P379EN

DeliverHighPowerwithCompleteDigitalControl&

TelemetryinCompactPCBArea

ByTonyArmstrongDirectorofProductMarketing

PowerProducts

LinearTechnologyCorporationtarmstrong@linear.com

IntroductionItshouldcomeasnosurprisetoanyonethatathecontinuingadoptionofDigitalPowerSystemManagement(DPSM)withinthecommunicationsandcomputerindustriescontinuestobedriven,inlargepart,tothehighcurrentlevelsdemandedbythesub-20nmASICsand/orFPGAsthatareatthecoreoftheirsystemarchitecture.Bywayofexample,considerthenewestASICsbeingusedinthenextgenerationdatacenterswitches;theyenableamoreflexiblesetofinterfacesforportscarrying100Mbitto100Gbit/secondEthernetand32Gbit/sfiberchanneltraffic.Thisallowshigherdensityof100Gportstobeplacedintoasinglerackunit.Coupledwiththis,thesesub-20nmprocessesallow20plusMbytesofmemorytobeplacedontheASICitself,therebypotentiallyeliminatingtheneedforexternalmemory,savingboardspaceandcost.Nevertheless,thesystemarchitectsofthistypeofcommunicationsequipmentareconstantlybeingpushedtoincreasethedatathroughputandperformanceoftheirsystemsaswellasaddfunctionalityandfeatures.Simultaneously,pressureisbeingappliedtodecreasethesystemsoverallpowerconsumption.Forexample,atypicalchallengeistoreduceoverallpowerconsumptionbyreschedulingtheworkflowandmovingjobstounderutilizedservers,therebyenablingshutdownofotherservers.Tomeetthesedemands,itisessentialtoknowthepowerconsumptionoftheend-userequipment.Thus,aproperlydesigneddigitalPSMcanprovidetheuserwithpowerconsumptiondata,allowingforsmartenergymanagementdecisionstobemade.ThisisakeyrequirementfordatacenterswheretheelectricitycostsfortheHVACsystemsthatkeeptheinternalambienttemperaturesundercontrolaresignificant.Inreality,thesecostscanbeinthemillionsofdollarsonamonthlybasis.

SystemChallengesWhendigitalpowerisdonecorrectly,itcanreduceequipmentpowerconsumption,shortentimetomarket,haveexcellentstabilityandtransientresponse,andincreaseoverallsystemreliability.AprincipalbenefitofDPSMisreduceddesigncostandfastertimetomarket.Complexmulti-railsystemscanbeefficientlydevelopedusingacomprehensivedevelopmentenvironmentwithintuitivegraphicaluserinterface(GUI).Suchsystemsalsosimplifyin-circuittesting(ICT)andboarddebugbyenablingchangesviatheGUIinsteadofsolderingin“whitewire”fixes.Anotherbenefitisthepotentialtopredictpowersystemfailuresandenablepreventivemeasures,thankstotheavailabilityofreal-timetelemetrydata.Perhapsmostsignificantly,DC/DCconverterswithdigitalmanagementfunctionalityenabledesignerstodevelop“green”powersystemsthatmeettargetperformance(computespeed,datarate,etc.)withminimumenergyusageatthepointofload,board,rackandeveninstallationlevels,reducinginfrastructurecostsandthetotalcostofownershipoverthelifeoftheproduct.Manytelecomanddatacomsystemsarepoweredviaa48Vbackplane.Thisvoltageisnormallysteppeddowntoalowerintermediatebusvoltageoftypically12Vto3.3Vtopowertheracksofboardswithinthesystem.However,mostofthesub-circuitsorICsontheseboardsarerequiredtooperateatvoltagesrangingfromsub-1Vto3.3Vatcurrentsrangingfromtensofmilliampstohundredsofamps.Asaresult,point-of-load(PoL)DC/DCconvertersarenecessarytostepdownfromtheintermediatebusvoltagetothedesiredvoltagerequiredbythesub-circuitsorICs.Theserailshavestrictrequirementsforsequencing,voltageaccuracy,marginingandsupervision.Therecanbeasanymanyas50PoLvoltagerailsinatelecomsystemandsystemarchitectsneedasimplewaytomanagetheserailswithregardstotheiroutputvoltage,sequencingandmaximumallowablecurrent.CertainprocessorsdemandthattheirI/Ovoltagerisebeforetheircorevoltage,alternativelycertainASICandDSPsrequiretheircorevoltagerisebeforetheirI/O.Powerdownsequencingisalsonecessary.DesignersneedaneasywaytomakechangestooptimizesystemperformanceandtostoreaspecificconfigurationforeachDC/DCconverterinordertosimplifythedesigneffort.Furthermore,inordertoprotectexpensiveFPGAs,ASICsandDSPsfromthepossibilityofanovervoltagecondition,high-speedcomparatorsmustmonitorthevoltagelevelsofeachrailandtakeimmediateprotectiveactionifarailgoesoutofitsspecifiedsafeoperatinglimits.Inadigitalpoweredsystem,thehostcanbenotifiedwhenafaultoccursviathePMBusalertlineanddependentrailscanbeshutdowntoprotectthepowereddevicessuchasanFPGA.Achievingthislevelofprotectionrequiresreasonableaccuracyandresponsetimesontheorderoftensofmicroseconds.ItwasbecauseofthesechallengesthatLinearTechnology’sdecidedtodevelopaPSMproductlinethatincludessynchronousstep-downDC/DCcontrollerswithintegratedpowerFETgatedriversandcomprehensivepowermanagementfeaturesaccessedviatheI2C-basedPMBus.

