Understanding Motor Driver Current Ratings - …

1 ApplicationReportSLVA505 ,especiallyasrelatedtotheselectionofa motordriverpartfora thatthereis nostandardwayofspecifyingcurrentratings, (OCP) MotorDriverThemaximumdrivecurrentobtaine dfroma givenmotordriverICis limitedbya ,butalsothePCBconstruction,ambienttemper ature, motordriverICis thoughtofasa switchorsetofswitches,it is nota dissipatedin themotordriverIC,primarilyduetoresistive losseswhichareproportionaltodrivecurrent , complexanda subjectofitsown(refertotheapplicationrep ortCalculatingMotorDriverPowerDissipatio n, SLVA504). Forthepurposesofthisdiscussion,wewillsim plifythepowerlosstothatwhichis dissipatedin theFETON-resistanceofthepowerstage,calle dRDS(ON).

2 1 SLVA505 February2012 UnderstandingMotorDriverCurrentRatingsSu bmitDocumentationFeedbackCopyright 2012,TexasInstrumentsIncorporatedFactors LimitingtheMaximumOutputCurrentofa resistivewhenit is conductingcurrent,it dissipatespoweraccordingtoOhm'slaw:P = I2R,whereI is theDCorRMScurrentflowingtotheloadandR is thesumoftheRDS(ON) ,whendrivingcurrent,therearetwoswitchesd issipatingpower;thehigh-sideswitchtothes upply, (inwatts)bythethermalresistancetoambient temperature,referredtoas JA. The JAvalueis variable,asit JAbasedona toomuchcurrentis driven,thedeviceheatsuptoa (allmotordriverICsfromTI)havea thermalshutdowncircuitdisablingtheoutput swhenthedietemperaturereachesa predefinedthreshold(typicallyaround150 C forTIparts).

3 Themaximumdietemperaturebeforeovertemper atureshutdownis a limitingfactorforhowmuchDCorRMScurrenta nottypicallya ,thethermallimitis thedominantfactorin determiningthemaximumcurrenta notsimpletocalculate,asit dependsgreatlyonconditionsthattheICmanuf acturerdoesnotcontrol, , (OCP)LimitationsThemotordriverICis protectedfrompossibledamageordegradation duetoexcessivecurrentbyincorporatingsome formofovercurrentprotection(OCP).Manymot ordriverICsandallofTI' levelthatis forspecificinformationaboutTI' : ananalogcurrentlimit disablinganindividualFETortheentiredevic ewhensomepresetcurrentlevelis reachedAttemptingtodrawmorecurrentthanis allowedbytheOCPcircuitresultsin a ,theOCPcircuitmaximumcurrentbecomesa important,forexample,whenconsideringthes tart-upcurrentofa whichmodea deglitchcircuitis implementedtoallowa highercurrenttoflowfora necessarytoallowthehighpeakcurrentneeded tochargeparasiticcapacitanceintheload, ,signalrouting,andICpackageofa motordriveraredesignedtosupporta.

4 Safeoperatingarea(SOA)oftheoutputdevices IClayoutconsiderationssuchasthemaximumcu rrent-carryingcapabilityofmetalrouting,v ias,andcontactsonthedie maximumcurrent-carryingcapabilityofthebo ndwiresthatconnectthedietothepackageA devicewithOCPtakescareofeachoftheselimit ations;therefore, February2012 SubmitDocumentationFeedbackCopyright 2012,TexasInstrumentsIncorporatedOvecurr ent DetectAnalog IlimitOVERCURRENT(to digital)OUTPUTFETAMP+-AMP+-PRE_DRIVECOMP +-VREFOUTVMHS_ON(from digital) a motordriverdoesnothaveOCP,donotexceedany absolutemaximumcurrentratings, robustOCPschemepreventingdamagetotheICin ,orsoftshorts,betweentheoutputs, 'sOCPimplementationtypicallyincludestwoc omponents: A fast-actinganalogcurrentlimit,typicallyt ensofnanoseconds,tolimitthecurrentin theoutputtoalevelthatis doesthisbyoperatingtheoutputFETin a linearregion,dissipatingsignificantpower .

5 A digitaltimeis startedassoonasthecurrentrisesabovea predefinedthreshold, ,typicallya fewmicroseconds,if thecurrentlevelis stillabovethethreshold,theoutputis ,soeachFETis protectedfromshortstoeitherthesupply,the ground, independentofanycurrentregulation(curren tchoppingorITRIP) showsa simplifiedschematicoftheanalogportionofa shown;thereis a showsanoscilloscopecaptureofa shortcircuiteventusinga ,theoutputwasenabledwitha theinputsignal,thebluetraceis thefault-outputsignal,andthepinktraceis , briefovershoot,whichis nota problemfortheoutputstage,thecurrentis limitedtoapproximately9 A s, astheOCPdeglitchtimeexpires,thecurrentis stillattheanalogcurrentlevelof9 A, ,theOCPlevelis approximately3 ,theoutputis ,thefaultsignalis February2012 UnderstandingMotorDriverCurrentRatingsSu bmitDocumentationFeedbackCopyright 2012, ShortCircuitEventUsingTIDRV8813 MotorDriverDependingontheindividualdevic e,afteranOCPeventoccurs.

