LM2574 datasheet(14/24 Pages) NSC

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LM2574 Series Buck Regulator Design Procedure (Continued)

PROCEDURE (Adjustable Output Voltage Versions)

EXAMPLE (Adjustable Output Voltage Versions)

Given:

OUT = Regulated Output Voltage

IN(Max) = Maximum Input Voltage

LOAD(Max) = Maximum Load Current

F = Switching Frequency

(Fixed at 52 kHz)

Given:

OUT = 24V

IN(Max) = 40V

LOAD(Max) = 0.4A

F = 52 kHz

1. Programming Output Voltage

(Selecting R1 and R2, as

shown in Figure 2)

Use the following formula to select the appropriate resistor

values.

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

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

1. Programming Output Voltage

(Selecting R1 and R2)

2 = 1k (19.51−1) = 18.51k, closest 1% value is 18.7k

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant,

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

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.

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

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.

2. Inductor Selection (L1)

A. Calculate E

• T(V • µs)

B. E

• T = 185 V • µs

C. I

LOAD(Max) = 0.4A

D. Inductance Region = 1000

E. Inductor Value = 1000 µH

Choose from Pulse Engineer-

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

DS011394-16

FIGURE 8. LM2574HV-ADJ Inductor Selection Guide

LM2574 Datasheet(PDF) 14 Page - National Semiconductor (TI)

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