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Transcript
ASensible30AEnergyMeter ByChristopherGobok SeniorProductMarketingEngineer,MixedSignalProducts LinearTechnologyCorporation Introduction Tellsomeonethatyouhaveasolarpanelsysteminstalledontheroofofyourhomeandyou’velikely openedyourselfuptoaslewofquestions,including“Howmanypanelsdoyouhave?”or“Whatsizeis yoursystem?”Mostpeopletryandgaugeyoursystem’spowerratinginasometimesroundaboutway. Then,theymayproceedtoaskhowefficientyourpanelsare,ifyouarestilltiedtothegrid,orhow muchthewholesystemcostyou–allperfectlyvalidquestions.However,ifyou’relikemeandliketo explainthe“bigpicture”first(andminimizethequestions),trystartingtheconversationwithan explanationofhowmuchenergyyouconsumeandgenerate,sinceunlikepower,energyistheendgame andtrueperformancemetricofanysystemfromanelectricalconsumptionandgenerationstandpoint. Monitoringenergyhassimilarbenefitsinmanyapplications,notjustsolar.Handheld,rack-mounted andin-lineenergymetersarewidelyavailableandcanbeusedbypeoplesuchasfacilitymanagersto trackandallocateenergyusedbyequipmentordepartmentsamongmanythings.Thismayalsoinclude loadprofiling,whereexpectedenergyconsumptionpatternsarecomparedtopresentusageandareas ofconcernareflaggedbasedondeviationsfrommodeledenergypatterns.Bysizingloads,peoplecan determinehowmanywidgets,includinglights,computers,batteries,canbeconnectedtoasystemat anytime.Electricbikesandvehiclescanreporttheirenergyusepermileandquantifytheenergybeing extractedfromorreturnedtoabattery. Althoughenergymonitoringapplicationsareplentiful,thereareveryfewenergymonitoringICsinthe market.ManysystemdesignershavegottenbywithusingpowermonitoringICs,suchasLinear Technology’s100VLTC2945powermonitor,andhavingamicroprocessorkeeptrackofbothpowerand timewhilealsocalculatingenergy;althoughcomplexcodingisnotrequired,themaindrawbackwith thissolutionisthatcomputingresourcesaretieddown.LinearTechnology’s100VLTC2946energy monitorisamoreelegantsolutionthatprovidesdirectenergymeasurements,whereusershavethe flexibilitytochoosetheirownsenseresistor;however,thechallengeariseswhenyouneedtomeasure highcurrents.EnterLinearTechnology’slongoverdueLTC2947energymonitorwithanintegrated30A senseresistor,whichmakesenergymonitoringextremelypracticalfortoday’smostdemanding applications. RevisitingtheSenseResistor WhendesigninginapowerorenergymonitoringICthatusesasenseresistorforitscurrentsense element,calculatingtherequiredsenseresistorisgenerallyastraightforwardtask.Yousimplyapply Ohm’slaw–thatis,youtakethedataconverter’sfullscalevoltageanddividebytheloadcurrent. Then,youcheckyourfavoriteelectronicsdistributor’swebsitetoseewhatrealresistorvaluesare available.Simpleenough,untilyoustartmeasuringdoubledigitcurrents. TaketheLTC2946widerangeI2C,power,chargeandenergymonitorforexample,whichhasafull-scale voltageofapproximately100mV.IftheLTC2946isusedtomeasurea30Arail,a3.3mΩsenseresistoris M130,EN required,whichisreadilyavailable,butitwillhavetodissipate2.9Wofpower!Veryfewpeopleinthe world,ifany,wouldbewillingtodissipatethismuchpowerforasimpleenergymeasurement. Furthermore,becauseofthehighpowerdissipation,thepackagewilllikelynotbe“standard”byany meansand,therefore,fairlyexpensive.Forexample,Digi-KeysellsVishay’sCSM3637P3.3mΩ±1%5W senseresistor,showninFigure1a,for$6.90in1kpcsquantities!Its6mmx3mmx0.6mmbulkmetal foilpackagepracticallymakesita5Wheatsinkthat’sdifficulttomount. Figure1a.LTC2946measuringenergyof Figure1b.LTC2946measuringenergyof 30Arailusing3.3mΩsenseresistor 6Arailusing16mΩsenseresistor Now,iftheLTC2946isusedtomeasurelowercurrents,saya6Arail,thena16mΩsenseresistoris requiredandthepowerdissipationwouldbeamoreacceptable0.57W.Digi-KeysellsPanasonic’s ERJ8CW16mΩ±1%1Wsenseresistor,showninFigure1b,forareasonable$0.09in1kpcsquantities;it ishousedinatiny3.2mmx1.6mmx0.65mm1206package,whichisn’tdifficulttomountandwouldn’t addmuchtotheoverallboardspace.Ifanything,thismakesfora2.4%energymonitoringsolutionthat