Download Using a Current Sharing Controller when the Sum Current of Both

UsingaCurrentSharingControllerwhentheSumCurrentofBothSupplies
isRequiredtoSupporttheLoad
ByBobSmith,MixedSignalProducts,LinearTechnologyCorp.
TheLTC4370isa2-supplycurrentsharing,diode-ORingcontrollerthatusesMOSFETstoformidealdiodes.Inthisway,theLTC4370
canactivelybalancetheoutputcurrentsoftwosupplies,eventhosewithunequaloutputvoltages.Fortwounequalvoltageinput
supplies,theforwardvoltageofthehighervoltagesupplydiodeisservoedtobalancethesharedloadcurrent.Themaximum
allowablevoltagedropisprogrammedbyaresistorattheRANGEpinoftheLTC4370.
NormallytheLTC4370isusedtocurrentsharetheoutputsoftwosupplies,eitherofwhichiscapableofprovidingtheentireload
currentwhenitsmatedropsbelowthesharingvoltagethresholdsetbytheRANGEpinoftheLTC4370(seesidebar).
Nevertheless,theadvantagesofusingthecurrentsharingfunctioncanstillbereapedinanon-redundantsystem,wherethesumload
currentoftwosuppliesisequalto,orinexcessofthatrequiredbytheload.
Innormaloperation,theLTC4370allowsthehighervoltagesupplytosourcealloftheloadcurrent,however,inthecasewherethe
remainingsupplyisincapableofprovidingthefullcurrent,itisnecessarytopreventsuchoperation.Thisarticledescribesasolution
thatdisablesthedownstreamloadwhenthissituationoccurs.
PRINCIPALOFOPERATION
Innormaloperation,theLTC4370monitorsthecurrentofbothsupplies.Normally,withaperfectdiode,thesupplywiththehigher
voltagewouldsourceallofthecurrenttotheload.TheLTC4370preventsthisbylinearlycontrollingtheMOSFETofthehigher
voltagesupplytoprovidecurrentequaltothatofthelowervoltagesupply.Themaximumvoltagedifferenceallowedisdetermined
bytheresistorbetweentheRANGEpinandground.
Whentheinputsupplyvoltagedifferencerisesbeyondtheprogrammedrange,theLTC4370disablesthecurrentsharingfunction.
Therearetwoalarmoutputs,eachmonitoringthecontrolvoltageatthegateofeachMOSFET.Innormaloperation,wheneither
MOSFETisturnedoff(indicatingavoltagedifferencebeyondtheprogrammedrange)itsassociatedFETONsignalissettoalogiclow.
IntheoryitseemsthatifthesesignalswerepassedthroughalogicANDfunction,theycouldbeusedtocontrolthedownstreamload,
disablingitwhenaMOSFETisturnedoff(indicatingthelossofcurrentsharing).Thesesignals,however,bothreverttologiclow
whenzerocurrentispassingthroughtheMOSFET.Inthissituation,withthedownstreamloaddisabledanddrawingnocurrent,the
systemwouldremaininthisstateindefinitely.
Thesolutiondescribedheresensesinputvoltagedifferencesanddisablesthedownstreamloadwhenadefinedvoltagedifference
betweenthesuppliesisdetected.ThisvoltagedifferenceisprogrammedtobebelowthemaximumvoltagethresholdoftheLTC4370.
Ifanoutofbalanceconditionisdetected,thedownstreampowersupplyisdisabled.Topreventanoscillatorycondition,thecircuit
entershiccupmode,wherethesupplyiscycledonfor200msevery3.2seconds.TheblockdiagramisshowninFigure1.
M133, EN
Figure1.Blockdiagramofcurrentsummingcircuitry
Asshownintheblockdiagram,twocomparatorsareusedtosensewhentheabsolutevalueofthedifferencebetweenpowersupply
inputsVINAandVINBisbeyondthatallowedforpropercurrentsharing.Whenthishappens,theoutputofthenegativetrueORgate
isalogichigh,enablingthehiccupcircuit.Normally,theoutputofthehiccupcircuitisalogichigh,enablingthedownstreamload.
Whentheoutofrangefaultconditionisdetected,thehiccupcircuitisactivated,causingalogiclowtodisablethedownstreamload.
