LM2574M-12_技术文档

June 1999

LM2574/LM2574HV

™

SIMPLE SWITCHER 0.5A Step-Down Voltage Regulator

General Description

Features

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

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.

n Adjustable version output voltage range, 1.23V to 37V

(57V for HV version) 4% max over line and load

conditions

n Guaranteed 0.5A output current

n Wide input voltage range, 40V, up to 60V for HV version

n Requires only 4 external components

n 52 kHz fixed frequency internal oscillator

n TTL shutdown capability, low power standby mode

n High efficiency

±

Requiring a minimum number of external components, these

regulators are simple to use and include internal frequency

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.

n Uses readily available standard inductors

n Thermal shutdown and current limit protection

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.

Applications

n Simple high-efficiency step-down (buck) regulator

n Efficient pre-regulator for linear regulators

n On-card switching regulators

±

Other features include a guaranteed 4% tolerance on out-

n Positive to negative converter (Buck-Boost)

put voltage within specified input voltages and output load

±

conditions, and 10% on the oscillator frequency. External

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

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

ing, as well as thermal shutdown for full protection under

fault conditions.

Typical Application (Fixed Output Voltage Versions)

DS011394-1

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

Patent Pending

™

SIMPLE SWITCHER is a trademark of National Semiconductor Corporation

DS011394

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Connection Diagrams

8-Lead DIP

14-Lead Wide

Surface Mount (WM)

DS011394-2

* No internal connection, but should be soldered to PC board for best heat

transfer.

Top View

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

DS011394-3

Top View

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 N08A

See NS Package Number M14B

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2

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Lead Temperature

(Soldering, 10 seconds)

Maximum Junction Temperature

Power Dissipation

260˚C

150˚C

Internally Limited

Maximum Supply Voltage

Operating Ratings

LM2574

45V

63V

LM2574HV

Temperature Range

LM2574/LM2574HV

Supply Voltage

LM2574

ON /OFF Pin Input Voltage

Output Voltage to Ground

(Steady State)

−0.3V ≤ V ≤ +V_IN

−40˚C ≤ T_J≤ +125˚C

−1V

40V

60V

Minimum ESD Rating

LM2574HV

=

(C 100 pF, R 1.5 kΩ)

2 kV

Storage Temperature Range

−65˚C to +150˚C

LM2574-3.3, LM2574HV-3.3

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2574-3.3

LM2574HV-3.3

Limit

Units

(Limits)

Typ

3.3

72

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2

=

V_OUT

η

Output Voltage

V_IN12V, I_LOAD100 mA

V

3.234

3.366

V(Min)

V(Max)

V

Output Voltage

LM2574

4.75V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 0.5A

4.75V ≤ V_IN≤ 60V, 0.1A ≤ I_LOAD≤ 0.5A

3.168/3.135

3.432/3.465

V(Min)

V(Max)

Output Voltage

LM2574HV

3.168/3.135

3.450/3.482

V(Min)

V(Max)

%

= =

V_IN12V, I_LOAD0.5A

Efficiency

LM2574-5.0, LM2574HV-5.0

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2574-5.0

LM2574HV-5.0

Limit

Units

(Limits)

Typ

5

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2

=

V_OUT

η

Output Voltage

V_IN12V, I_LOAD100 mA

V

4.900

5.100

V(Min)

V(Max)

V

Output Voltage

LM2574

7V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 0.5A

7V ≤ V_IN≤ 60V, 0.1A ≤ I_LOAD≤ 0.5A

5

4.800/4.750

5.200/5.250

V(Min)

V(Max)

Output Voltage

LM2574HV

5

4.800/4.750

5.225/5.275

V(Min)

V(Max)

%

=

Efficiency

V_IN12V, I_LOAD0.5A

77

3

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LM2574-12, LM2574HV-12

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2574-12

LM2574HV-12

Limit

Units

(Limits)

Typ

12

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2

=

V_OUT

η

Output Voltage

V_IN25V, I_LOAD100 mA

V

11.76

12.24

V(Min)

V(Max)

V

Output Voltage

LM2574

15V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 0.5A

15V ≤ V_IN≤ 60V, 0.1A ≤ I_LOAD≤ 0.5A

12

11.52/11.40

12.48/12.60

V(Min)

V(Max)

Output Voltage

LM2574HV

12

11.52/11.40

12.54/12.66

V(Min)

V(Max)

%

=

Efficiency

V_IN15V, I_LOAD0.5A

88

LM2574-15, LM2574HV-15

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

ture Range.

