 HER102

11JQ05

MBR160

60V SR106

11DQ06

11JQ06

90V 11DQ09

FIGURE 9. Diode Selection Guide

To further simplify the buck regulator design procedure, National

Semiconductor is making available computer design software to

be used with the Simple Switcher line of switching regulators.

Switchers Made Simple (version 3.3) is available on a (3(/2×)

diskette for IBM compatible computers from a National

Semiconductor sales office in your area.

Application Hints

INPUT CAPACITOR (CIN)

To maintain stability, the regulator input pin must be by-

passed with at least a 22 mF electrolytic capacitor. The ca-

pacitor’s leads must be kept short, and located near the

regulator.

If the operating temperature range includes temperatures

below b25§C, the input capacitor value may need to be

larger. With most electrolytic capacitors, the capacitance

value decreases and the ESR increases with lower temper-

atures and age. Paralleling a ceramic or solid tantalum ca-

pacitor will increase the regulator stability at cold tempera-

tures. For maximum capacitor operating lifetime, the capaci-

tor’s RMS ripple current rating should be greater than

1.2 c#tON

TJcILOAD

where tON

TeVOUT

VIN

for a buck regulator

and tON

VOUT

aVIN

for a buck-boost regulator.

INDUCTOR SELECTION

All switching regulators have two basic modes of operation:

continuous and discontinuous. The difference between the

two types relates to the inductor current, whether it is flow-

ing continuously, or if it drops to zero for a period of time in

the normal switching cycle. Each mode has distinctively dif-

ferent operating characteristics, which can affect the regula-

tor performance and requirements.

The LM2574 (or any of the Simple Switcher family) can be

used for both continuous and discontinuous modes of oper-

ation.

In many cases the preferred mode of operation is in the

continuous mode. It offers better load regulation, lower peak

switch, inductor and diode currents, and can have lower out-

put ripple voltage. But it does require relatively large induc-

tor values to keep the inductor current flowing continuously,

especially at low output load currents.

To simplify the inductor selection process, an inductor se-

lection guide (nomograph) was designed (see

Figures 4

through

). This guide assumes continuous mode opera-

tion, and selects an inductor that will allow a peak-to-peak

inductor ripple current (DIIND) to be a certain percentage of

the maximum design load current. In the LM2574 SIMPLE

SWITCHER, the peak-to-peak inductor ripple current per-

centage (of load current) is allowed to change as different

design load currents are selected. By allowing the percent-

age of inductor ripple current to increase for lower current

applications, the inductor size and value can be kept rela-

tively low.

INDUCTOR RIPPLE CURRENT

When the switcher is operating in the continuous mode, the

inductor current waveform ranges from a triangular to a

sawtooth type of waveform (depending on the input volt-

age). For a given input voltage and output voltage, the peak-

to-peak amplitude of this inductor current waveform remains

constant. As the load current rises or falls, the entire saw-

tooth current waveform also rises or falls. The average DC

value of this waveform is equal to the DC load current (in

the buck regulator configuration).

If the load current drops to a low enough level, the bottom

of the sawtooth current waveform will reach zero, and the

switcher will change to a discontinuous mode of operation.

This is a perfectly acceptable mode of operation. Any buck

switching regulator (no matter how large the inductor value

is) will be forced to run discontinuous if the load current is

light enough.

The curve shown in

Figure 10

illustrates how the peak-to-

peak inductor ripple current (DIIND) is allowed to change as

different maximum load currents are selected, and also how

it changes as the operating point varies from the upper bor-

der to the lower border within an inductance region (see

Inductor Selection guides).

