PM6681ATR datasheet(32/47 Pages) STMICROELECTRONICS

32 / 47 page

Design guidelines

PM6681A

32/47

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 18

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.

9.5

Power MOSFETs

Logic-level MOSFETs are recommended, since low side and high side gate drivers are

In notebook applications, power management efficiency is a high level requirement. The

power dissipation on the power switches becomes an important factor in switching

selections. Losses of high-side and low-side MOSFETs depend on their working conditions.

The power dissipation of the high-side MOSFET is given by:

Equation 19

Table 12.

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 13.

Input capacitor manufacturer

Manufacturer

Series

Capacitor value (uF)

Rated voltage (V)

Tayio yuden

UMK432 X5506MM-T

TDK

C3225X5R1E106M

)

(

)

(

CinRMS

−

switching

conduction

DHighSide

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