Symbol

Parameter

Conditions

LM2574-15

LM2574HV-15

Limit

Units

(Limits)

Typ

15

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2

=

V_OUT

η

Output Voltage

V_IN30V, I_LOAD100 mA

V

14.70

15.30

V(Min)

V(Max)

V

Output Voltage

LM2574

18V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 0.5A

18V ≤ V_IN≤ 60V, 0.1A ≤ I_LOAD≤ 0.5A

15

14.40/14.25

15.60/15.75

V(Min)

V(Max)

Output Voltage

LM2574HV

15

14.40/14.25

15.68/15.83

V(Min)

V(Max)

%

=

Efficiency

V_IN18V, I_LOAD0.5A

88

LM2574-ADJ, LM2574HV-ADJ

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

=

ture Range. Unless otherwise specified, V_IN12V, I_LOAD100 mA.

Symbol Parameter Conditions

LM2574-ADJ

Units

(Limits)

LM2574HV-ADJ

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2

=

V_FB

Feedback Voltage

V_IN12V, I_LOAD100 mA

1.230

V

1.217

1.243

V(Min)

V(Max)

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4

LM2574-ADJ, LM2574HV-ADJ

Electrical Characteristics ^(Continued)

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

=

ture Range. Unless otherwise specified, V_IN12V, I_LOAD100 mA.

Symbol Parameter Conditions

LM2574-ADJ

Units

(Limits)

LM2574HV-ADJ

Typ

Limit

(Note 2)

SYSTEM PARAMETERS (Note 3) Test Circuit Figure 2

V_FB

η

Feedback Voltage

LM2574

7V ≤ V_IN≤ 40V, 0.1A ≤ I_LOAD≤ 0.5A

1.230

77

V

V_OUTProgrammed for 5V. Circuit of Figure 2

1.193/1.180

1.267/1.280

V(Min)

V(Max)

Feedback Voltage

LM2574HV

7V ≤ V_IN≤ 60V, 0.1A ≤ I_LOAD≤ 0.5A

V_OUTProgrammed for 5V. Circuit of Figure 2

1.193/1.180

1.273/1.286

V(Min)

V(Max)

%

= = =

V_IN12V, V_OUT5V, I_LOAD0.5A

Efficiency

All Output Voltage Versions

Electrical Characteristics

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

=

ture Range. Unless otherwise specified, V_IN12V for the 3.3V, 5V, and Adjustable version, V_IN25V for the 12V version,

=

and V_IN30V for the 15V version. I_LOAD100 mA.

Symbol Parameter Conditions

LM2574-XX

LM2574HV-XX

Limit

Units

(Limits)

Typ

(Note 2)

DEVICE PARAMETERS

=

I_b

Feedback Bias

Current

Adjustable Version Only, V_OUT5V

50

52

100/500

nA

f_O

Oscillator Frequency

Saturation Voltage

(see Note 10)

kHz

kHz(Min)

kHz(Max)

V

47/42

58/63

=

V_SAT

DC

I_OUT0.5A (Note 4)

0.9

98

1.2/1.4

V(max)

%

Max Duty Cycle

(ON)

(Note 5)

93

%(Min)

A

I_CL

Current Limit

Peak Current, (Notes 4, 10)

1.0

0.7/0.65

1.6/1.8

2

A(Min)

A(Max)

mA(Max)

mA

=

Output 0V

I_L

Output Leakage

Current

(Notes 6, 7)

(Note 6)

=

Output −1V

7.5

5

=

Output −1V

30

10

mA(Max)

mA

I_Q

Quiescent Current

mA(Max)

µA

=

I_STBY

Standby Quiescent

Current

ON /OFF Pin 5V (OFF)

50

200

µA(Max)

θ_JA

Thermal Resistance

N Package, Junction to Ambient (Note 8)

N Package, Junction to Ambient (Note 9)

M Package, Junction to Ambient (Note 8)

M Package, Junction to Ambient (Note 9)

92

72

˚C/W

102

78

5

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All Output Voltage Versions

Electrical Characteristics ^(Continued)

=

Specifications with standard type face are for T_J25˚C, and those with boldface type apply over full Operating Tempera-

=

ture Range. Unless otherwise specified, V_IN12V for the 3.3V, 5V, and Adjustable version, V_IN25V for the 12V version,

and V_IN30V for the 15V version. I_LOAD100 mA.

