PM6675AS datasheet(39/48 Pages) STMICROELECTRONICS

39 / 48 page

PM6675AS

Application information

39/48

In Table 13 some tested polymer capacitors are listed.

8.1.4

MOSFETs selection

In SMPS converters, power management efficiency is a high level requirement, so the

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

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

where RDSon is the MOSFET drain-source on-resistance.

Switching losses are approximately given by:

Equation 42

where tON and tOFF are the turn-on and turn-off times of the MOSFET and depend on the

gate-driver current capability and the gate charge Qgate. A greater efficiency is achieved with

low RDSon. Unfortunately low RDSon MOSFETs have a great gate charge.

As general rule, the RDSon

. Q

gate product should be minimized to find out the suitable

MOSFET.

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

greater than the maximum input voltage VINmax.

Below some tested high-side MOSFETs are listed.

Table 13.

Evaluated output capacitors

Manufacturer

Series

Capacitance

(µF)

Rated voltage

(V)

ESR max

@100kHz (m

Ω)

SANYO

4TPE220MF

220

4 V

15 to 25

4TPE150MI

150

4 V

4TPC220M

220

4 V

HITACHI

TNCB OE227MTRYF

220

2.5 V

switching

conduction

DHighSide

MAX

LOAD

min

OUT

DSon

conduction

⋅

)

(max)

(

)

(max)

(

off

LOAD

switching

⋅

∆

⋅

∆

−

⋅

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