M138, EN

LowIQIdealDiodeControllerwithReverseInputProtectionfor

AutomotiveandTelecomPowerSolutionsByMeilissaLum,DesignEngineer,MixedSignalProducts,LinearTechnologyCorp.

Blockingdiodesarewidelyusedinpowersuppliestosolveavarietyofproblems.Inautomotivesystems,aseriesblockingdiodeprotectsagainstaccidentalreversebatteryconnectionswhenthebatteryisreplacedorthecarisjumpstarted.Highavailabilitysystemsandtelecompowerdistributionsemployblockingdiodestoachieveredundancybyparallelingpowersupplies.Diodesarealsousedtopreventdischargeofreservoircapacitorsinsituationswheresometemporaryholdupofoutputvoltageisnecessarytoridethroughinputdropoutsornoisespikes,ortoallowtheloadtogracefullypowerdownwhentheinputsupplyabruptlyfails.Whileblockingdiodesareeasytounderstandandapply,theirforwarddropresultsinsignificantpowerdissipation,makingthemunsuitableinbothlowvoltageandhighcurrentapplications.Inlowvoltageapplications,theforwardvoltagedropbecomesalimitingfactorforacircuit’soperatingrange,evenwhenusingaSchottkybarrierdiode.Atleast500mVofsupplyheadroomislostacrossaseriesdiode—asubstantialdegradationin12Vautomotivesystemswherethesupplycandroptoaslowas4Vduringcoldcrank.Sincediodesoperateatafixedvoltagedropof400mVto700mVminimum,regardlessofcurrentrating,powerdissipationbecomesanissueinthe1A–2Arange,forsurfacemountapplications.Inapplicationsgreaterthan5A,powerdissipationbecomesamajorissue,requiringelaboratethermallayoutsorcostlyheatsinkstokeepthediodecool.Circuitdesignersneedabettersolution.OnesolutionistoreplacediodeswithMOSFETswitches.TheMOSFETisconnectedsothatitsbodydiodepointsinthesamedirectionasthediodeitreplaces,butduringforwardconductiontheMOSFETisturnedon,shortingthebodydiodewithalowlosspaththroughtheMOSFETchannel.Whenthecurrentreverses,theMOSFETisturnedoff,andthebodydiodeblockstheflowofcurrent,thusmaintainingthediodebehavior.Theforwarddropandpowerdissipationarereducedbyasmuchasafactorof10.Thisformsthebasisofan“ideal”diode,whencomparedtoconventionalp-norSchottkybarrierdiodes.TheLTC4357andLTC4359areidealdiodecontrollers,designedtodriveN-channelMOSFETsinawidevarietyofpowersupplyreverseblocking,ORingandholdupapplications.MOSFETswithRDS(ON)specificationsaslowas1mΩarereadilyavailable,soidealdiodescanbebuilttohandlecurrentsinexcessof50Ausingasinglepassdevicewhilemaintainingvoltageandpowerlosslevels10timesbetterthananydiodesolution.TheLTC4357andLTC4359bothreplaceadiode,butthelatterhasawideroperatingrangedownto4Vanditsquiescentcurrentisaquarteroftheformer.TheLTC4359’s/SHDNpinreducesthequiescentcurrentandturnstheLTC4359solutionintoaloadswitch,afeaturetheLTC4357anddiodesolutionsdonothave.Table1highlightsthefeaturesoftheLTC4357andLTC4359.TheLTC4359isalowquiescentcurrentcontrollerwithawideoperatingrangeof4V–80V.The4Vendoftheoperatingrangeisparticularlyimportantinlowvoltageapplications,wherethediodedropisnottolerable,whilethe80Vratingallowsittooperateandsurvivetransientsin48Vtelecomsystemsandautomotiveenvironments.TheLTC4359protectsdownstreamcircuitryfromreverseinputsdownto

−40V,seenwhenbatteryterminalsaremisconnected.Whenoperatingfromabattery,minimizingdischargecurrentisimportantinnormaloperation,andbecomescrucialwhentheloadisoff.TheLTC4359featuresalowquiescentcurrentof155µAtypical,whichcanbefurtherreducedto14µAwhenplacedinshutdown.AlthoughtheMOSFETisturnedoffinshutdown,itsbodydiodecanstillconductforwardcurrent.Someapplicationsrequiretheabilitytoturnon/offaloadortocontrolthedeliveryofpowerindependentofsupplyvoltage.TheLTC4359accomplishesthisbydrivingtwoN-channelMOSFETsasaloadswitchtoblockforwardandreversecurrent.