=

Symbol Parameter Conditions

LM2574-XX

LM2574HV-XX

Limit

Units

(Limits)

Typ

(Note 2)

ON /OFF CONTROL Test Circuit Figure 2

=

V_IH

V_IL

I_H

ON /OFF Pin Logic

Input Level

V_OUT0V

1.4

1.2

12

2.2/2.4

1.0/0.8

V(Min)

V(Max)

µA

=

V_OUTNominal Output Voltage

=

ON /OFF Pin 5V (OFF)

ON /OFF Pin Input

Current

30

10

µA(Max)

µA

=

I_IL

ON /OFF Pin 0V (ON)

0

µA(Max)

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

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

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

tion tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. 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 +12V for the Adjustable, 3.3V, and 5V versions, and +25V for the 12V and 15V versions, to force the

output transistor OFF.

=

40V (60V for high voltage version).

Note 7:

V

IN

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. Ad-

ditional 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

DS011394-27

DS011394-28

DS011394-29

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6

Typical Performance Characteristics (Circuit of Figure 2) (Continued)

Current Limit

Supply Current

Standby

Quiescent Current

DS011394-30

DS011394-31

DS011394-32

Oscillator Frequency

Switch Saturation

Voltage

Efficiency

DS011394-33

DS011394-35

DS011394-34

Minimum Operating Voltage

Supply Current

vs Duty Cycle

Feedback Voltage

vs Duty Cycle

DS011394-36

DS011394-37

DS011394-38

7

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Typical Performance Characteristics (Circuit of Figure 2) (Continued)

Feedback

Pin Current

Junction to Ambient

Thermal Resistance

DS011394-40

DS011394-39

Continuous Mode Switching Waveforms

Discontinuous Mode Switching Waveforms

=

V_OUT5V, 500 mA Load Current, L 330 µH

=

V_OUT5V, 100 mA Load Current, L 100 µH

DS011394-6

DS011394-7

Notes:

A: Output Pin Voltage, 10V/div

B: Inductor Current, 0.2 A/div

C: Output Ripple Voltage, 20 mV/div,

AC-Coupled

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 µs/div

500 mA Load Transient Response for Continuous

250 mA Load Transient Response for Discontinuous

=

Mode Operation. L 330 µH, C_OUT300 µF

Mode Operation. L 68 µH, C_OUT470 µF

DS011394-8

DS011394-9

Notes:

A: Output Voltage, 50 mV/div.

AC Coupled

A: Output Voltage, 50 mV/div.

AC Coupled

B: 100 mA to 500 mA Load Pulse

Horizontal Time Base: 200 µs/div

B: 50 mA to 250 mA Load Pulse

Horizontal Time Base: 200 µs/div

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8

Block Diagram

DS011394-10

=

R1 1k

=

3.3V, R2 1.7k

=

5V, R2 3.1k

=

12V, R2 8.84k

=

15V, R2 11.3k

For Adj. Version

=

R1 Open, R2 0Ω

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

FIGURE 1.

9

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Test Circuit and Layout Guidelines

Fixed Output Voltage Versions

DS011394-11

C

—

22 µF, 75V

Aluminum Electrolytic

220 µF, 25V

IN

—

OUT

Aluminum Electrolytic

D1

L1

—

Schottky, 11DQ06

330 µH, 52627

(for 5V in, 3.3V out, use

100 µH, RL-1284-100)

2k, 0.1%

R1

R2

—

6.12k, 0.1%

Adjustable Output Voltage Version

DS011394-12

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 pos-

sible. Single-point grounding (as indicated) or ground plane

construction should be used for best results. When using the

Adjustable version, physically locate the programming resis-

tors near the regulator, to keep the sensitive feedback wiring

short.

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10

Test Circuit and Layout Guidelines

U.S. Source

(Continued)

Note 1: Pulse Engineering,

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

(619) 674-8100

(516) 586-5566

Inductor

Value

Pulse Eng.

