PM6680A datasheet(32/48 Pages) STMICROELECTRONICS

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Design guidelines

PM6680A

32/48

9.4

Input capacitors selection

In a buck topology converter the current that flows into the input capacitor is a pulsed current

with zero average value. The input RMS current of the two switching sections can be roughly

estimated as follows:

Equation 17

Where D1, D2 are the duty cycles and I1, I2 are the maximum load currents of the two

sections.

Input capacitor should be chosen with an RMS rated current higher than the maximum RMS

current given by both sections.

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 the charge. Ceramic capacitors have

usually a higher RMS current rating with smaller size and they remain the best choice.

Below there is a list of some ceramic capacitor manufacturers.

Table 11.

Output capacitor manufacturer

Manufacturer

Series

Capacitor value

(uF)

Rated voltage (V)

ESR max (m

Ω)

SANYO

POSCAP TPB, TPD,

TPE

100 to 470

2.5 to 6.3

12 to 65

PANASONIC

SPCAP UD, UE

100 to 470

2 to 6.3

7 to 18

Table 12.

Input capacitor manufacturer

Manufacturer

Series

Capacitor value (uF)

Rated voltage (V)

TAYIO YUDEN

UMK325BJ106KM-T

TAYIO YUDEN

GMK325BJ106MN

)

(

)

(

CinRMS

−

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