Calculating Motor Driver Power Dissipation - TI.com

1 ApplicationReportSLVA504 motordriverICfora particularapplication, givenapplication,anestimationofthetotalm otordriverpowerdissipationis MotorDriverTherearea numberofsourcesofpowerdissipationinsidea ,likethepowerdissipatedin theFETON-resistance, , (ON)DissipationThebiggestsourceofpowerdi ssipatedinsidea motordriverICis thepowerdissipatedin theFETON-resistance,orRDS(ON). Thepowerdissipatedin anH-bridge(consistingofa high-sideFETanda low-sideFET)is calculatedwiththefollowing:PRDS= (HS-RDS(ON) (IOUT(RMS))2) + (LS-RDS(ON) (IOUT(RMS))2)wherePRDSis thepowerdissipatedin theoutputFETs,HS-RDS(ON)is theresistanceofthehigh-sideFET,LS-RDS(ON )is theresistanceofthelowsideFET,andIOUT(RMS )is (ON)increaseswithtemperature,asthedevice heats, a devicehastwoH-bridges,likea typicalsteppingmotordriver, ,theoutputdevicestraversea a functionofthefollowing.

2 Riseandfalltimesoftheoutput,howquicklyth eoutputswingsfromoneextremetotheother supplyvoltage outputcurrent frequencythattheoutputis switchingInsomecases,suchasa DCmotordriverthatis onlyturnedonoroff(notsubjectedtoPWMspeed controlorcurrentcontrol), ,forexamplea steppermotordriverusingcurrentcontrol, February2012 CalculatingMotorDriverPowerDissipationSu bmitDocumentationFeedbackCopyright 2012,TexasInstrumentsIncorporatedSources ofPowerDissipationin a ,switchinglossesforeachoutputarecalculat edasfollows:PSW= PSW_RISE+ PSW_FALL wherePSWis thetotalswitchingloss(inwatts)foroneoutp ut,andPSW_RISEis thepowerdissipatedduringtherisingedge,an dPSW_FALLis :PSW_RISE= VM IOUT tR fSWandPSW_FALL= VM IOUT tF fSWwhereVMis thesupplyvoltage(involts),IOUTis theoutputcurrent(inamps),tRis therisetime(inseconds),tFis thefalltime(inseconds),andfSWis theswitchingfrequency(inHz).

3 ThevaluesoftRandtFaregenerallylistedin ,if themotordriveris performingcurrentregulation(aswhendrivin gasteppermotor),ormaybeanexternallyappli edPWMfrequency(aswhendoingDCmotorspeedco ntrolfroma processor).InanH-bridge,it is ,witha DCmotordriverora steppermotorusingslowdecaymode,onlyonesi deoftheH-bridgeis beingpulse-widthmodulated,whiletheothers ideremainsata steppermotordriverrunningin fastdecay,however, , ,thisis calculatedusing:PIVM= VM usedtoprovidesomereferencecurrent, ,powerdissipationis calculatedasfollows:PLDO= ILDO_OUT (VM VOUT)2 CalculatingMotorDriverPowerDissipationSL VA504 February2012 SubmitDocumentationFeedbackCopyright 2012, steppermotorusingcurrentcontrol,twoH-bri dgesarealwaysconductingcurrent,andarealw aysbeingswitched(toregulatecurrentusingP WM).

4 Inthisexample,usinga DRV8825steppermotorcontroller,weusethefo llowingoperatingparameters: VM(supplyvoltage)= 24V IOUT= Onlyslowdecayis used(soonlyonesideofeachH-bridgeis switching) 2-mAcurrentis drawnfromtheLDOregulator(V3P3 OUT)Forestimationpurposes,usethetypical8 5 C RDS(ON)number,andtheworst-case(maximum) calculatedasfollows:PTOT= (2 PRDS) + (2 PSW) + PIVM+ PLDOT hefactorof2 onthePRDSandPSWtermscomesfromthefactthat therearetwoH-bridgesin theDRV8825,eachdrivingonephaseofthestepp ermotor,andonesideofeachH-bridgeis ,usingdatafromtheDRV8825datasheet:PRDS= (HS-RDS(ON) (IOUT(RMS))2) + (LS-RDS(ON) (IOUT(RMS))2)PRDS= ( ( )2) + ( ( )2)PRDS= PSW_RISE+ PSW_FALLPSW= ( VM IOUT tR fSW) + ( VM IOUT tF fSW)PSW= ( 24V 200ns 30kHz)+ ( 24V 200ns 30kHz)PSW= VM IVM= 24V 5 mAPLDO= ILDO_OUT (VM VOUT) = 2 mA (24V )= ,PTOT= (2 PRDS) + (2 PSW) + PIVM+ PLDOPTOT= 2 + 2 + + theapproximateamountofpowerdissipatedin February2012 CalculatingMotorDriverPowerDissipationSu bmitDocumentationFeedbackCopyright 2012, thedriver, JAandtheambienttemperatureasfollows.

5 TDIE= TAMB+ ( JA PTOT)Inthisexample,if theDRV8825weremountedona JEDEC-standardPCB,the JAvalueis 25 C ambienttemperatureis:TDIE= TAMB+ ( JA PTOT)TDIE= 25 C + ( C/W )= CThistemperatureis wellbelowtheovertemperatureshutdowntempe ratureof150 ,anambienttemperatureof70 C producesthefollowing:= 70 C + ( C/W )= CAtanambienttemperatureof70 C, :WhenmountedontheapplicationPCB,theactua l JAvalueis probablydifferentthanthe JAvaluelistedonthedatasheetfora (SLVA505) February2012 SubmitDocumentationFeedbackCopyright 2012,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,modifications,enhancements,improvement s, s accordancewithTI s ,testingofallparametersofeachproductis , ,eitherexpressorimplied,is grantedunderanyTIpatentright,copyright,m askworkright,orotherTIintellectualproper tyrightrelatingtoanycombination,machine, orprocessin licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, safety-criticalapplications(suchaslifesu pport)

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