(Note 1)

*

Renco

NPI

(Note 3)

NP5915

NP5916

NP5917

NP5918/5919

NP5920/5921

NP5922

NP5923

*

(Note 2)

Note 2: Renco Electronics Inc.,

68 µH

RL-1284-68-43

RL-1284-100-43

RL-1284-150-43

RL-1284-220-43

RL-1284-330-43

RL-1284-470-43

RL-1283-680-43

RL-1283-1000-43

RL-1283-1500-43

RL-1283-2200-43

60 Jeffryn Blvd. East, Deer Park, NY 11729

*

100 µH

150 µH

220 µH

330 µH

470 µH

680 µH

1000 µH

1500 µH

2200 µH

*

Contact Manufacturer

52625

52626

52627

52628

52629

52631

*

European Source

Note 3: NPI/APC

+44 (0) 634 290588

47 Riverside, Medway City Estate

Strood, Rochester, Kent ME2 4DP.

UK

*

Contact Manufacturer

*

FIGURE 3. Inductor Selection by

Manufacturer’s Part Number

11

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LM2574 Series Buck Regulator Design Procedure

PROCEDURE (Fixed Output Voltage Versions)

EXAMPLE (Fixed Output Voltage Versions)

Given:

=

V_OUT5V

V_OUTRegulated Output Voltage (3.3V, 5V, 12V, or 15V)

=

V_IN(Max) 15V

V_IN(Max) Maximum Input Voltage

=

I_LOAD(Max) Maximum Load Current

I_LOAD(Max) 0.4A

1. Inductor Selection (L1)

A. Select the correct Inductor value selection guide from Fig-

ures 4, 5, 6, or Figure 7. (Output voltages of 3.3V, 5V, 12V or

15V respectively). For other output voltages, see the design

procedure for the adjustable version.

A. Use the selection guide shown in Figure 5.

B. From the selection guide, the inductance area intersected

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

C. Inductor value required is 330 µH. From the table in Figure

3, choose Pulse Engineering PE-52627, Renco RL-1284-330,

or NPI NP5920/5921.

B. From the inductor value selection guide, identify the induc-

tance region intersected by V_IN(Max) and I_LOAD(Max).

C. Select an appropriate inductor from the table shown in Fig-

ure 3. Part numbers are listed for three inductor manufactur-

ers. The inductor chosen must be rated for operation at the

LM2574 switching frequency (52 kHz) and for a current rating

of 1.5 x I_LOAD. For additional inductor information, see the in-

ductor section in the Application Hints section of this data

sheet.

2. Output Capacitor Selection (C_OUT

)

2. Output Capacitor Selection (C_OUT)

=

A. C_OUT100 µF to 470 µF standard aluminum electrolytic.

A. The value of the output capacitor together with the inductor

defines the dominate pole-pair of the switching regulator loop.

For stable operation and an acceptable output ripple voltage,

(approximately 1% of the output voltage) a value between

100 µF and 470 µF is recommended.

=

B. Capacitor voltage rating 20V.

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 recom-

mended.

Higher voltage electrolytic capacitors generally have lower

ESR numbers, and for this reason it may be necessary to se-

lect a capacitor rated for a higher voltage than would normally

be needed.

3. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.5 times

greater than the maximum load current. Also, if the power

supply design must withstand a continuous output short, the

diode should have a current rating equal to the maximum cur-

rent limit of the LM2574. The most stressful condition for this

diode is an overload or shorted output condition.

A. For this example, a 1A current rating is adequate.

B. Use a 20V 1N5817 or SR102 Schottky diode, or any of the

suggested fast-recovery diodes shown in Figure 9.

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

1.25 times the maximum input voltage.

4. Input Capacitor (C_IN

)

4. Input Capacitor (C_IN)

An aluminum or tantalum electrolytic bypass capacitor located

close to the regulator is needed for stable operation.

A 22 µF aluminum electrolytic capacitor located near the input

and ground pins provides sufficient bypassing.

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12

LM2574 Series Buck Regulator Design Procedure (Continued)

INDUCTOR VALUE SELECTION GUIDES (For Continuous Mode Operation)

DS011394-26

FIGURE 4. LM2574HV-3.3 Inductor Selection Guide

DS011394-14

FIGURE 6. LM2574HV-12 Inductor Selection Guide

DS011394-13

DS011394-15

FIGURE 5. LM2574HV-5.0 Inductor Selection Guide

FIGURE 7. LM2574HV-15 Inductor Selection Guide

13

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LM2574 Series Buck Regulator Design Procedure (Continued)

DS011394-16

FIGURE 8. LM2574HV-ADJ Inductor Selection Guide

PROCEDURE (Adjustable Output Voltage Versions)

Given:

EXAMPLE (Adjustable Output Voltage Versions)

Given:

=

V_OUT24V

V_OUTRegulated Output Voltage

=

V_IN(Max) 40V

V_IN(Max) Maximum Input Voltage

=

I_LOAD(Max) 0.4A

I_LOAD(Max) Maximum Load Current

=

F

Switching Frequency (Fixed at 52 kHz)

F

52 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 resistor

values.

