PM6641 datasheet(37/47 Pages) STMICROELECTRONICS

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PM6641

Components selection

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The losses due to the input capacitor are thus maximized when the duty-cycle is 0.5:

Equation 18

The input capacitor should be selected with a RMS rated current higher than ICinRMS,MAX.

Tantalum capacitors are good in term of low ESR and small size, but they occasionally can

burn out if subjected to very high current during operation. Multi-Layers-Ceramic-Capacitors

(MLCC) have usually a higher RMS current rating with smaller size and very low ESR.

When only one common input capacitor is chosen for the application, instead of one

dedicated capacitor for each regulator (close to each input power supply pins), the total

input current can be quite different from the arithmetic sum of the buck regulators RMS input

currents, if phase management is allowed (see Chapter 7.9: Phase management on

page 30

section for details).

8.3

Output capacitor selection

Using tantalum or electrolytic capacitors, the selection is made referring to ESR and voltage

rating rather than by a specific capacitance value.

The output capacitor has to satisfy the output voltage ripple requirements. At a given

switching frequency, small inductor values are useful to reduce the size of the choke but

increase the inductor current ripple. Thus, to reduce the output voltage ripple a low ESR

capacitor is required:

Equation 19

where VRIPPLE is the maximum tolerable ripple voltage.

The zero introduced by the output capacitor ESR must be higher than the switching

frequency or must be compensated (see Chapter 7.3: SW regulators control loop on

page 24

section for details):

Equation 20

In order to minimize the output voltage ripple, ceramic capacitors are suggested.

(

MAX

LOAD

Cin

MAX

CinRMS

Cin

loss

ESR

⋅

MAX

RIPPLE

ESR

∆

≤

π ESR C

OUT

⋅⋅

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