Applications Information (Cont.)

9

© 2006 Semtech Corp.

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SC198

POWER MANAGEMENT

Inductor Selection

The SC198 was designed for optimum performance when

using a 4.7μH inductor. The magnitude of the inductor

current ripple is dependent on the inductor value and can

be determined by the following equation:

V

OUT

V

OUT

ΔI

L × f

OSC

V

IN

This equation demonstrates the relationship between in-

put voltage, output voltage, and inductor ripple current.

The inductor should also have a low DCR to minimize the

conduction losses and maximize efficiency. The minimum

DC current rating of the inductor should be equal to the

maximum load current plus half of the inductor current

ripple as shown by the following equation:

ΔI

L

I

2

Final inductor selection will depend on various design con-

siderations such as efficiency, EMI, size and cost. Table B

lists some manufactuers of practical inductor options.

Capacitor Selection

Input Capacitor

The source input current to a buck converter is non-on-

tinuous. To prevent large input voltage ripple a low ESR

ceramic capacitor is required. A minimum value of 4.7μF

should be used for sufficient input voltage filtering and a

22μF MLCC may be used for optimum input voltage filter-

ing.

Output Capacitor

A 4.7μF ceramic capacitor is the minimum recommended

for the output filter capacitor. A capacitor value of at least

10μF is recommended if the part will be used in power-

save mode. Output voltage ripple is dominated by the fil-

ter capacitance ESR as shown in the following equation:

V

Capacitors with X7R or X5R ceramic dielectric are strongly

recommended for their low ESR and superior temperature

and voltage characteristics. Y5V capacitors should not be

used as their temperature coefficients make them unsuit-

able for this application. Table C lists some manufactur-

ers of recommended capacitor options.