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LTC3868

3868fb

applicaTions inForMaTion

TheequivalentresistanceR1||R2isscaledtotheroom

temperatureinductanceandmaximumDCR:

R R

DCR at

C C

1 2

•

(

)

Thesenseresistorvaluesare:

R R

1 2

;

•

–

ThemaximumpowerlossinR1isrelatedtodutycycle,

andwilloccurincontinuousmodeatthemaximuminput

voltage:

LOSS

IN MAX

OUT

–

•

(

)

= (

)

EnsurethatR1hasapowerratinghigherthanthisvalue.

Ifhighefficiencyisnecessaryatlightloads,considerthis

powerlosswhendecidingwhethertouseDCRsensingor

senseresistors.Lightloadpowerlosscanbemodestly

higherwithaDCRnetworkthanwithasenseresistor,due

totheextraswitchinglossesincurredthroughR1.However,

DCRsensingeliminatesasenseresistor,reducesconduc-

tionlossesandprovideshigherefficiencyatheavyloads.

Peakefficiencyisaboutthesamewitheithermethod.

Inductor Value Calculation

Theoperatingfrequencyandinductorselectionareinter-

relatedinthathigheroperatingfrequenciesallowtheuse

ofsmallerinductorandcapacitorvalues.Sowhywould

anyoneeverchoosetooperateatlowerfrequencieswith

largercomponents?Theanswerisefficiency.Ahigher

frequencygenerallyresultsinlowerefficiencybecause

ofMOSFETgatechargelosses.Inadditiontothisbasic

trade-off,theeffectofinductorvalueonripplecurrentand

lowcurrentoperationmustalsobeconsidered.

Theinductorvaluehasadirecteffectonripplecurrent.The

inductorripplecurrent

∆ILdecreaseswithhigherinduc-

tanceorhigherfrequencyandincreaseswithhigherVIN:

IL =

( ) L

( )

VOUT 1–

VOUT

VIN













Accepting larger values of

∆IL allows the use of low

inductances,butresultsinhigheroutputvoltageripple

andgreatercorelosses.Areasonablestartingpointfor

settingripplecurrentis

∆IL=0.3(IMAX).Themaximum

∆ILoccursatthemaximuminputvoltage.

Theinductorvaluealsohassecondaryeffects.Thetran-

sitiontoBurstModeoperationbeginswhentheaverage

inductorcurrentrequiredresultsinapeakcurrentbelow

30%ofthecurrentlimitdeterminedbyRSENSE.Lower

inductorvalues(higher

∆IL)willcausethistooccurat

lowerloadcurrents,whichcancauseadipinefficiencyin

theupperrangeoflowcurrentoperation.InBurstMode

operation,lowerinductancevalueswillcausetheburst

frequencytodecrease.

Inductor Core Selection

OncethevalueforLisknown,thetypeofinductormust

beselected.Highefficiencyconvertersgenerallycannot

affordthecorelossfoundinlowcostpowderedironcores,

forcingtheuseofmoreexpensiveferriteormolypermalloy

cores.Actualcorelossisindependentofcoresizefora

fixedinductorvalue,butitisverydependentoninductance

valueselected.Asinductanceincreases,corelossesgo

down.Unfortunately,increasedinductancerequiresmore

turnsofwireandthereforecopperlosseswillincrease.

Ferritedesignshaveverylowcorelossandarepreferred

athighswitchingfrequencies,sodesigngoalscancon-

centrateoncopperlossandpreventingsaturation.Ferrite

corematerialsaturates

hard,whichmeansthatinduc-

tancecollapsesabruptlywhenthepeakdesigncurrentis

exceeded.Thisresultsinanabruptincreaseininductor

ripplecurrentandconsequentoutputvoltageripple.Do

notallowthecoretosaturate!

Power MOSFET and Schottky Diode (Optional)

Selection

TwoexternalpowerMOSFETsmustbeselectedforeach

controllerintheLTC3868:oneN-channelMOSFETforthe

top(main)switch,andoneN-channelMOSFETforthe

bottom(synchronous)switch.

Thepeak-to-peakdrivelevelsaresetbytheINTVCCvoltage.

Thisvoltageistypically5.2Vduringstart-up(seeEXTVCC

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