TL/H/11394–18

FIGURE 10. Inductor Ripple Current (DIIND) Range

Based on Selection Guides from

Figures 4

–

Consider the following example:

VOUT e5V @0.4A

VIN e10V minimum up to 20V maximum

The selection guide in

Figure 5

shows that for a 0.4A load

current, and an input voltage range between 10V and 20V,

the inductance region selected by the guide is 330 mH. This

value of inductance will allow a peak-to-peak inductor ripple

current (DIIND) to flow that will be a percentage of the maxi-

mum load current. For this inductor value, the DIIND will also

vary depending on the input voltage. As the input voltage

increases to 20V, it approaches the upper border of the

inductance region, and the inductor ripple current increases.

Referring to the curve in

Figure 10

, it can be seen that at the

0.4A load current level, and operating near the upper border

of the 330 mH inductance region, the DIIND will be 53% of

0.4A, or 212 mA p-p.

This DIIND is important because from this number the peak

inductor current rating can be determined, the minimum

load current required before the circuit goes to discontinu-

ous operation, and also, knowing the ESR of the output

capacitor, the output ripple voltage can be calculated, or

conversely, measuring the output ripple voltage and know-

ing the DIIND, the ESR can be calculated.

Application Hints (Continued)

From the previous example, the Peak-to-peak Inductor Rip-

ple Current (DIIND)e212 mA p-p. Once the DIND value is

known, the following three formulas can be used to calcu-

late additional information about the switching regulator cir-

cuit:

1. Peak Inductor or peak switch current

e#ILOAD aDIIND

2Je#0.4A a212

2Je506 mA

2. Mimimum load current before the circuit becomes discon-

tinuous

eDIIND

2e212

2e106 mA

3. Output Ripple Voltage e(DIIND)c(ESR of COUT)

The selection guide chooses inductor values suitable for

continuous mode operation, but if the inductor value chosen

is prohibitively high, the designer should investigate the pos-

sibility of discontinuous operation. The computer design

software

Switchers Made Simple

will provide all compo-

nent values for discontinuous (as well as continuous) mode

of operation.

Inductors are available in different styles such as pot core,

toroid, E-frame, bobbin core, etc., as well as different core

materials, such as ferrites and powdered iron. The least ex-

pensive, the bobbin core type, consists of wire wrapped on

a ferrite rod core. This type of construction makes for an

inexpensive inductor, but since the magnetic flux is not com-

pletely contained within the core, it generates more electro-

magnetic interference (EMI). This EMl can cause problems

in sensitive circuits, or can give incorrect scope readings

because of induced voltages in the scope probe.

The inductors listed in the selection chart include powdered

iron toroid for Pulse Engineering, and ferrite bobbin core for

Renco.

An inductor should not be operated beyond its maximum

rated current because it may saturate. When an inductor

begins to saturate, the inductance decreases rapidly and

the inductor begins to look mainly resistive (the DC resist-

ance of the winding). This can cause the inductor current to

rise very rapidly and will affect the energy storage capabili-

ties of the inductor and could cause inductor overheating.

Different inductor types have different saturation character-

istics, and this should be kept in mind when selecting an

inductor. The inductor manufacturers’ data sheets include

current and energy limits to avoid inductor saturation.

OUTPUT CAPACITOR

An output capacitor is required to filter the output voltage

and is needed for loop stability. The capacitor should be

located near the LM2574 using short pc board traces. Stan-

dard aluminum electrolytics are usually adequate, but low

ESR types are recommended for low output ripple voltage

and good stability. The ESR of a capacitor depends on

many factors, some which are: the value, the voltage rating,

physical size and the type of construction. In general, low

value or low voltage (less than 12V) electrolytic capacitors

usually have higher ESR numbers.

The amount of output ripple voltage is primarily a function of

the ESR (Equivalent Series Resistance) of the output ca-

pacitor and the amplitude of the inductor ripple current

(DIIND). See the section on inductor ripple current in Appli-

cation Hints.

The lower capacitor values (100 mF– 330 mF) will allow typi-

cally 50 mV to 150 mV of output ripple voltage, while larger-

value capacitors will reduce the ripple to approximately

20 mV to 50 mV.