Thehiccupcircuitmonitorsthevoltagedifferentialduringthe200msonperiodandisdisabledwhenthefaultconditioniscleared.
CIRCUITDESCRIPTION
Figure2showsthecompletesolution.InFigure2,U2andU3areLT1716Over-The-Topvoltagecomparatorsusedtodetectvoltage
differencesbetweenVINAandVINB.
Figure2.Completeloadsharingdesign
ThresholdoffsetvoltagetothecomparatorsisprovidedbycurrentsinktransistorsQ5andQ6incombinationwithR8andR9.
CurrentatthecollectorsofQ5andQ6isstabilizedat100µAbytransistorsQ1,Q2,Q3,andU6,anLT6650voltagereference.Inthis
case,R8andR9aresetto3.01k,resultinginanoffsetof300mV.Theseresistorvaluescanbechangedtoprovideadifferentoffsetto
matchthatoftheLTC4370.
WheneithercomparatorU2orU3reachesthethresholddeterminedbytheoffset,theiroutputbecomeslogiclow,enablingthe
hiccupcircuit.
U4isa74HC132quadCMOSNANDgatewithhysteresisoneachinput.U5isa74HC1634-bitprogrammableCMOScounter.
TheoutputofU4AislogiclowwhenVINAandVINBiswithinthethresholddeterminedbyR8andR9.WhenVINAandVINBis
beyondthisthreshold,thecorrespondingcomparatoroutputbecomeslogiclow,causingtheoutputofU4Atobecomealogichigh.
AlogichighoutputofU4AisinvertedbyU4B,producingalogiclowatoneinputofNORgateU4C.Theresultinglogichighoutputof
U4CcausescounterU5tobegincounting.Thefirstcountiszero,causingtheTC(terminalcount)pintobecomelogiclow.Thisoutput
remainslowforthenext15countsregardlessoftheinputfromU11B,duetothefeedbackfromitsoutputtotheotherinputofNOR
gateU4C.Oncount16,theTCbecomeshighforaperiodof200ms.Duringthisperiod,thedownstreamloadisenabled.Ifthe
comparatorsdeterminethevoltagedifferenceiswithinlimits,thecounterstopswiththeTCoutputremaininglogichigh,enabling
theload.Ifthevoltagedifferenceisnotwithinlimits,thecounterbeginsagain,countingto15withtheTCoutputalogiclow.Inthis
way,theloadisenabledfor200msevery3.2secondsuntilthefaultconditioniscleared.
TheclockisprovidedbyU4D,ahystereticrelaxationoscillatorwithaperiodof200msdeterminedbyR14andC7.
U1istheLTC4370,whichprovidesthecurrentsharingfunction.Thethresholdissetto300mVbyR1.Operationofthisdeviceis
describedinthedatasheet.
PowerfortheadditionalcircuitryisderivedfromVCCoftheLTC4370.
CONCLUSION
TheLTC4370isdesignedprimarilyasacurrentsharing,diode-ORcontrollerfortworedundantsupplies.Withafewadditional
components,itcanbeeasilyusedinanon-redundantsupplyenvironmentasarobustloadsharingcontroller,wherebothofthe
suppliesarenecessarytosupporttheentireload.Thesolutiondescribedhereprovidesthatfunction.
SIDEBAR:LOADSUMMINGADVANTAGE
Atypicaldiode-ORsystemisawinner-take-allsystemwherethehighestvoltagesupplysourcestheentireloadcurrent.Thisonesupply-at-a-timeschemeunderutilizesthetwosupplies.TheLTC4370’scurrentsharingdiode-ORsolution,ontheotherhand,reaps
thebenefitsofsourcingandsharingcurrentfrombothsupplies:
•
•
•
Supply lifetimes are extended if each takes on half the load, spreading the supply heat and reducing thermal stresses on supply
components.
Because the lower voltage supply is always operational, there is no surprise when transitioning to a backup supply that may have
already silently failed—a possibility in a simple diode-OR system.
The recovery dynamics on supply failure are smoother and faster, since the supply changes are on the order of less and more, not
off and on.
ADC/DCconverterformedbytwosuppliesrunningathalfcapacityhasbetteroverallconversionefficiencythanasinglesupply
runningnearfullcapacity.