worksoverthe˗40°Cto85°Coperatingtemperaturerange,whichmaybetooinaccurateforsome applications;the2.4%doesnotincludeanyinaccuracyduetothethermocoupleeffectscreatedbythe externalconnections(i.e.,leadsandtraces)betweentheLTC2946andsenseresistor. Whetheryouwanttomeasurea30Arailor6Arail,asimpleralternativetotheLTC2946wouldbethe LTC2947energymonitor,showninFigure2,whichintegratesa300µΩsenseresistorandeliminatesthe headachesofusingexternalsenseresistorstomeasurehighcurrents,includingpowerdissipation, accuracy,temperaturedriftandsizeissues.Whenmeasuringafull-scalecurrentof30A,thevoltage dropovertheLTC2947’sintegratedsenseresistorisonly10mV,causingapowerdissipationofonly aboutaquarterWattor10mWwhenmeasuringa6Arail.Inadditiontolowpowerdissipation,the LTC2947offershighdynamicrangeduetoitslowoffsetofonly-9mA(or2.7µV).Temperature compensatedenergyreadingsareguaranteedtobe1.2%accurateatroomtemperatureor1.5% accurateovertheentire-40°Cto85°Coperatingtemperaturerange.Furthermore,theLTC2947is housedina4mmx6mm32-pinQFNpackage,makingitveryeffectiveinspace-constraineddesigns. Figure2.LTC2947power/energymonitorwithintegratedsenseresistor EnergyMeasurements TheLTC2947measuresmanyparameters,includingcurrent,voltage,power,charge,energy, temperatureandtime.RefertotheblockdiagraminFigure3.TheLTC2947usesthreeΔ∑ADCs,twoof whichmeasurevoltageandcurrentwhilethethirdADCcalculatespower.Incontinuousmode,the ADCscontinuouslyandsimultaneouslymeasurecurrent,voltage,powerandtemperatureandupdate correspondinginternalregistersevery100ms.Asingleshotmodetriggersasinglesetofround-robin measurements.Whennomeasurementsneedtobemade,theLTC2947canparkitselfineither shutdownmode,wheretotalcurrentconsumptionisreducedtolessthan10µA,oridlemode,whereall circuitrystaysactiveandreadytogointocontinuous,singleshotorshutdownmode. Figure3.LTC2947blockdiagram TheLTC2947’s1.3%accurateenergymeasurementsarereallyowedtotheuniquewayitcalculates power.UnlikeexistingpowermonitorswherepowerismultipliedatanADC’sconversionrate,the LTC2947implementsauniquemeasurementschemethatresultsinmaximumpowermeasurement accuracy.EachofthethreeADCsintheLTC2947istailoredforaspecifictask.ThefirstADCmeasures currentfrom-30Ato30Aandusesacontinuousoffsetcalibrationtoensurethatallinputsamplesare averagedwithequalweightandnosamplesaremissed. ThesecondADCmeasuresbothinternaltemperatureanddifferentialvoltageatthesametimethefirst ADCismeasuringcurrent.TemperatureisbothreportedtothehostandusedinternallybytheLTC2947 tocompensateforthetemperaturedriftoftheinternalcurrentsenseresistor,resultinginmore accuratecurrentmeasurements.Becauseofthe0Vto15Vrail-to-railoperatingrange,theLTC2947is usefulinmanytypesofsystems.Notonlydo20Vabs-max-ratedsupplyandsensepinsprovidealotof headroomfor12Vapplications,butthezerovoltsensemonitoringcapabilityisjustasusefulin monitoringcurrentlevelsduringshortcircuitorblackoutsituations.Faultcurrentlevelsatzerovolts canimmediatelyindicatewhetherthepowersupplyorloadhasgonebadwithoutadditionalcircuitry. TheLTC2947’s“secretsauce”whenmeasuringpowerandenergyreallyresidesinthethirdADC,which multipliescurrentandvoltageata5MHzsamplingfrequency,priortoanyconversionaveraging.You see,intypicalpowerorenergymonitoringICs,oneortwoADCsareusedtomeasurebothcurrentand voltage,andtheresultsaremultipliedtoobtainpower;however,becauseΔ∑ADCsarenormallyused, thevaluesbeingmultipliedareinherentlyaveragecurrentandaveragevoltage,whichwillalways contributetosomepowererror.Insteadofmultiplyingaveragedvalues,theLTC2947multipliesraw (pre-decimationfilter)readingsofcurrentandvoltage,thenconvertstheresult.Thisenablesthe LTC2947toaccuratelymeasurepowerinthepresenceofcurrentandvoltagevariationsupto50kHz– farbeyonditsconversionfrequency.Thismighthappenif,forexample,powerisdrawnfromabattery withsignificantimpedance. Figure4showsanexampleofacurrentandvoltagewaveformthatarechangingphasesovera20µs interval,aswellashowpowerwouldbecalculateddifferentlyintypicalpowerorenergymonitoringICs