=

R₂1k (19.51−1) 18.51k, closest 1% value is 18.7k

R₁can be between 1k and 5k. (For best temperature coeffi-

cient and stability with time, use 1% metal film resistors)

2. Inductor Selection (L1)

A. Calculate the inductor Volt • microsecond constant,

E • T (V • µs), from the following formula:

A. Calculate E • T (V • µs)

=

B. E • T 185 V • µs

=

C. I_LOAD(Max) 0.4A

B. Use the E • T value from the previous formula and match

it with the E • T number on the vertical axis of the Inductor

Value Selection Guide shown in Figure 8.

=

D. Inductance Region 1000

=

E. Inductor Value 1000 µH Choose from Pulse Engineer-

C. On the horizontal axis, select the maximum load current.

ing Part #PE-52631, or Renco Part #RL-1283-1000.

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

value and the maximum load current value, and note the in-

ductor value for that region.

E. Select an appropriate inductor from the table shown in Fig-

ure 3. Part numbers are listed for three inductor manufactur-

ers. The inductor chosen must be rated for operation at the

LM2574 switching frequency (52 kHz) and for a current rating

of 1.5 x I_LOAD. For additional inductor information, see the in-

ductor section in the application hints section of this data

sheet.

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14

LM2574 Series Buck Regulator Design Procedure (Continued)

PROCEDURE (Adjustable Output Voltage Versions)

3. Output Capacitor Selection (C_OUT

EXAMPLE (Adjustable Output Voltage Versions)

3. Output Capacitor Selection (C_OUT

)

A. The value of the output capacitor together with the inductor

defines the dominate pole-pair of the switching regulator loop.

For stable operation, the capacitor must satisfy the following

requirement:

However, for acceptable output ripple voltage select

C_OUT≥ 100 µF

=

C_OUT100 µF electrolytic capacitor

The above formula yields capacitor values between 5 µF and

1000 µF that will satisfy the loop requirements for stable op-

eration. But to achieve an acceptable output ripple voltage,

(approximately 1% of the output voltage) and transient re-

sponse, 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 se-

lect a capacitor rate for a higher voltage than would normally

be needed.

4. Catch Diode Selection (D1)

A. The catch-diode current rating must be at least 1.5 times

greater than the maximum load current. Also, if the power

supply design must withstand a continuous output short, the

diode should have a current rating equal to the maximum cur-

rent limit of the LM2574. The most stressful condition for this

diode is an overload or shorted output condition. Suitable di-

odes are shown in the selection guide of Figure 9.

A. For this example, a 1A current rating is adequate.

B. Use a 50V MBR150 or 11DQ05 Schottky diode, or any of

the suggested fast-recovery diodes in Figure 9.

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

1.25 times the maximum input voltage.

5. Input Capacitor (C_IN

)

5. Input Capacitor (C_IN)

An aluminum or tantalum electrolytic bypass capacitor located

close to the regulator is needed for stable operation.

A 22 µF aluminum electrolytic capacitor located near the input

and ground pins provides sufficient bypassing. See (Figure 9).

To further simplify the buck regulator design procedure, Na-

tional 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 (3¹⁄

tional Semiconductor sales office in your area.

2

") diskette for IBM compatible computers from a Na-

15

www.national.com

LM2574 Series Buck Regulator Design Procedure (Continued)

V_R

1 Amp Diodes

Schottky

1N5817

SR102

Fast Recovery

20V

MBR120P

1N5818

SR103

30V

40V

11DQ03

MBR130P

10JQ030

1N5819

SR104

The

following

diodes

are all

rated to

100V

11DQ04

11JQ04

MBR140P

MBR150

SR105

50V

60V

90V

11DF1

10JF1

11DQ05

11JQ05

MBR160

SR106

MUR110

HER102

11DQ06

11JQ06

11DQ09

FIGURE 9. Diode Selection Guide

INDUCTOR SELECTION

Application Hints

All switching regulators have two basic modes of operation:

continuous and discontinuous. The difference between the

two types relates to the inductor current, whether it is flowing

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

normal switching cycle. Each mode has distinctively different

operating characteristics, which can affect the regulator per-

formance and requirements.

