LM2598 datasheet(10/33 Pages) NSC

10 / 33 page

Test Circuit and Layout Guidelines

(Continued)

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

idly switching currents associated with wiring inductance can

generate voltage transients which can cause problems. For

minimal inductance and ground loops, the wires indicated by

heavy lines should be wide printed circuit traces and

should be kept as short as possible. For best results,

external components should be located as close to the

switcher lC as possible using ground plane construction or

single point grounding.

If open core inductors are used, special care must be

taken as to the location and positioning of this type of induc-

tor. Allowing the inductor flux to intersect sensitive feedback,

lC groundpath and C

OUT wiring can cause problems.

When using the adjustable version, special care must be

taken as to the location of the feedback resistors and the

associated wiring. Physically locate both resistors near the

IC, and route the wiring away from the inductor, especially an

open core type of inductor. (See application section for more

information.)

LM2598 Series Buck Regulator Design Procedure (Fixed Output)

PROCEDURE (Fixed Output Voltage Version)

EXAMPLE (Fixed Output Voltage Version)

Given:

OUT = Regulated Output Voltage (3.3V, 5V or 12V)

OUT =5V

IN(max) = Maximum DC Input Voltage

IN(max) = 12V

LOAD(max) = Maximum Load Current

LOAD(max) = 1A

1. Inductor Selection (L1)

A. Select the correct inductor value selection guide from

Figures

Figure 4, Figure 5,or Figure 6 (Output voltages of

3.3V, 5V, or 12V respectively.) For all other voltages, see the

design procedure for the adjustable version.

A. Use the inductor selection guide for the 5V version shown

Figure 5.

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

ductance region intersected by the Maximum Input Voltage

line and the Maximum Load Current line. Each region is

identified by an inductance value and an inductor code

(LXX).

B. From the inductor value selection guide shown in

Figure 5,

the inductance region intersected by the 12V horizontal line

and the 1A vertical line is 68 µH, and the inductor code is

L30.

C. Select an appropriate inductor from the four manufactur-

er’s part numbers listed in

Figure 8.

C. The inductance value required is 68 µH. From the table in

Figure 8, go to the L30 line and choose an inductor part

number from any of the four manufacturers shown. (In most

instance, both through hole and surface mount inductors are

available.)

2. Output Capacitor Selection (C

OUT)

2. Output Capacitor Selection (C

OUT)

A. In the majority of applications, low ESR (Equivalent Series

Resistance) electrolytic capacitors between 47 µF and 330

µF and low ESR solid tantalum capacitors between 56 µF

and 270 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 330 µF.

For additional information, see section on output capaci-

tors in application information section.

A. See section on output capacitors in application infor-

mation section.

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