PM8908 datasheet(21/28 Pages) STMICROELECTRONICS

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PM8908

Application information

6.5

Output capacitors

The output capacitors are basic components to define the ripple voltage across the output

and for the fast transient response of the power supply. They depend on the output voltage

ripple requirements, as well as any output voltage deviation requirement during a load

transient.

During steady-state conditions, the output voltage ripple is influenced by both the ESR and

the capacitive value of the output capacitors as follows:

Equation 6

Equation 7

where

I

L is the inductor current ripple. In particular, the expression that defines VOUT_C

takes into consideration the output capacitor charge and discharge as a consequence of the

inductor current ripple.

6.6

Input capacitors

The input capacitor bank is designed considering, mainly, the input RMS current that

depends on the output deliverable current (IOUT) and the duty cycle (D) for the regulation as

follows:

Equation 8

The equation reaches its maximum value, IOUT/2, with D = 0.5. The losses depend on the

input capacitor ESR and, in the worst case, are:

Equation 9

V

OUT_ESR

I

ESR



V

OUT_C

I

OUT



---------------------------------------



rms

OUT

–





PESR

OUT 2







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