INPUT CAPACITOR (C_IN

)

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

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

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

regulator.

If the operating temperature range includes temperatures

below −25˚C, the input capacitor value may need to be

larger. With most electrolytic capacitors, the capacitance

value decreases and the ESR increases with lower tempera-

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

pacitor will increase the regulator stability at cold tempera-

tures. For maximum capacitor operating lifetime, the

capacitor’s RMS ripple current rating should be greater than

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

used for both continuous and discontinuous modes of opera-

tion.

In many cases the preferred mode of operation is in the con-

tinuous 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 inductor

values to keep the inductor current flowing continuously, es-

pecially at low output load currents.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see Figure 4 through

Figure 8). This guide assumes continuous mode operation,

and selects an inductor that will allow a peak-to-peak induc-

tor ripple current (∆I_IND) 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.

www.national.com

16

the output ripple voltage can be calculated, or conversely,

Application Hints (Continued)

measuring the output ripple voltage and knowing the ∆I_IND

,

INDUCTOR RIPPLE CURRENT

the ESR can be calculated.

When the switcher is operating in the continuous mode, the

inductor current waveform ranges from a triangular to a saw-

tooth type of waveform (depending on the input voltage). For

a given input voltage and output voltage, the peak-to-peak

amplitude of this inductor current waveform remains con-

stant. As the load current rises or falls, the entire sawtooth

current waveform also rises or falls. The average DC value

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

regulator configuration).

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

=

Ripple Current (∆I_IND

)

212 mA p-p. Once the ∆_INDvalue is

known, the following three formulas can be used to calculate

additional information about the switching regulator circuit:

1. Peak Inductor or peak switch current

2. Minimum load current before the circuit becomes dis-

continuous

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.

=

3. Output Ripple Voltage (∆I_IND) x (ESR of C_OUT

)

The selection guide chooses inductor values suitable for

continuous mode operation, but if the inductor value chosen

is prohibitively high, the designer should investigate the pos-

sibility of discontinuous operation. The computer design soft-

ware Switchers Made Simple will provide all component

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

eration.

The curve shown in Figure 10 illustrates how the peak-to-

peak inductor ripple current (∆I_IND) 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 In-

ductor Selection guides).

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 inex-

pensive 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 be-

cause 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 be-

gins to saturate, the inductance decreases rapidly and the

inductor begins to look mainly resistive (the DC resistance of

the winding). This can cause the inductor current to rise very

rapidly and will affect the energy storage capabilities of the

inductor and could cause inductor overheating. Different in-

ductor types have different saturation characteristics, and

this should be kept in mind when selecting an inductor. The

inductor manufacturers’ data sheets include current and en-

ergy limits to avoid inductor saturation.

DS011394-18

FIGURE 10. Inductor Ripple Current (∆I_IND) Range

Based on Selection Guides from Figure 4 through

Figure 8.

Consider the following example:

=

@

V_OUT5V 0.4A

=

V_IN10V 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 µH. This

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

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

mum load current. For this inductor value, the ∆I_INDwill also

vary depending on the input voltage. As the input voltage in-

creases to 20V, it approaches the upper border of the induc-

tance region, and the inductor ripple current increases. Re-

ferring 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 µH inductance region, the ∆I_INDwill be 53% of

0.4A, or 212 mA p-p.

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. Standard alu-

minum electrolytics are usually adequate, but low ESR types

are recommended for low output ripple voltage and good

stability. The ESR of a capacitor depends on many factors,

some which are: the value, the voltage rating, physical size

and the type of construction. In general, low value or low

voltage (less than 12V) electrolytic capacitors usually have

higher ESR numbers.

This ∆I_INDis important because from this number the peak

inductor current rating can be determined, the minimum load

current required before the circuit goes to discontinuous op-

eration, and also, knowing the ESR of the output capacitor,

The amount of output ripple voltage is primarily a function of

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

17

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FEEDBACK CONNECTION

Application Hints (Continued)

The LM2574 (fixed voltage versions) feedback pin must be

wired to the output voltage point of the switching power sup-

ply. When using the adjustable version, physically locate

both output voltage programming resistors near the LM2574

to avoid picking up unwanted noise. Avoid using resistors

greater than 100 kΩ because of the increased chance of

noise pickup.

pacitor and the amplitude of the inductor ripple current

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

tion Hints.