Output Ripple Voltage e(DIIND) (ESR of COUT)

To further reduce the output ripple voltage, several standard

electrolytic capacitors may be paralleled, or a higher-grade

capacitor may be used. Such capacitors are often called

‘‘high-frequency,’’ ‘‘low-inductance,’’ or ‘‘low-ESR.’’ These

will reduce the output ripple to 10 mV or 20 mV. However,

when operating in the continuous mode, reducing the ESR

below 0.03Xcan cause instability in the regulator.

Tantalum capacitors can have a very low ESR, and should

be carefully evaluated if it is the only output capacitor. Be-

cause of their good low temperature characteristics, a tanta-

lum can be used in parallel with aluminum electrolytics, with

the tantalum making up 10% or 20% of the total capaci-

tance.

The capacitor’s ripple current rating at 52 kHz should be at

least 50% higher than the peak-to-peak inductor ripple cur-

rent.

CATCH DIODE

Buck regulators require a diode to provide a return path for

the inductor current when the switch is off. This diode

should be located close to the LM2574 using short leads

and short printed circuit traces.

Because of their fast switching speed and low forward volt-

age drop, Schottky diodes provide the best efficiency, espe-

cially in low output voltage switching regulators (less than

5V). Fast-Recovery, High-Efficiency, or Ultra-Fast Recovery

diodes are also suitable, but some types with an abrupt turn-

off characteristic may cause instability and EMI problems. A

fast-recovery diode with soft recovery characteristics is a

better choice. Standard 60 Hz diodes (e.g., 1N4001 or

1N5400, etc.) are also not suitable. See

Figure 9

for

Schottky and ‘‘soft’’ fast-recovery diode selection guide.

OUTPUT VOLTAGE RIPPLE AND TRANSIENTS

The output voltage of a switching power supply will contain

a sawtooth ripple voltage at the switcher frequency, typically

about 1% of the output voltage, and may also contain short

voltage spikes at the peaks of the sawtooth waveform.

The output ripple voltage is due mainly to the inductor saw-

tooth ripple current multiplied by the ESR of the output ca-

pacitor. (See the inductor selection in the application hints.)

The voltage spikes are present because of the the fast

switching action of the output switch, and the parasitic in-

ductance of the output filter capacitor. To minimize these

voltage spikes, special low inductance capacitors can be

used, and their lead lengths must be kept short. Wiring in-

ductance, stray capacitance, as well as the scope probe

used to evaluate these transients, all contribute to the am-

plitude of these spikes.

An additional small LC filter (20 mH & 100 mF) can be added

to the output (as shown in

Figure 16

) to further reduce the

amount of output ripple and transients. A 10 creduction in

output ripple voltage and transients is possible with this fil-

ter.

Application Hints (Continued)

FEEDBACK CONNECTION

The LM2574 (fixed voltage versions) feedback pin must be

wired to the output voltage point of the switching power

supply. When using the adjustable version, physically locate

both output voltage programming resistors near the LM2574

to avoid picking up unwanted noise. Avoid using resistors

greater than 100 kXbecause of the increased chance of

noise pickup.

ON/OFF INPUT

For normal operation, the ON/OFF pin should be grounded

or driven with a low-level TTL voltage (typically below 1.6V).

To put the regulator into standby mode, drive this pin with a

high-level TTL or CMOS signal. The ON/OFF pin can be

safely pulled up to aVIN without a resistor in series with it.

The ON/OFF pin should not be left open.

GROUNDING

The 8-pin molded DIP and the 14-pin surface mount pack-

age have separate power and signal ground pins. Both

ground pins should be soldered directly to wide printed cir-

cuit board copper traces to assure low inductance connec-

tions and good thermal properties.

