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PowerSupplySequencingSimplified ByNathanEnger,SeniorApplicationsEngineer,MixedSignalProducts,LinearTechnologyCorp. Thechallengesindesigningamultiplepowersupplymultiplywitheachadditionalsupplyrail.Thedesignermust considerthedynamicenvironmentofcoordinatedpowersupplysequencingandtiming,generatingpower-on reset,monitoringforfaultsandrespondingappropriatelytoprotectthesystem.Anexperienceddesigner recognizesthatflexibilityiskeytosuccessfullynavigatingtheebbandflowasaprojectmovesfromprototypeto production.Theidealsolutionminimizesthenumberofhardwareandsoftwarechangesduringdevelopment. TheidealmultisupplydesigntoolisasingleICthatresidesinadesignfrombeginningtoend,requiringnowiring changesthroughthelifecycleoftheproduct.Itautonomouslysupervisesandsequencesmultiplepowerrails, cooperatingwithotherICstoseamlesslysupervisemanypowerregulatorsinthesystem,andprovidesfaultand resetmanagement.ThedesignercanusepowerfulPC-basedsoftwaretoconfigure,visualizeanddebugsystem behaviorinrealtimewhenconnectedtoanI2Cbus. TheLTC2937fitsthisbill.Itisa6-channelvoltagesequencerandhighaccuracysupervisorwithEEPROM.Eachof thesixchannelshastwodedicatedcomparatorstoaccuratelymonitorover-andundervoltageconditionstowithin ±0.75%.Thecomparatorthresholdsareindividuallyprogrammableoverarangeof0.2Vto6Vwith8-bit resolution.Thecomparatorsarefast,withdeglitchedpropagationdelaysof10μs.Eachsequencerchannelhasan enableoutputthatcancontrolanexternalregulator,orthegateofapassFET.Allaspectsofsupervisorvoltageand sequencertimingareindividuallyconfigurable,includingup-anddown-sequenceorder,sequencetiming parameters,andfaultresponse.Thebuilt-inEEPROMmakesthepartcompletelyautonomousandabletopower-up inthecorrectstatetocontrolthesystem.Inaddition,multipleLTC2937scancooperatetoautonomouslysequence upto300suppliesinasystem,allusingasingle-wirecommunicationbus. Powersupplyfaultsarecontrollable,visibleandmanageablethroughtheLTC2937’sautonomousfaultresponse behaviors,andthroughdebugregisters.TheLTC2937automaticallydetectsfaultconditionsandcanpowerdown thesysteminacoordinatedmanner.Itcanremainoff,orattempttoresequencethesuppliesafterthefault.Ina systemwithamicrocontrollerandanI2C/SMBus,theLTC2937providesdetailedinformationregardingthetype andcauseofthefault,andthestateofthesystem.Themicrocontrollercanmakedecisionsabouthowtorespond, orallowtheLTC2937torespondonitsown. Figure1.LTC2937sequencingsixsupplies P395, EN THREESTEPSOFPOWERSUPPLYCONTROL Apowersupplycyclehasthreeoperatingsteps:sequence-up,monitoringandsequence-down.Figure2shows thesephasesforatypicalsystem.Duringup-sequencing,eachpowersupplymustwaititsturn,andthenpowerup tothecorrectvoltageinadesignatedamountoftime.Duringthemonitoringphaseeachpowersupplymustremain withindesignatedover-andundervoltagelimits.Duringdown-sequencing,eachsupplymustwaititsturn(oftenin adifferentorderfromup-sequencing),thenpowerdownwithinaconfiguredtimelimit.Atanypoint,something cangowrong,causingafaultinthesystem.Thedesignchallengeistocreateasysteminwhichallofthesesteps, andallofthevariables,areeasilyconfigurable,butcarefullycontrolled. Figure2.Powersupplysequencingwaveforms Sequence-upbeginswhentheONinputtransitionstoactive.TheLTC2937advancesthroughitsup-sequence, enablingeachsupplyinturn,andmonitoringtoensurethatthesupplyvoltagerisesabovetheconfiguredthreshold beforetheconfiguredtime.Anysupplythatfailstomeetitsassignedtimingtriggersasequencingfault. AuniquebenefitoftheLTC2937isitssequencepositionclock.Eachchannelisassignedtoasequenceposition(1– 1023),andreceivesitsenablesignalwhentheLTC2937countstothegivennumberinthesequence.Achannel withsequenceposition1isalwaysenabledbeforeachannelwithsequenceposition2.Ifasystemspecification changes,requiringthesetwochannelstosequenceinadifferentorder,thenthesequencepositionscanbe