The lower capacitor values (100 µF- 330 µF) 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.

ON /OFF INPUT

=

Output Ripple Voltage (∆I_IND) (ESR of C_OUT

)

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

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

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

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

safely pulled up to +V_INwithout a resistor in series with it.

The ON /OFF pin should not be left open.

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

0.03Ω can cause instability in the regulator.

GROUNDING

Tantalum capacitors can have a very low ESR, and should

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

cause of their good low temperature characteristics, a tanta-

lum can be used in parallel with aluminum electrolytics, with

the tantalum making up 10% or 20% of the total capacitance.

The 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.

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

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

rent.

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 generous

amounts of printed circuit board copper, such as a ground

plane. Large areas of copper provide the best transfer 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 intro-

duces, and the resultant higher junction temperature.

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 Schot-

tky and “soft” fast-recovery diode selection guide.

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.

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.

A curve displaying thermal resistance vs. pc board area for

the two packages is shown in the Typical Performance Char-

acteristics curves section of this data sheet.

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.)

These thermal resistance numbers are approximate, and

there can be many factors that will affect the final thermal re-

sistance. Some of these factors include board size, shape,

thickness, position, location, and board temperature. Other

factors are, the area of printed circuit copper, copper thick-

ness, 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. Further-

more, 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 increases

because of the increased air current activity at the higher

power levels, and the lower surface to air resistance coeffi-

cient at higher temperatures.

The voltage spikes are present because of the the fast

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

tance of the output filter capacitor. To minimize these voltage

spikes, special low inductance capacitors can be used, and

their lead lengths must be kept short. Wiring inductance,

stray capacitance, as well as the scope probe used to evalu-

ate these transients, all contribute to the amplitude of these

spikes.

An additional small LC filter (20 µH & 100 µF) can be added

to the output (as shown in Figure 16 ) to further reduce the

amount of output ripple and transients. A 10 x reduction in

output ripple voltage and transients is possible with this filter.

www.national.com

18

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

it can be estimated by using the formula:

Application Hints (Continued)

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 fol-

lowing equation.

Where I_Sis obtained from the typical supply current curve

(adjustable version use the supply current vs. duty cycle

curve).

=

T_jT_cu+ (θ_j-cux P_D)

With the switcher operating under worst case conditions and

all other components on the board in the intended enclosure,

measure the copper temperature (T_cu) near the IC. This can

be done by temporarily soldering a small thermocouple to

the pc board copper near the IC, or by holding a small ther-

mocouple on the pc board copper using thermal grease for

good thermal conduction.

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 regu-

lates it to −12V.

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

=

θ_j-cu42˚C/W for the N-8 package

=

θ_j-cu52˚C/W for the M-14 package

DS011394-19

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

FIGURE 11. Inverting Buck-Boost Develops −12V

=

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.

Where f_osc52 kHz. Under normal continuous inductor cur-

rent operating conditions, the minimum V_INrepresents the

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

rent anticipated.

The switch currents in this buck-boost configuration are

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

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

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

mode regulator, and this may overload an input power

source with a current limit less than 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.

Also, the maximum voltage appearing across the regulator is

the absolute sum of the input and output voltage. For a −12V

output, the maximum input voltage for the LM2574 is +28V,

or +48V 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

Because of the structural differences between the buck and

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

sign procedure section can not be used to to select the in-

ductor or the output capacitor. The recommended range of

inductor values for the buck-boost design is between 68 µH

and 220 µH, and the output capacitor values must be larger

than what is normally required for buck designs. Low input

voltages or high output currents require a large value output

capacitor (in the thousands of micro Farads).

Another variation on the buck-boost topology is the negative

boost configuration. The circuit in Figure 12 accepts an input

voltage ranging from −5V to −12V and provides a regulated

−12V output. Input voltages greater than −12V will cause the

output to rise above −12V, but will not damage the regulator.

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

switch current, can be calculated from the following formula:

19

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Additional Applications (Continued)

DS011394-21

Note: Complete circuit not shown.