HOWITWORKSTheLTC4359controlsanN-channelMOSFET,shownasQ1intheblockdiagramofFigure1.TheMOSFETsourceisconnectedtotheinputsupplyandactsliketheanodeofthediode,whilethedrainisthecathode.Whenpowerisfirstapplied,theloadcurrentinitiallyflowsthroughthebodydiodeoftheMOSFET.TheLTC4359sensesthevoltagedropfromIN-OUTanddrivestheMOSFETon.Theinternalamplifier(GATEAMP)andchargepumptrytomaintain30mVdropacrosstheMOSFET.Iftheloadcurrentcausesmorethan30mVofvoltagedrop,theMOSFETisdrivenfullyon,andtheforwarddropincreasesaccordingtoRDS(ON)•ILOAD.

Figure 1. Block diagram of the LTC4359

Iftheloadcurrentisreduced,theGATEAMPdrivestheMOSFETgatelowertomaintaina30mVdrop.Iftheforwardcurrentisreducedtoapointwhere30mVcannotbesupported,theGATEAMPdrivestheMOSFEToff.ThispreventsDCreversecurrentandallowssmoothswitchoverwithoutoscillationinredundantpowersupplyapplications.Intheeventofaninputshort,thecurrentquicklyreversesandissuppliedbyoutputcapacitanceoranothersupply.Thefastpull-downcomparator(FPDCOMP)sensesreversecurrentbymeasuringthedropacrosstheMOSFETbetweenINandOUT.Whenthereismorethan−30mVacrosstheMOSFET,theFPDCOMPcomparatorrespondsbypullingtheMOSFETgatelowinlessthan500ns.TheSHDNpincontrolstheICandtheexternalMOSFETs.Pulling/SHDNpinlowturnsofftheICandexternalMOSFETs,whilereducingthecurrenttoamere14µA.ToturntheICon,the/SHDNpincaneitherbeleftfloatingordrivenhigh.Ifleftfloating,aninternal2.6µAcurrentsourcepullsup/SHDN.

BETTERTHANASCHOTTKYDIODEAMOSFETbaseddiodesolutionreducesthepowerdissipationandforwardvoltagedropoveraSchottkydiode,andismoreversatile,withavastselectionofMOSFETsavailableforvirtuallyanyvoltageandcurrentcombination.Figures2and3comparethepowerdissipationandforwardvoltagedropofanSBG2040CTSchottkydiodetoaBSC028N06NSMOSFET.At20A,theBSC028N06NS2.8mΩMOSFETdissipatesonly1W,saving8WofpowerdissipationovertheSBG2040CTSchottkydiode.TheMOSFET’sgreatlyreducedforwardvoltagedropofRDS(ON)•ILOAD=56mV,comparedto450mVwiththeSchottkydiode,enablescircuitstooperateatalowervoltage.

Figure 2. Power dissipation vs load current

Figure 3. Load current vs forward voltage drop

12V/20AAUTOMOTIVEDIODEWITHREVERSEINPUTPROTECTION

Figure4showsatypical12V,20Aapplicationthatcanhandlereverseinputsto−40V.Theforwarddropisamere56mVatfullloadcurrent,duetotheMOSFET’slowon-resistanceof2.8mΩ.

Figure 4. 12V/20A ideal diode with reverse input protection

Duringinputshorts,potentiallydestructivetransientscanappearattheIN,SOURCEandOUTpins.D1andD2protectINandSOURCEbyclampingthevoltagetransientstolessthan−40V.Q1,a60VBVDSSMOSFETwithavalancheratingof50A,absorbstheinductiveenergyandpreventsIN,SOURCEandOUTfromexceedingtheirabsolutemaximumratings.Downstreamcircuitry,suchasDC/DCconvertersandlinearregulatorsrequireprotectionagainstvoltagesseenbyreverseinputsandmisconnectedbatteryterminals.TheLTC4359’sinputpinsareratedto−40V.TokeeptheMOSFEToff,aninternalNEGATIVECOMPcomparatorsenseswhentheSOURCEpinisnegativewithrespecttoVSSbyatleast1.7V,andpullsdownontheGATEpin.WiththeMOSFEToff,thenegativevoltageispreventedfromreachingtheload.Reverseinputprotectionislimitedtoabout−40VbythedissipationinR1.

DIODEASALOADSWITCH

TheLTC4359canbeusedasaswitchtocontroldeliveryofpowertotheload.Adiode,whetherit’saSchottkydiodeorthecircuitofFigure4,alwaysconductsforwardcurrent.Inshutdown,theLTC4359turnsofftheMOSFET,butitsbodydiodestillconductsforwardcurrent.Toblockforwardcurrent,anadditionalMOSFET,Q2,isaddedasshowninFigure5.The/SHDNpinservesasthecontrolsignaltoturnon/offtheloadswitch.Pulling/SHDNlowturnsoffbothMOSFETs:Q2blocksforwardcurrent,whileQ1preventsreversecurrent.TheMOSFETbodydiodespointinoppositedirections,whichblocksforwardandreversecurrentflow.Floatingordriving/SHDNhighturnsontheICandenablesdiodebehaviorintheMOSFETs.Duringturn-on,inrushcurrentcanbelimitedbycontrollingtheslewrateattheGATEpinwiththegatecapacitor,C1,andtheLTC4359’scontrolledgatecurrent.

