PM6675AS datasheet(37/48 Pages) STMICROELECTRONICS

37 / 48 page

PM6675AS

Application information

37/48

In Pulse-Skip Mode, low inductance values produce a better efficiency versus load curve,

while higher values result in higher full-load efficiency because of the smaller current ripple.

8.1.2

Input capacitor selection

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

with zero average value. The RMS input current can be calculated as follows:

Equation 34

Neglecting the second term, the equation 34 is reduced to:

Equation 35

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

Equation 36

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

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 they remain the best

choice. The drawback is their quite high cost.

It must be taken in account that MLCC capacitance decreases when the operating voltage is

near the rated voltage. In table 12 some MLCC suitable for most of applications are listed.

Table 12.

Evaluated MLCC for input filtering

Manufacturer

Series

Capacitance

(µF)

Rated voltage

(V)

Maximum Irms

@100 kHz (A)

TAIYO YUDEN

UMK325BJ106KM-T

TAIYO YUDEN

GMK316F106ZL-T

2.2

TAIYO YUDEN

GMK325F106ZH-T

2.2

TAIYO YUDEN

GMK325BJ106KN

2.5

TDK

C3225X5R1E106M

LOAD

RMS

Cin

)

(

)

(

∆

⋅

−

⋅

)

(

LOAD

RMS

Cin

−

⋅

LOAD

Cin

CinRMS

Cin

loss

(max))

(

ESR

(max)

ESR

⋅

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