THERMAL CONSIDERATIONS

The 8-pin DIP (N) package and the 14-pin Surface Mount

(M) package are molded plastic packages with solid copper

lead frames. The copper lead frame conducts the majority

of the heat from the die, through the leads, to the printed

circuit board copper, which acts as the heat sink. For best

thermal performance, wide copper traces should be used,

and all ground and unused pins should be soldered to gen-

erous amounts of printed circuit board copper, such as a

ground plane. Large areas of copper provide the best trans-

fer of heat (lower thermal resistance) to the surrounding air,

and even double-sided or multilayer boards provide better

heat paths to the surrounding air. Unless the power levels

are small, using a socket for the 8-pin package is not recom-

mended because of the additional thermal resistance it in-

troduces, and the resultant higher junction temperature.

Because of the 0.5A current rating of the LM2574, the total

package power dissipation for this switcher is quite low,

ranging from approximately 0.1W up to 0.75W under varying

conditions. In a carefully engineered printed circuit board,

both the N and the M package can easily dissipate up to

0.75W, even at ambient temperatures of 60§C, and still keep

the maximum junction temperature below 125§C.

A curve displaying thermal resistance vs. pc board area for

the two packages is shown in the Typical Performance

Characteristics curves section of this data sheet.

These thermal resistance numbers are approximate, and

there can be many factors that will affect the final thermal

resistance. Some of these factors include board size,

shape, thickness, position, location, and board temperature.

Other factors are, the area of printed circuit copper, copper

thickness, trace width, multi-layer, single- or double-sided,

and the amount of solder on the board. The effectiveness of

the pc board to dissipate heat also depends on the size,

number and spacing of other components on the board.

Furthermore, some of these components, such as the catch

diode and inductor will generate some additional heat. Also,

the thermal resistance decreases as the power level in-

creases because of the increased air current activity at the

higher power levels, and the lower surface to air resistance

coefficient at higher temperatures.

The data sheet thermal resistance curves and the thermal

model in

Switchers Made Simple

software (version 3.3)

can estimate the maximum junction temperature based on

operating conditions. ln addition, the junction temperature

can be estimated in actual circuit operation by using the

following equation.

TjeTcu a(ij-cu cPD)

With the switcher operating under worst case conditions

and all other components on the board in the intended en-

closure, measure the copper temperature (Tcu ) near the IC.

This can be done by temporarily soldering a small thermo-

couple to the pc board copper near the IC, or by holding a

small thermocouple on the pc board copper using thermal

grease for good thermal conduction.

The thermal resistance (ij-cu) for the two packages is:

ij-cu e42§C/W for the N-8 package

ij-cu e52§C/W for the M-14 package

The power dissipation (PD) for the IC could be measured, or

it can be estimated by using the formula:

PDe(VIN)(I

)a#V

IN J(ILOAD)(V

SAT)

Where ISis obtained from the typical supply current curve

(adjustable version use the supply current vs. duty cycle

curve).

Additional Applications

INVERTING REGULATOR

Figure 11

shows a LM2574-12 in a buck-boost configuration

to generate a negative 12V output from a positive input volt-

age. This circuit bootstraps the regulator’s ground pin to the

negative output voltage, then by grounding the feedback

pin, the regulator senses the inverted output voltage and

regulates it to b12V.

Note: Pin numbers are for the 8-pin DIP package. TL/H/11394–19

FIGURE 11. Inverting Buck-Boost Develops b12V

Additional Applications (Continued)

For an input voltage of 8V or more, the maximum available

output current in this configuration is approximately 100 mA.

At lighter loads, the minimum input voltage required drops to

approximately 4.7V.

The switch currents in this buck-boost configuration are

higher than in the standard buck-mode design, thus lower-

ing the available output current. Also, the start-up input cur-

rent of the buck-boost converter is higher than the standard

buck-mode regulator, and this may overload an input power

source with a current limit less than 0.6A. Using a delayed

turn-on or an undervoltage lockout circuit (described in the

next section) would allow the input voltage to rise to a high

enough level before the switcher would be allowed to turn

on.