Theseincludeprecisionreferencesandtemperature-compensatedanalogcurrent-modecontrolloopsoffering±0.5%DCaccuracy,easycompensationthatiscalibratedtobeindependentofoperatingconditions,cycle-by-cyclecurrentlimit,andfastandaccuratecurrentsharingandresponsetolineandloadtransientswithoutanyoftheADCquantization-relatederrorsfoundinproductsutilizing“digital”control.Someofthesealsoincorporate16-bitdataacquisitionsystemsthatprovidedigitalreadbackofinputandoutputvoltagesandcurrents,dutycycleandtemperature.Alsoincludeisafaultloggingcapabilityviaaninterruptflagalongwitha“blackbox”recorderthatstoresthestateoftheconverteroperatingconditionsjustpriortoafault.Finally,multi-railsystemdevelopmentisfacilitatedthoughLinear’sLTpowerPlay®developmentsoftwareandGUIinterface.SimpleSolutionsforComplexProblemsOK,sowhatdoesthesystemarchitecthavetodoinordertoconfigureadigitalpowersystemmanagementsolutionfortheirendproduct?Oneoftheprimaryobjectiveswillbetodesignasystemsothatitcanbeeasilyconfiguredandmonitoredviaadigitalcommunicationsbus.Thiswillbeachievedbyutilizingoneofthefollowing;I2C,SMBusorPMBus.Eitherofthesebusescanenableon-demandtelemetrycapabilitytoset,monitor,changeandlogpowerparametersofanyPoLconverterconfigurationwithinthesystem.AsimplifiedsnapshotofsuchasystemisshowninFigure1.

