SP7661 datasheet(11/20 Pages) SIPEX

11 / 20 page

Mar -07 Rev N

SP766 Wide Input Voltage Range 3A, 600kHz Buck Regulator

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

CiN(rMS)

• reSr(CiN)

Thiscanbecomeasignificantpartofpower

lossesinaconverterandhurttheoverall

energy transfer efficiency. The input volt-

agerippleprimarilydependsontheinput

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