Because of the structural differences between the buck and

the buck-boost regulator topologies, the buck regulator de-

sign procedure section can not be used to to select the

inductor or the output capacitor. The recommended range

of inductor values for the buck-boost design is between

68 mH and 220 mH, and the output capacitor values must be

larger than what is normally required for buck designs. Low

input voltages or high output currents require a large value

output capacitor (in the thousands of micro Farads).

The peak inductor current, which is the same as the peak

switch current, can be calculated from the following formula:

Ip&ILOAD (VIN a

)

VIN

aVIN

VINa

2L1fosc

Where fosc e52 kHz. Under normal continuous inductor

current operating conditions, the minimum VIN represents

the worst case. Select an inductor that is rated for the peak

current anticipated.

Also, the maximum voltage appearing across the regulator

is the absolute sum of the input and output voltage. For a

b12V output, the maximum input voltage for the LM2574 is

a28V, or a48V for the LM2574HV.

The

Switchers Made Simple

(version 3.3) design software

can be used to determine the feasibility of regulator designs

using different topologies, different input-output parameters,

different components, etc.

NEGATIVE BOOST REGULATOR

Another variation on the buck-boost topology is the nega-

tive boost configuration. The circuit in

Figure 12

accepts an

input voltage ranging from b5V to b12V and provides a

regulated b12V output. Input voltages greater than b12V

will cause the output to rise above b12V, but will not dam-

age the regulator.

TL/H/11394–20

Note: Pin numbers are for 8-pin DIP package.

FIGURE 12. Negative Boost

Because of the boosting function of this type of regulator,

the switch current is relatively high, especially at low input

voltages. Output load current limitations are a result of the

maximum current rating of the switch. Also, boost regulators

can not provide current limiting load protection in the event

of a shorted load, so some other means (such as a fuse)

may be necessary.

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off

until the input voltage reaches a certain threshold. An un-

dervoltage lockout circuit which accomplishes this task is

shown in

Figure 13

, while

Figure 14

shows the same circuit

applied to a buck-boost configuration. These circuits keep

the regulator off until the input voltage reaches a predeter-

mined level.

VTH &VZ1 a2VBE (Q1)

TL/H/11394–21

Note: Complete circuit not shown.

Note: Pin numbers are for 8-pin DIP package.

FIGURE 13. Undervoltage Lockout for Buck Circuit

TL/H/11394–22

Note: Complete circuit not shown (see Figure 11).

Note: Pin numbers are for 8-pin DIP package.

FIGURE 14. Undervoltage Lockout

for Buck-Boost Circuit

Additional Applications (Continued)

DELAYED STARTUP

The ON/OFF pin can be used to provide a delayed startup

feature as shown in

Figure 15

. With an input voltage of 20V

and for the part values shown, the circuit provides approxi-

mately 10 ms of delay time before the circuit begins switch-

ing. Increasing the RC time constant can provide longer de-

lay times. But excessively large RC time constants can

cause problems with input voltages that are high in 60 Hz or

120 Hz ripple, by coupling the ripple into the ON/OFF pin.

ADJUSTABLE OUTPUT, LOW-RIPPLE

POWER SUPPLY

A 500 mA power supply that features an adjustable output

voltage is shown in

Figure 16

. An additional L-C filter that

reduces the output ripple by a factor of 10 or more is includ-

ed in this circuit.

TL/H/11394–23

Note: Complete circuit not shown.

Note: Pin numbers are for 8-pin DIP package.

FIGURE 15. Delayed Startup

TL/H/11394–24

Note: Pin numbers are for 8-pin DIP package.

FIGURE 16. 1.2V to 55V Adjustable 500 mA Power Supply with Low Output Ripple

Definition of Terms

BUCK REGULATOR

A switching regulator topology in which a higher voltage is

converted to a lower voltage. Also known as a step-down

switching regulator.

BUCK-BOOST REGULATOR

A switching regulator topology in which a positive voltage is

converted to a negative voltage without a transformer.

