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SP7661 数据表(PDF) 11 Page - Sipex Corporation |
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SP7661 数据表(HTML) 11 Page - Sipex Corporation |
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11 / 20 page  Mar -07 Rev N SP766 Wide Input Voltage Range 3A, 600kHz Buck Regulator © 2007 Sipex Corporation APPLICATIONS INFORMATION Output Capacitor Selection TherequiredESR(EquivalentSeriesRe- sistance)andcapacitancedrivetheselec- tionofthetypeandquantityoftheoutput capacitors.TheESRmustbesmallenough thatboththeresistivevoltagedeviationdue toastepchangeintheloadcurrentand the output ripple voltage do not exceed thetolerancelimitsexpectedontheoutput voltage. During an output load transient, theoutputcapacitormustsupplyallthead- ditionalcurrentdemandedbytheloaduntil theSP7661adjuststheinductorcurrentto thenewvalue. InordertomaintainVOut,thecapacitance must be large enough so that the output voltageisheldupwhiletheinductorcur- rent ramps to the value corresponding to the new load current.Additionally, the ESRintheoutputcapacitorcausesastep intheoutputvoltageequaltothecurrent. Becauseofthefasttransientresponseand inherent100%to0%dutycyclecapability providedbytheSP7661whenexposedto outputloadtransients,theoutputcapacitor istypicallychosenforESR,notforcapaci- tancevalue. TheESRoftheoutputcapacitor,combined withtheinductorripplecurrent,istypically themaincontributortooutputvoltageripple. ThemaximumallowableESRrequiredto maintainaspecifiedoutputvoltageripplecan becalculatedby: ReSr ≤ ∆VOut IPk-Pk where: ∆VOut = Peak-to-Peak Output Voltage Ripple IPk-Pk = Peak-to-Peak Inductor Ripple Current Thetotaloutputrippleisacombinationof theESRandtheoutputcapacitancevalue andcanbecalculatedasfollows: ∆VOut = √(IPP•(1–D))2+(IPP•ReSr)2 ƒs•COut ƒs=SwitchingFrequency D=DutyCycle COut = Output Capacitance Value Input Capacitor Selection Theinputcapacitorshouldbeselectedfor ripple current rating, capacitance and volt- agerating.Theinputcapacitormustmeet theripplecurrentrequirementimposed bytheswitchingcurrent.Incontinuous conductionmode,thesourcecurrentof thehigh-sideMOSFETisapproximatelya squarewaveofdutycycleVOut/ViN.More accurately,thecurrentwaveformistrap- ezoidal,givenafiniteturn-onandturn-off, switchtransitionslope.Mostofthiscurrent issuppliedbytheinputbypasscapaci- tors.TheRMScurrenthandlingcapability oftheinputcapacitorsisdeterminedat maximumoutputcurrentandunderthe assumptionthatthepeak-to-peakinductor ripplecurrentislow,itisgivenby: ICiN(rMS) = IOut(MAx) • √D(1 - D) Theworstcaseoccurswhenthedutycycle Dis50%andgivesanRMScurrentvalue equaltoIOUT/2.Selectinputcapacitorswith adequateripplecurrentratingtoensurereli- ableoperation. Thepowerdissipatedintheinputcapaci- toris: PCiN = I 2 CiN(rMS) • reSr(CiN) Thiscanbecomeasignificantpartofpower lossesinaconverterandhurttheoverall energy transfer efficiency. The input volt- agerippleprimarilydependsontheinput |
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