Figure 1. Typical Digital Power System Management System Configuration

Ascanbeseeninthisexample,thePoLconvertersillustrate3differenttopologyconfigurations.Atthetopofthefigure,apowersystemmanagerchipisusedalongsideaconventionalDC/DCconverter.TheDC/DCconvertercanbeofanytopologyandhaveanydegreeofintegrationsinceitisthepowersystemmanagerthatwillallowittobeinterfaced,controlledandmonitoredviathecommunicationbus.ThemiddlePoLconverterdemonstratesanincreasedlevelofintegration,namely,thattheDC/DCconverterhasthepowersystemmanagementbuilt-in(inthesamepackage).Andfinally,thebottomPoLconverterisacompactmodulewhichincorporatesthepowersystemmanager,theDC/DCconverterandallitsassociatedexternalcomponentsintoasingleformfactor(LinearcallstheseaµModule®regulator).Linear’suModuleDC/DCregulatorscanprovideasimpleandeffectivewaytodeliverbothhighpoweroutputsandDPSMcapability.SincemanyoftheµModuleregulatorscanbeeasilyparalleledforhighcurrentoutputswithperchannelcurrentmatchingwithinanominal1%ofeachother,therebymitigatingthepotentialsforhotspots.Moreover,onlyoneoftheµModuleregulatorsneedstocontainDPSMcapability,sinceitcansupplythecompletedigitalinterfaceeveniftheuModulesinparalleldoesnothaveDPSMinherentwithinthem.Figure2showsanapplicationschematicofoneLTM4677(36ADPSMµModule)inparallelwith3LTM4650s(50AµModules).

Figure2.ACombinationof1LTM4677DPSMModule&3LTM4650ModulesDelivers186Aat1VfromaNominal12VInput

LTpowerPlayUniversalGUIforDPSMProductsAkeyadvantageofasystemarchitectedwithDPSMproductsisthatwiththerightGUIitiseasytocommunicatewitheachindividualPoLconverterwithininthesystem.So,fromtheonset,LineardecidedtodevelopaGUIthatwasacompletedevelopmentplatformwhichcouldbeeasilyusedwithallofthedifferenttypesofproductsitofferedinitsDPSMproductportfolio–theLTpowerPlaywindowsbaseddevelopmentenvironment.Notonlydoesthissoftwaremake

iseasytocontrolandmonitormultipleLinearPMBus-enableddevicessimultaneously,italsoallowsmodificationoftheDC/DCconvertersconfigurationsinrealtimebedownloadingsystemparameterstotheinternalEEPROMoftheindividualdevices.Thisreducesdesigndevelopmenttimebyallowingsystemconfigurationstobeadjustedinsoftwareratherthanresortingtothetimehonoredtraditionofswappingoutcomponentsandrewiringboardswithasolderiron!Butitdoesn’tstopthere,onceanendsystemhasbeendeployedinthefield,suchasadatacenter,thenthesystemcanbeadjustedrealtimebyanoverseerbysimpleupdatingoperationparametersofthePoLconvertersviaanapplicableinterfaceusingthisGUI.Figure3showsatypicalscreenshotofLinear’sLTpowerPlaydashboardthatauserwouldseewheninterrogatingtheirsystem.AllLinearPMBusproductsaresupportedbythissoftwaredevelopmentsystemwhichcanhelpdesignersquicklydebugsystemsbothduringinitialdevelopmentandthenwhentheyareinstalledatauser’ssite.Itquicklyandeasilyallowsmonitoring,controlandadjustmentofsupplyvoltages,limitsandsequencing.Furthermore,productionmargintestingiseasilyperformedusingacoupleofstandardPMBuscommands.

Figure3.ASnapshotofaTypicalDashboardScreenShotoftheLTpowerPlayGUI ConclusionHavingDPSMcapabilityintoday’sdatacomandtelecomsystemsprovidesasystemarchitectwithansimpleandpowerfulwaytodeliverover180Wattsofpowertothe1.xVcorevoltagesofthenewestsub-20nmASICsandFPGAswithjust4µModuleregulators.UtilizingacombinationoftheLTM4677and3LTM4650sinamultiphaseconfigurationasshowninFigure

2,notonlysavesexpensivePCBrealestate,butreducestheamountofrequiredcoolingduetoitsoveralloperatingefficiency.Furthermore,thesoftwareprogrammabilityofDPSMsignificantlyreducesthedebugtimenormallyassociatedwithsuchendeavors.Thisreducesinfrastructurecosts,aswellasthetotalcostofownershipoverthelifeoftheproduct.Andwhileitmighttakethefunoutofthetraditionalsolderironandrewiringapproachtodebugginghardwaresystems,itsconvenienceandtimesavingattributesaretoocompellingtobeignored.