6 Thedevicemaylatchin anoffstateuntilsomeinterventionis madebythesystem(liketheapplicationofa resetsignal),ortheoutputmayautomatically re-enableaftera thedeviceusesanautomaticretryandis operatingintoa continuousshortcircuit, , ,thedeviceis , , ,usuallyliststherecommendedmaximumoutput currentforthedevice:FEATURES Dual-H-BridgeCurrent-ControlMotorDriver CapableofDrivingTwoDCMotorsorOneStepperM otor LowMOSFETOn-Resistance ,2-APeakperH-Bridge(atVM= 5 V,25 C) February2012 SubmitDocumentationFeedbackCopyright 2012, ,theRMS(orDC)maximumcurrentis calculatedasthecurrentthatthedevicecanpr ovideat25 C ambienttemperature,whenmountedona standardJEDEC-specifiedPCB,beforeit notattainedathigherambienttemperatures, ,it maynotbepossibletodrivethismuchcurrent.

7 Todeterminetheactualmaximumcurrentin a specificapplication, specifiedin ,butit ,if theRDS(ON)oftheFETsis low, ,bothpeakandRMS/DCcurrentis limitedbytheOCPcurrent, ,themaximumDCorRMScurrentdecreases, ,if exceeded:ABSOLUTEMAXIMUMRATINGSVALUEUNIT VMP owersupplyvoltagerange 40to150 CTstgStoragetemperaturerange 60to150 CNotethatthepeakmotordriveoutputcurrenti s notspecifiedasa number,onlythatit becausethisdevicehasOCPprotection;it is theoutputsareshorted, 25 C (unlessotherwisenoted)MINNOMMAXUNITVMM otorpowersupplyvoltagerange(1) (2) (1)NotethatRDS(ON)increasesandmaximumout putcurrentis reducedatVMsupplyvoltagesbelow5 V.

8 (2)VM= 5 V, basedon25 C ambienttemperature,ona ,statingRDS(ON)increasesathighertemperat ures,andNote2, PCBthatcannotdissipatepoweraswellastheJE DEC board,thiscurrentlevelis February2012 UnderstandingMotorDriverCurrentRatingsSu bmitDocumentationFeedbackCopyright 2012, givensetofpower-dissipationconditions:TH ERMALINFORMATIONPWPRTYTHERMALMETRICUNITS 16 PINS16 PINS JCtopJunction-to-case(top) C/W JCbotJunction-to-case(bottom) JAnumberforthechosenpackagegivesanestima teofhowmuchtemperatureriseis expectedif thedevicewasmountedona , ,includingTIapplicationreportsICPackageT hermalMetrics(SPRA935A)andUsingNewTherma lMetrics(SBVA025).

9 RDS(ON):MINTYPMAXUNITH-BRIDGEFETSVM= 5 V,IO= 500mA,TJ= 25 C200VM= 5 V,IO= 500mA,TJ= 85 C325 HSFETON-resistancem VM= ,IO= 500mA,TJ= 25 C250VM= ,IO= 500mA,TJ= 85 C350 RDS(ON)VM= 5 V,IO= 500mA,TJ= 25 C160VM= 5 V,IO= 500mA,TJ= 85 C275 LSFETON-resistancem VM= ,IO= 500mA,TJ= 25 C200VM= ,IO= 500mA,TJ= 85 C300 Inthiscase,theRDS(ON)is specifiedseparatelyforhigh-sideandlow-si deFETs, usedtoestimatethepowerdissipationinsidet hedevice,applyingOhm'slawasdescribedin C6 UnderstandingMotorDriverCurrentRatingsSL VA505 February2012 SubmitDocumentationFeedbackCopyright 2012, (IOCP), themaximumamountofcurrentthatthedeviceca ndrivewithoutactivatingtheOCPcircuit,is theresultingoutputcurrentremainsaboveIOC P foratleasttDEG, thenOCPis theeventofOCP, listedhereastOCP, alsolistedin thetemperature,asmeasuredonthedie,is ,thedeviceautomaticallyre-enablesitselfw henthetemperaturefallstoa safelevel,10 40 C (SLVA504) (SPRA935A) (SBVA025) (SLMA002G)7 SLVA505 February2012 UnderstandingMotorDriverCurrentRatingsSu bmitDocumentationFeedbackCopyright 2012,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)

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