PM6670ASTR datasheet(44/54 Pages) STMICROELECTRONICS

44 / 54 page

Application information

PM6670AS

44/54

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

negligible. Then the inductance could 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 38

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

capacitor voltage before the load transient.

In Table 8 are listed some tested polymer capacitors are listed.

8.1.4

MOSFETs selection

In most of the applications, power management efficiency is a high level requirement. Power

dissipation on the power switches becomes animportant 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 39

Maximum conduction losses are approximately given by:

Equation 40

Table 8. 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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