元器件交易网

SGLS371–JANUARY2007

Where:

DIpp=DV´D´

tSLl

DV=VIN-VOUT-(DCR+RDSON)´IOUT

RDSON=FETon-stateresistanceDCR=InductordcresistanceD=Dutycycle

tS=Reciprocaloftheswitchingfrequency

Usingthevaluesinthisexample,thedcpowerlossis129mW.TheremainingFETlossesare: PSW–PowerdissipatedwhileswitchingtheFETonandoff Pgate–PowerdissipateddrivingtheFETgatecapacitance PCOSS–PowerswitchingtheFEToutputcapacitance

ThetotalpowerdissipatedbytheFETisthesumofthesecontributions:PFET=PSW+Pgate+PCOSS+PRDSON

TheP-channelFETusedinthisapplicationisanFDC654P,withthefollowingcharacteristics:trise=13×10–9tfall=6×10–9RDSON=0.1 Qgd=1.2×10–9

COSS=83×10–12Qg=9nCVgate=1.9VQgs=1.0×10–9

Usingthesedevicecharacteristicsandthefollowingformulasproduces:

öfSæf

÷PSW=S´ç=10mWVt´´VIN´Ipk´tCHOFFIpkINCHON÷+

2ç2øè

()

(10)

Where:

tCHON=and

QGD´RG

VIN-VTHQGD´RG

VIN

tCHOFF=

aretheswitchingtimesforthepowerFET.

PGATE=QG´VGATE´fS=22mW

PCOSS=

COSS´VIN_MAX´fS

2

2

=2mW

IG=QG×fS=2.7mAisthegatecurrent

Thesumoftheswitchinglossesis34mWandiscomparabletothe129-mWdclosses.Ataddedexpense,aslightlylargerFETwouldbebetterbecausethedclosswoulddropandtheaclosseswouldincrease,withbothmovingtowardtheoptimumpointofequallosses.

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