DS011394-20

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

FIGURE 13. Undervoltage Lockout for Buck Circuit

FIGURE 12. Negative Boost

Because of the boosting function of this type of regulator, the

switch current is relatively high, especially at low input volt-

ages. Output load current limitations are a result of the maxi-

mum current rating of the switch. Also, boost regulators can

not provide current limiting load protection in the event of a

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

necessary.

UNDERVOLTAGE LOCKOUT

In some applications it is desirable to keep the regulator off

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

voltage lockout circuit which accomplishes this task is shown

in Figure 13 while Figure 14 shows the same circuit applied

to a buck-boost configuration. These circuits keep the regu-

lator off until the input voltage reaches a predetermined

level.

DS011394-22

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

V_TH≈ V_Z1+ 2V_BE(Q1)

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 re-

duces the output ripple by a factor of 10 or more is included

in this circuit.

DS011394-23

Note: Complete circuit not shown.

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

FIGURE 15. Delayed Startup

www.national.com

20

Additional Applications (Continued)

DS011394-24

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

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

grade capacitors (“low-ESR”, “high-frequency”, or “low-

inductance”) in the 100 µF–1000 µF range generally have

ESR of less than 0.15Ω.

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.

EQUIVALENT SERIES INDUCTANCE (ESL)

The pure inductance component of a capacitor (see Figure

17). The amount of inductance is determined to a large ex-

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

unwanted inductance causes voltage spikes to appear on

the output.

BUCK-BOOST REGULATOR

A switching regulator topology in which a positive voltage is

converted to a negative voltage without a transformer.

OUTPUT RIPPLE VOLTAGE

DUTY CYCLE (D)

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 (∆I_IND). 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.

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

CAPACITOR RIPPLE CURRENT

RMS value of the maximum allowable alternating current at

which a capacitor can be operated continuously at a speci-

fied temperature.

CATCH DIODE OR CURRENT STEERING DIODE

The diode which provides a return path for the load current

when the LM2574 switch is OFF.

STANDBY QUIESCENT CURRENT (I_STBY

)

EFFICIENCY (η)

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).

The proportion of input power actually delivered to the load.

INDUCTOR RIPPLE CURRENT (∆I_IND

)

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

typically a sawtooth waveform when the regulator is operat-

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

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 operat-

ing lifetime. When used as a switching regulator output filter,

higher ESR values result in higher output ripple voltages.

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.

DS011394-25

FIGURE 17. Simple Model of a Real Capacitor

Most standard aluminum electrolytic capacitors in the

100 µF–1000 µF range have 0.5Ω to 0.1Ω ESR. Higher-

21

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OPERATING VOLT MICROSECOND CONSTANT (E•T_op

)

Definition of Terms (Continued)

The product (in VoIt•µs) of the voltage applied to the inductor

and the time the voltage is applied. This E•T_opconstant is a

measure of the energy handling capability of an inductor and

is dependent upon the type of core, the core area, the num-

ber of turns, and the duty cycle.

INDUCTOR SATURATION

The condition which exists when an inductor cannot hold any

more magnetic flux. When an inductor saturates, the induc-

tor appears less inductive and the resistive component domi-

nates. Inductor current is then limited only by the DC resis-

tance of the wire and the available source current.

www.national.com

22

Physical Dimensions inches (millimeters) unless otherwise noted

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

23

www.national.com

Physical Dimensions inches (millimeters) unless otherwise noted (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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DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL

COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant

into the body, or (b) support or sustain life, and

whose failure to perform when properly used in

accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a

significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform

can be reasonably expected to cause the failure of

the life support device or system, or to affect its

safety or effectiveness.

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Corporation

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Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: support@nsc.com

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Response Group

Tel: 65-2544466

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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.


型号：	LM2574M-12
厂家：	National Semiconductor
描述：	SIMPLE SWITCHER⑩ 0.5A Step-Down Voltage Regulator SIMPLE SWITCHER⑩ 0.5A降压稳压器稳压器开关光电二极管
文件：	总24页 (文件大小：642K)
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LM2574M-12 [NSC]

相关型号：

SI9130DB

SI9135LG-T1

SI9135LG-T1-E3

SI9135_11

SI9136_11

SI9130CG-T1-E3

SI9130LG-T1-E3

SI9130_11

SI9137

SI9137DB

SI9137LG

SI9122E