PM6675STR datasheet(43/53 Pages) STMICROELECTRONICS

43 / 53 page

PM6675S

Application information

43/53

If ceramic capacitors are used, the output voltage ripple due to inductor current ripple is

negligible. Then the inductance should be smaller, reducing the size of the choke. In this

case it is important that output capacitor can adsorb the inductor energy without generating

an over-voltage condition when the system changes from a full load to a no load condition.

The minimum output capacitance can be chosen by the following equation:

Equation 39

where Vf is the output capacitor voltage after the load transient, while Vi is the output

capacitor voltage before the load transient.

In Table 13 are listed some tested polymer capacitors.

8.1.4

MOSFETs selection

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

The power dissipation on the power switches becomes an important factor in the selection

of switches. Losses of high-side and low-side MOSFETs depend on their working condition.

Considering the high-side MOSFET, the power dissipation is calculated as:

Equation 40

Maximum conduction losses are approximately given by:

Equation 41

Table 13.

Evaluated output capacitors

Manufacturer

Series

Capacitance

(µF)

Rated voltage

(V)

ESR max @100 kHz

Ω)

SANYO

4TPE220MF

220

4 V

15 to 25

4TPE150MI

220

4 V

4TPC220M

220

4 V

HITACHI

TNCB OE227MTRYF

220

2.5 V

MAX

LOAD

min

OUT

−

⋅

switching

conduction

DHighSide

MAX

LOAD

min

OUT

DSon

conduction

⋅

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