LM2598 datasheet(13/33 Pages) NSC

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LM2598 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) = 1A

F = Switching Frequency

(Fixed at a nominal 150 kHz).

1. Programming Output Voltage (Selecting R

1 and R2,as

shown in

Figure 1)

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 1)

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 = 1V

and V

D = diode forward voltage drop = 0.5V

2. Inductor Selection (L1)

A. Calculate the inductor Volt

• microsecond constant

• T),

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 7.

B. E

• T = 34.8 (V • µs)

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

C. I

LOAD(max) = 1A

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).

D. From the inductor value selection guide shown in

Figure 7,

the inductance region intersected by the 35 (V

• µs) horizon-

tal line and the 1A vertical line is 100 µH, and the inductor

code is L29.

E. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in

Figure 8.

E. From the table in

Figure 8, locate line L29, and select an

inductor part number from the list of manufacturers part

numbers.

3. Output Capacitor Selection (C

OUT)

3. Output Capacitor SeIection (C

OUT)

A. In the majority of applications, low ESR electrolytic or solid

tantalum capacitors between 82 µF and 220 µF provide the

best results. This capacitor should be located close to the IC

using short capacitor leads and short copper traces. Do not

use capacitors larger than 220 µF. For additional informa-

tion, see section on output capacitors in application

information section.

A. See section on C

OUT in Application Information section.

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