TPS40200-EP中文资料(21)

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SGLS371–JANUARY2007

InductorSelectionCriteria

TheTPS40200P-channelFETdriverfacilitatesswitchingthepowerFETatahighfrequency.This,inturn,enablestheuseofsmaller,less-expensiveinductorsasshowninthis300-kHzapplication.Ferrite,withitsgoodhigh-frequencyproperties,isthematerialofchoice.Severalmanufacturersprovidecatalogswithinductorsaturationcurrents,inductancevalues,andLSRs(internalresistance)fortheirvarious-sizedferrites.

Inthisapplication,thepartmustdeliveramaximumcurrentof2.5A.Thisrequiresthattheoutputinductorsaturationcurrentbeabove2.5Aplusone-halftheripplecurrentcausedduringinductorswitching.Thevalueoftheinductordeterminesthisripplecurrent.Alowvalueofinductancehasahigherripplecurrentthatcontributestoripplevoltageacrosstheresistanceoftheoutputcapacitors.Theadvantagesofalowinductanceareahighertransientresponse,lowerDCR,highersaturationcurrent,andasmaller,less-expensivepart.Toolowaninductor,however,leadstohigherpeakcurrentsthatultimatelyareboundedbytheovercurrentlimitsettoprotecttheoutputFETorbyoutputripplevoltage.Fortunately,withlow-ESRceramiccapacitorsontheoutput,theresultingripplevoltageforrelativelyhighripplecurrentscanbesmall.

Forexample,asingle1-µF1206-sized6.3-Vceramiccapacitorhasaninternalresistanceof2 at1MHz.Forthis2.5-Aapplication,a10%ripplecurrentof0.25Aproducesa50-mVripplevoltage.Thisripplevoltagemaybefurtherreducedbyadditionalparallelcapacitors.

Theotherboundoninductanceistheminimumcurrentatwhichthecontrollerentersdiscontinuousconduction.Atthispoint,inductorcurrentiszero.Theminimumoutputcurrentforthisapplicationisspecifiedat0.125A.Thisaveragecurrentisone-halfthepeakcurrentthatmustdevelopduringaminimumontime.Theconditionsforminimumontimearehighlineandlowload,using:

V-VOUT

LMAX=IN´tON= 32mH

IPEAK(14)Where:VIN=16VVOUT=3.3VIPEAK=0.25AtON=0.686µst3.3V1

´

isgivenby300kHz16V

Theinductorusedinthecircuitisthecloseststandardvalueof33µH.Thisisthemaximuminductancethatcanbeusedintheconvertertodelivertheminimumcurrent,whilemaintainingcontinuousconduction.

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