LM2597 datasheet(12/34 Pages) NSC

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LM2597/LM2597HV Series Buck Regulator Design Procedure (Adjustable

Output)

PROCEDURE (Adjustable Output Voltage Version)

EXAMPLE (Adjustable Output Voltage Version)

Given:

OUT = Regulated Output Voltage

IN(max) = Maximum Input Voltage

LOAD(max) = Maximum Load Current

F = Switching Frequency

(Fixed at a nominal 150 kHz).

Given:

OUT = 20V

IN(max) = 28V

LOAD(max) = 0.5A

F = Switching Frequency

(Fixed at a nominal 150 kHz).

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in

Figure 12)

Use the following formula to select the appropriate resistor

values.

Select a value for R

1 between 240Ω and 1.5 kΩ. The lower

resistor values minimize noise pickup in the sensitive feed-

back pin. (For the lowest temperature coefficient and the best

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

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in

Figure 12)

Select R

1 to be 1 kΩ, 1%. Solve for R2.

2 = 1k (16.26 − 1) = 15.26k, closest 1% value is 15.4 kΩ.

2 = 15.4 kΩ.

2. Inductor Selection (L1)

A. Calculate the inductor Volt microsecond constant E

• T

• µs), from the following formula:

where V

SAT = internal switch saturation voltage = 0.9V

and V

D = diode forward voltage drop = 0.5V

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 6.

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. Each region is

identified by an inductance value and an inductor code (LXX).

E. Select an appropriate inductor from the four manufacturer’s

part numbers listed in

Figure 7.

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant (E • T),

B. E

• T = 35.2 (V • µs)

C. I

LOAD(max) = 0.5A

D. From the inductor value selection guide shown in

Figure 6,

the inductance region intersected by the 35 (V

• µs) horizontal

line and the 0.5A vertical line is 150 µH, and the inductor code

is L19.

E. From the table in

Figure 7, locate line L19, and select an

inductor part number from the list of manufacturers part num-

bers.

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