smallerwww.4xxpp.com

ApplicationBriefAdvantagesofIntegratedCurrentSensingRyanKehrMotorDrivesMonitoringandregulatingcurrentisamustformanybrushedandsteppermotorapplications.
Forbrushedmotors,thecurrentinformationcanbeusedtodeterminechangesinloadconditionsorforlimitingstartupandstallcurrents.
Forsteppermotors,highlevelsofmicro-steppingrequirecurrenttoberegulatedforeachstep.
InFigure1,currentisplottedversustimedepictingthestartupprofileforabrushedDCmotor.
Inthiscase,currentislimitedtoapproximately2amperesbeforethemotorreachesasteadystateconditionoflessthan1ampere.
Withoutcurrentregulation,thissamemotorpeakstoover14amperes.
Thisnotonlyrequiresanover-designedpowersupplytosupportthistransient,butthemotordriveralsoneedstoberatedtoreliablyhandlethepeakcurrents.
Figure1.
DCMotorCurrentvs.
TimeTraditionally,thiscurrentlimitingwasachievedusingexternalshuntresistorsinthegroundpath.
Thevoltagedropacrosstheseshuntswasmonitoredandcomparedtoareferencevoltageeitherprovidedinternallyorexternallydependingonthedevice.
Sincethefullloadcurrentpassesthroughtheseshunts,theresistorsmustbepowerresistorsandcanbeupto1206incasesizewhichcanbealmostaslargeastheintegratedcircuititselfwitha3.
2mmby1.
6mmdimension.
Figure2comparesthesizeofashunttoaDRV8870DCmotordriver.
Figure3depictstheDRV8871withintegratedcurrentsensingwhereasingle0402resistorisusedtosetthecurrentlimitingthreshold.
AsisclearlyshowninFigure3,integratedcurrentsenseleadstoboardandcomponentsavings.
Alowwattagesignalpathresistorcomparedtoapowerresistorrequiresmuchsmallerboardspaceandlowersthebillofmaterials(BOM).
Inadditiontothesebenefits,thereisnopowerlossacrosstheresistorandonemoreheatsourceiseliminatedfromadesignthatismostlikelyalreadyunderthermalbudgetpressure.
Figure2.
DRV8870withExternalShuntFigure3.
DRV8871withIntegratedCurrentSensingwww.
ti.
comTITechNoteSLVAE68A–MARCH2019–REVISEDOCTOBER2020SubmitDocumentFeedbackAdvantagesofIntegratedCurrentSensing1Copyright2021TexasInstrumentsIncorporatedNowthatcurrentlimitingandintegratedcurrentsensingbenefitsarewellunderstood,wecanalsoaddmonitoringcapabilityinournewestfamilyofbrushedDCmotordriversthroughtheIPROPIpin.
TheDRV8873isthenewestproductintheBrushedMotorDriverfamilythatprovidesananalogcurrentoutputproportionaltothecurrentflowingthroughthebridge.
Bypassingthisproportionalcurrentthroughasmallresistor,avoltageproportionaltocurrentcanbemonitoredwithanexternalmicrocontrollertodeterminechangesinloadconditionsandactaccordingly.
Thisinternalcurrentmonitoringfeatureeliminatestheneedforanexternalpowerresistorandconditioningcircuitslikeanoperationalamplifier.
Finally,wegettosteppermotorsthatfurtherbenefitfromthePCBsizeandBOMadvantagesofintegratedcurrentsensingsincesteppermotorshavetraditionallyrequiredtwoexternalsenseresistorstofeedbackthecurrenttothedriver.
Figure4comparesthePCBfootprintofatraditionaldriverliketheDRV8825tothenewestDRV8424.
TheboardwithDRV8424is60%smallerthantheboardwithDRV8825Figure4.
BoardLayoutReductionOnelesstangiblebenefitthatisonlyrealizedduringPCBlayoutisthesimplicityofthelayoutwhenchoosingadevicewiththeintegratedsensingfeature.
Itisnolongernecessarytoroutearoundthelargeexternalshuntcomponentsandbreakuppower,output,andGNDroutingthatrequirelargertracesforcarryingdevicecurrent.
Insummary,integratedcurrentsensinginTI'slatestfamilyofmotordriversremovescostlypowerresistorsfromtheBOM,reducesboardsize,componentcount,andsimplifiesPCBrouting.
Table1.
AlternateDeviceTableDeviceNameDeviceDescriptionDRV842435V,2.
5AStepperMotordriverwithintegratedcurrentsensingDRV842635V,1.
5AStepperMotordriverwithintegratedcurrentsensingDRV842835V,1AStepperMotordriverwithintegratedcurrentsensingDRV843650V,1.
5AStepperMotordriverwithintegratedcurrentsensingDRV887337V,10ABrushedDCMotordriverwithIntegratedcurrentsensingTable1.
AlternateDeviceTable(continued)DeviceNameDeviceDescriptionDRV887437V,6ABrushedDCMotordriverwithIntegratedcurrentsensingDRV887637V,3.
5ABrushedDCMotordriverwithIntegratedcurrentsensingHowtouseintegratedcurrentsensinginbrushedDCmotorsHowtouseintegratedcurrentsensinginsteppermotorsReferenceswww.
ti.
com2AdvantagesofIntegratedCurrentSensingSLVAE68A–MARCH2019–REVISEDOCTOBER2020SubmitDocumentFeedbackCopyright2021TexasInstrumentsIncorporatedIMPORTANTNOTICEANDDISCLAIMERTIPROVIDESTECHNICALANDRELIABILITYDATA(INCLUDINGDATASHEETS),DESIGNRESOURCES(INCLUDINGREFERENCEDESIGNS),APPLICATIONOROTHERDESIGNADVICE,WEBTOOLS,SAFETYINFORMATION,ANDOTHERRESOURCES"ASIS"ANDWITHALLFAULTS,ANDDISCLAIMSALLWARRANTIES,EXPRESSANDIMPLIED,INCLUDINGWITHOUTLIMITATIONANYIMPLIEDWARRANTIESOFMERCHANTABILITY,FITNESSFORAPARTICULARPURPOSEORNON-INFRINGEMENTOFTHIRDPARTYINTELLECTUALPROPERTYRIGHTS.
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