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SP7661 Datasheet(PDF) 10 Page - Sipex Corporation |
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SP7661 Datasheet(HTML) 10 Page - Sipex Corporation |
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10 / 20 page  0 Mar -07 Rev N SP766 Wide Input Voltage Range 3A, 600kHz Buck Regulator © 2007 Sipex Corporation APPLICATIONS INFORMATION andprovidelowcorelossatthehigh switchingfrequency.Lowcostpowdered- ironcoreshaveagradualsaturationchar- acteristicbutcanintroduceconsiderable ACcoreloss,especiallywhentheinduc- torvalueisrelativelylowandtheripple currentishigh.Ferritematerials,although moreexpensive,haveanabruptsatura- tioncharacteristicwiththeinductance droppingsharplywhenthepeakdesign currentisexceeded.Nevertheless,they arepreferredathighswitchingfrequen- ciesbecausetheypresentverylowcore losswhilethedesignerisonlyrequired to preventsaturation.Ingeneral,ferrite ormolypermalloymaterialsareabetter choiceforallbutthemostcostsensitive applications. Optimizing Efficiency Thepowerdissipatedintheinductorisequal tothesumofthecoreandcopperlosses. To minimize copper losses, the winding resistanceneedstobeminimized,butthis usuallycomesattheexpenseofalarger inductor.Corelosseshaveamoresignificant contributionatlowoutputcurrentwherethe copperlossesareataminimum,andcan typicallybeneglectedathigheroutputcur- rents where the copper losses dominate. Core loss information is usually available fromthemagneticsvendor.Properinduc- torselectioncanaffecttheresultingpower supplyefficiencybymorethan15%! Thecopperlossintheinductorcanbecal- culatedusingthefollowingequation: PL(Cu) = I 2 L(rMS) • rwiNDiNg whereIL(rMS)istheRMSinductorcurrent thatcanbecalculatedasfollows: IL(rMS) = IOut(MAx) • √+ (. IPP )2 3 IOut(MAx) Inductor Selection Therearemanyfactorstoconsiderinse- lectingtheinductorincludingcorematerial, inductancevs.frequency,currenthandling capability,efficiency,sizeandEMI.Inatypi- calSP7661circuit,theinductorischosen primarilyforvalue,saturationcurrentandDC resistance.Increasingtheinductorvaluewill decreaseoutputvoltageripple,butdegrade transientresponse.Lowinductorvaluespro- videthesmallestsize,butcauselargeripple currents,poorefficiencyandrequiremore outputcapacitancetosmoothoutthelarger ripple current. The inductor must be able tohandlethepeakcurrentattheswitching frequencywithoutsaturating,andthecopper resistanceinthewindingshouldbekeptas lowaspossibletominimizeresistivepower loss.Agoodcompromisebetweensize,loss andcostistosettheinductorripplecurrent tobewithin20%to40%ofthemaximum outputcurrent. Theswitchingfrequencyandtheinductor operatingpointdeterminetheinductorvalue asfollows: L= . VOut•(ViN(MAx) - VOut) ViN(MAx) • ƒs•Kr • IOut(MAx) where: ƒs=switchingfrequency Kr=ratiooftheACinductorripplecurrent tothemaximumoutputcurrent Thepeak-to-peakinductorripplecurrentis: IPP = . VOut•(ViN(MAx) - VOut) ViN(MAx) • ƒs•L Oncetherequiredinductorvalueisselected, theproperselectionofcorematerialisbased onpeakinductorcurrentandefficiencyre- quirements.Thecoremustbelargeenough nottosaturateatthepeakinductorcurrent IPeAk = IOut(MAx) + IPP 2 |
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