PM6686 datasheet(39/50 Pages) STMICROELECTRONICS

39 / 50 page

PM6686

Application information

Doc ID 15281 Rev 4

39/50

10.1.2

Input capacitor

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 12

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.

10.1.3

Output capacitor

The selection of the output capacitor is based on the ESR value and on the voltage rating

rather than on the capacitor value Cout.

The output capacitor has to satisfy the output voltage ripple requirements. Lower inductor

value can reduce the size of the choke but increases the inductor current ripp

le ΔIL.

Since the voltage ripple VRIPPLEout is given by:

Equation 13

A low ESR capacitor is required to reduce the output voltage ripple. Switching sections can

work correctly even with 15mV output ripple.

Table 14.

Inductor manufacturer

Manufacturer

Series

Inductor value (µH)

RMS current (A)

Saturation current (A)

COILCRAFT

MSS-1048

3,3

7,22

7,6

COILCRAFT

MSS-1260

3,3

9,7

COILCRAFT

MLC-1550

2,5

16,5

11,4

Table 15.

Input capacitor manufacturer

Manufacturer

Series

Capacitor value (µF)

Rated voltage (V)

TAYIO YUDEN

UMK325BJ106 KM-T

TAYIO YUDEN

GMK325BJ106MN

)

(

)

(

CinRMS

−

out

RIPPLEout

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