swapped,poweringthesecondchannelinsequenceposition1,andthefirstinposition2.MultipleLTC2937scan sharesequencepositioninformation,sothatsequencepositionNhappensatthesametimeforallLTC2937chips, andchannelscontrolledbydifferentchipscanparticipateinthesamesequence(seeFigure3). Figure3.TypicalconnectionsbetweenmultipleLTC2937s Themonitoringphasebeginswhenthelastchannelsequencesupandcrossesitsundervoltagethreshold.During monitoring,theLTC2937usesitshighaccuracycomparatorstocontinuouslymonitorthevoltageateachinput againstover-andundervoltagethresholds.Itignoresminorglitchesontheinputs,onlytriggeringifthevoltage crossesthethresholdwithsufficientmagnitudeforsufficienttime.WhentheLTC2937detectsafault,itresponds immediatelyaccordingtoitsconfiguredsupervisorfaultresponse.Inatypicalscenario,itshutsdownallsupplies simultaneously,assertingRESETBtothesystem,thenitattemptstoresequenceupaccordingtothenormalstartupsequence.Thispreventsthesuppliesfrompoweringpartsofthesystemwhileothersareunpowered,or executinganuncoordinatedrecoveryafterthefault.MultipleLTC2937sinasystemcansharefaultstate,and respondtoeachother’sfaults,maintainingcompletecoherencebetweencooperatingchannelsduringfault recovery.TheLTC2937offersnumerousprogrammablefaultresponsebehaviorstosatisfymanydifferentsystem configurations. Thesequence-downphasebeginswhentheONinputtransitionslow.Thesequencepositionclockbeginsitscount againtobringthesuppliesdown,butallofthesequence-downparametersareindependentfromthesequence-up parameters.Channelscansequencedowninanyorder,andmultipleLTC2937chipscoordinatesequencingofall controlledsupplies.Duringthedown-sequence,eachsupplymustfallbelowitsdischargethresholdwithinits configuredtimelimit,ortriggerasequencingfault.TheLTC2937canpulldownonthesupplywithanoptional currentsourcetoactivelydischargeslowmovingsupplies. Thesequencepositionclockenforcesevent-basedsequenceorder,witheacheventwaitingforprecedingevents beforeitcancontinue.TheLTC2937alsoallowstime-basedsequencing,andcanparticipateinsystemsthatenable supplyrailsatpredeterminedtimepoints.Reconfigurableregistersfunctionineithertime-basedorevent-based mode. LTpowerPlayMAKESITSIMPLE TheLTC2937’sextensiveregistersetispowerful,yetmasteringitissimple.TheLTpowerPlaygraphicaluser interface(GUI)displaysallofthestatusanddebugregisterinformationinoneconvenientinterface.TheGUI communicateswithanyLinearTechnologypowersystemmanagementIC(includingtheLTC2937)onthe I2C/SMBus.ConfiguringoneormoreLTC2937sisassimpleasafewclicksofthemouse. LTpowerPlaysavessettingsonthePC,andcanwritethemintotheLTC2937EEPROM.TheGUIalsoshowsallofthe debuginformationforsystemmalfunctions.LTpowerPlaycanshowwhenanysupplyisover-orundervoltage,orif asupplyhasfailedsequencetiming.Afterafault,theGUIallowscompletecontroloverrestartingthesystem.In everystageofthedesign—start-up,configuration,debug,andoperation—LTpowerPlayisanindispensable windowintosystemperformance. Figure4.LTpowerPlaygraphicaluserinterface(GUI)displaysallofthestatusanddebugregisterinformation inoneconvenientinterface.ConfiguringoneormoreLTC2937sisassimpleasafewclicksofthemouse. LTpowerPlaysavessettingsonthePC,andcanwritethemintotheLTC2937EEPROM. CONCLUSION TheLTC2937simplifiespowersystemsequencingandsupervision.Itrequiresverylittleboardrealestatefora completesystem.Itisflexibleandreconfigurable,yetautonomousthroughitsEEPROMmemory.Itcanoperateon itsown,orinconcertwithotherchipsinalargesystem,seamlesslyorchestratingtheoperationsofupto300 powersupplies.n Table1.Programmable6-channelsequencerandsupervisorswithEEPROM LTC2933 LTC2936 LTC2937 Seque ncer No No Yes Compa rator Output s No Yes No Thres hold Range 1Vto13.9V (1×)0.2Vto 5.8V(5×) 0.2Vto5.8V(6×) 0.2Vto6V(6×) Thres hold Accura cy ±1% ±1% ±0.75% Power Supply 3.4Vto13.9V 3.13Vto13.9V 2.9Vto16.5V Packa ge (mm× mm) 5×4DFN-16, SSOP-16 4×5QFN-24, SSOP-24 5×6QFN-28