DUTY CYCLE (D)

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

for buck regulator

DetON

TeVOUT

VIN

for buck-boost regulator

DetON

aVIN

CATCH DIODE OR CURRENT STEERING DIODE

The diode which provides a return path for the load current

when the LM2574 switch is OFF.

EFFICIENCY (h)

The proportion of input power actually delivered to the load.

hePOUT

ePOUT

POUT aPLOSS

CAPACITOR EQUIVALENT SERIES RESISTANCE (ESR)

The purely resistive component of a real capacitor’s imped-

ance (see

Figure 17

). It causes power loss resulting in ca-

pacitor heating, which directly affects the capacitor’s oper-

ating lifetime. When used as a switching regulator output

filter, higher ESR values result in higher output ripple volt-

ages.

TL/H/11394–25

FIGURE 17. Simple Model of a Real Capacitor

Most standard aluminum electrolytic capacitors in the

100 mF– 1000 mF range have 0.5Xto 0.1XESR. Higher-

grade capacitors (‘‘low-ESR’’, ‘‘high-frequency’’, or ‘‘low-in-

ductance’’’) in the 100 mF–1000 mF range generally have

ESR of less than 0.15X.

EQUIVALENT SERIES INDUCTANCE (ESL)

The pure inductance component of a capacitor (see

Figure

). The amount of inductance is determined to a large

extent on the capacitor’s construction. In a buck regulator,

this unwanted inductance causes voltage spikes to appear

on the output.

Definition of Terms (Continued)

OUTPUT RIPPLE VOLTAGE

The AC component of the switching regulator’s output volt-

age. It is usually dominated by the output capacitor’s ESR

multiplied by the inductor’s ripple current (DIIND). The peak-

to-peak value of this sawtooth ripple current can be deter-

mined by reading the Inductor Ripple Current section of the

Application hints.

CAPACITOR RIPPLE CURRENT

RMS value of the maximum allowable alternating current at

which a capacitor can be operated continuously at a speci-

fied temperature.

STANDBY QUIESCENT CURRENT (ISTBY)

Supply current required by the LM2574 when in the standby

mode (ON/OFF pin is driven to TTL-high voltage, thus turn-

ing the output switch OFF).

INDUCTOR RIPPLE CURRENT (DIIND)

The peak-to-peak value of the inductor current waveform,

typically a sawtooth waveform when the regulator is operat-

ing in the continuous mode (vs. discontinuous mode).

CONTINUOUS/DISCONTINUOUS MODE OPERATION

Relates to the inductor current. In the continuous mode, the

inductor current is always flowing and never drops to zero,

vs. the discontinuous mode, where the inductor current

drops to zero for a period of time in the normal switching

cycle.

INDUCTOR SATURATION

The condition which exists when an inductor cannot hold

any more magnetic flux. When an inductor saturates, the

inductor appears less inductive and the resistive component

dominates. Inductor current is then limited only by the DC

resistance of the wire and the available source current.

OPERATING VOLT MICROSECOND CONSTANT (E#Top)

The product (in VoIt#ms) of the voltage applied to the induc-

tor and the time the voltage is applied. This E#Top 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.

Physical Dimensions inches (millimeters)

14-Lead Wide Surface Mount (WM)

Order Number LM2574M-3.3, LM2574HVM-3.3, LM2574M-5.0,

LM2574HVM-5.0, LM2574M-12, LM2574HVM-12, LM2574M-15,

LM2574HVM-15, LM2574M-ADJ or LM2574HVM-ADJ

NS Package Number M14B

LM2574/LM2574HV Series SIMPLE SWITCHER 0.5A Step-Down Voltage Regulator

Physical Dimensions inches (millimeters) (Continued)

8-Lead DIP (N)

Order Number LM2574M-3.3, LM2574HVM-3.3, LM2574HVN-5.0, LM2574HVN-12,

LM2574HVN-15, LM2574HVN-ADJ, LM2574N-5.0,

LM2574N-12, LM2574N-15 or LM2574N-ADJ

NS Package Number N08A

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