Figure 5. 28V load switch and ideal diode with reverse input protection

Formultiplepowersupplies,duplicatingFigure5enablesactivepowersourceselectionregardlessofrelativesourcevoltage.Thisisincontrasttoapassiveselectionschemewherestrictdiodebehaviorsimplyselectstheinputsourcewiththehighestvoltagesupply.

PARALLELINGSUPPLIESMultipleLTC4359scanbeusedtocombinetheoutputsoftwoormoresuppliesforredundancyorfordroopsharing,asshowninFigure6.Forredundantsupplies,thesupplywiththehighestoutputvoltagesourcesmostoralloftheloadcurrent.Ifthesupply’soutputisshortedtogroundwhiledeliveringloadcurrent,thecurrenttemporarilyreverses,flowingbackwardthroughtheMOSFET.TheLTC4359sensesthisreversecurrentandactivatesthefastpull-downcomparator(FPDCOMP)andturnsofftheMOSFETin500ns.

Figure 6. Redundant power supplies

Iftheother,initiallylower,supplyisnotdeliveringanyloadcurrentatthetimeofthefault,theoutputfallsuntilthebodydiodeofitsORingMOSFETconducts.Meanwhile,theLTC4359chargestheMOSFETgatewith10µAuntiltheforwarddropreducesto30mV.Ifthissupplyissharingloadcurrentatthetimeofthefault,itsassociatedORingMOSFETsimplydrivestheMOSFETgateharderinanefforttomaintainadropof30mV.Droopsharingcanbeaccomplishedifbothpowersupplyoutputvoltagesandoutputimpedancesarenearlyequal.The30mVregulationtechniqueensuressmoothloadsharingbetweenoutputswithoutoscillation.ThedegreeofsharingisafunctionofMOSFETRDS(ON),theoutputimpedanceofthesuppliesandtheirinitialoutputvoltages,asprescribedbyOhm’slaw.

EXTENDINGREVERSEINPUTPROTECTIONRANGEFigure7showstheLTC4359configuredasa48Videaldiodeprotectedagainstreverseinputvoltage.R2isaddedtoextendtheVIN–VOUTrangeto−100VDCwiththeeffectofreducingtheforwardregulationby10mV.Inapplicationswheretheoutputisheldupat+48Vbyasecondsupplyorbychargedcapacitors,Q1willblockareversed48Vinputsupply.Innon-redundantapplicationswheretheoutputcanbeexpectedtofalltozerowhentheinputsupplyisremovedoraccidentlyreversed,inputsofupto−100VDCaresuccessfullyblockedfromreachingtheoutput.

Figure 7. 48V ideal diode with reverse input protection

R2isapulse-ratedcomponentsothatVIN–VOUTtransientsinexcessof−100Vareeasilytolerated.Q1wasselectedforitscombinationof250VBVDSSandexceptionallylowRDS(ON)of20mΩ,butitsavalancheratingisamodest320mJwith47Amaximumavalanchecurrent.IntheeventthereversecurrentexceedstheMOSFETavalanchecurrentrating,D6canbeaddedtoprotectQ1byabsorbinganyavalancheenergy,andthislimitsthepeakVIN–VOUTvoltageto−150V.BeyondthispointD6breaksdownandpassestransientcurrentpulsesthroughtotheoutput.

CONCLUSIONTheLTC4359idealdiodecontrollerreplacesSchottkydiodes,andalsocandrivealoadswitch.Atcurrentsof1A–2Aormore,theLTC4359issuperiortoSchottkydiodesolutions.Withitswide4V–80Voperatingrangeandreverseinputcapability,theLTC4359maintainslowforwarddropinlowvoltage

applicationsthroughautomotivecoldcrank,andprotectstheloadfromreversebatteryconnections.Shutdownmodefurtherreducesthealreadylowquiescentcurrentof155µAdownto14µAandcanbeusedasanon/offcontrolsignalforaloadswitch.TheLTC4359isanexcellentfitforautomotiveaswellastelecomandredundantpowersupplyapplications.

Table1.Idealdiodecontrollers

PARTNUMBER

OPERATINGVOLTAGE

SUPPLYCURRENTAT12V

IGATE(UP) FEATURES

LTC4357 9V–80V 650µA 20µA 0.5µsturn-offtimewith2Agatepull-down

LTC4359 4V–80V 155µA 10µALowIQshutdownmode,reverseinputprotectionto−40V,controlssingleorback-to-backMOSFETs