Download Power Supply Sequencing Simplified

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