versustheLTC2947.IntypicalpowerorenergymonitoringICs,poweriscalculatedastheaverage currentmultipliedbytheaveragevoltage.IntheLTC2947,poweriscalculatedastheaverageof multipliedsamples(inthisexample,twosamplesareused).TheLTC29470.218Wpowercalculation morecloselyresemblesactualpowerandthetypicalpowerorenergymonitoringIC’s0.234Wpower calculationisa7.3%error.TheLTC2947avoidsthiserrorandmaintainsaccuracywithupto50kHz signals. Figure4.Exampleoftypicalpowercalculationvs.LTC2947powercalculation Sincechargeistheamountofcurrentconsumedovertimeandenergyistheamountofpower consumedovertime,theLTC2947integratescurrentandpowerovertimetocalculatechargeand energyflowingtoorfromtheload.Italsokeepstrackofthetotalaccumulatedtimethathasbeenused forintegration,wheretheintegrationtimebasecanbeprovidedeitherbythe1%accurateinternal clockoranexternal100kHzto25MHztimebase.Chargedatacanbeespeciallyusefulinbattery applicationswherechargeisjustoneofmanyprerequisitestoaccuratelydeterminingthestateof charge(SOC)ofabattery.Moreover,energydataisprovingtobemorecommonineveryday applications,asitallowsfordynamicloadingversusrelyingonstaticpowerreadingsforactivities. DigitalConvenience TheLTC2947includesahostofconvenientdigitalfeaturesthatsimplifiesdesigns.Themostapparent digitalfeatureistheintegrationofamultiplierandaccumulatorwhichprovideuserswith24-bitpower and48-bitenergyandchargevalues,alleviatingthehostofpollingvoltageandcurrentdataand performingextracomputations.Aseparate1.8Vto5.5Vdigitalsupplyallowsuserstorunlogiclevelsat avoltagedifferentfromthesupplybeingmonitored. TheLTC2947hasminimumandmaximumregistersforcurrent,voltage,powerandtemperature,which eliminatetheneedforcontinuoussoftwarepollingandfreethebusandhosttoperformothertasks.In additiontodetectingandstoringmin/maxvalues,theLTC2947hasthresholdregistersthatcanbeused toissueanalertintheeventanyofthethresholdsareexceeded,again,eliminatingtheneedforthe microprocessortoconstantlypolltheLTC2947andanalyzedata.TheLTC2947canalsobeconfiguredto generateanoverflowalertafteraspecifiedamountofenergyorchargehasbeendeliveredorwhena presetamountoftimehaselapsed.Foranenergymonitor,analertresponsecanbeequallyasvaluable asminimumandmaximumregisters,soaseparatealertregisterisavailableandallowsuserstoselect whichparameterswillrespondinaccordancewiththeSMBusalertresponseprotocol,wheretheAlert ResponseAddress(ARA)isbroadcastedandthe/ALERTpinispulledlowtonotifythehostofanalert event. Userscanpin-configuretheLTC2947tosupportastandardSPIorI2Cinterfacetocommunicatewiththe outsideworld.SixI2CdeviceaddressesareavailablesomultipleLTC2947scanbeeasilydesignedinto thesamesystem.Astuck-busresettimerresetstheinternalI2Cstatemachinetoallownormal communicationtoresumeintheeventthatI2Csignalsareheldlowforover50ms(stuckbuscondition) foranyreason;thissought-afterstuckbusprotectionfeaturepreventsahostfrommanually troubleshootingabusstucklow,whichmightresultinasystemresetthatisdisruptive,costlyandtime consuming.TheLTC2947alsoprovidesasplitI2Cdataline,whichconvenientlyeliminatestheneedto useI2Csplittersorcombinersforbidirectionaltransmissionandreceivingofdataacrossanisolation boundary. Conclusion TheLTC2947isaconvenientboardlevelenergymonitorthatintegratesa300µΩsenseresistorto eliminatecommonsenseresistorchallengesthattendtoarisewhenmeasuringhighcurrents.Atany currentlevel,threeADCsareuniquelydesignedtoprovideuserswithhighlyaccuratereadingsof current,voltage,power,energy,charge,temperatureandtime.The˗30Ato30Acurrentrangeand0V to15VvoltagerangeallowstheLTC2947toworkinawidevarietyofapplications,includingapplications wherebidirectionalcurrentsarepresent.TheLTC2947’sanalogprowessisequallymatchedbyitshost resource-reducingdigitalfeatures,includingamultiplier,accumulator,min/maxregisters,configurable alertsandaverycapableSPIorI2Cinterface.Occupyingonly24mm2ofboardarea,theLTC2947isthe mostsensibledeviceinLinearTechnology’spowermonitoringportfoliotodate.
