高侧电流检测的测量：集成电路和原则-High-Side Cu

高侧电流检测的测量：集成电路和原则-High-SideCu

Abstract:Thisappl_icatj_onnotedescribestheuseofcurrentsense

amplifiers,differentialamplifiersandinstrumentationamplifiersto

measurebatterychargeanddischargecurrentsinportableequipment,

computernotebooksandUSBaccessories.Itcompareshighsidecurrent

senseamplifierswithlowsidedifferentialamplifiersandrecommends

selectioncriteriaforcurrentsenseresistorsAhighvoltagecircuit

breakercircuitisdescribedtoprovidesystemovercurrent

protectionduetofaultsandshortcircuits.Applicationcircuitsfor

avariablelinearcurrentsourceandaprogrammable0-5Acurrent

sourceisincluded.

Currentmeasurement(monitoringcurrentflowintoandoutof

electroniccircuits)isanessentialskillinthedesigner*s

repertoireandnecessaryinawiderangeofapplications.Examples

includeovercurrent-protectionandsupervisingdevices,4-20mA

systems,programmablecurrentsources,linearandswitch-modepower

supplies,batterychargers,andbattery-operatedcircuitsforwhich

youmustknowtheratioofcurrentflowintoandoutofa

rechargeablebattery(thatis,thegaugefunction).

Asmoreapplicationsbecomeportable,thedemandincreasesfor

dedicatedcurrentmonitorsthataccomplishtheirtaskinasmall

packageandwithlowquiescentcurrent.Thefollowingdiscussion

coverslow-sideandhigh-sidecurrentmonitorsandincludestheir

architecturesandapplications.

High-orLow-SideMonitor?

Mostcurrent-measurementapplicationsemployeitherthelow-side

principle,inwhichthesenseresistorconnectsinserieswiththe

groundpath(Figure1),orthehigh-sideprinciple,inwhichit

connectsinserieswiththehotwire(Figure2).Thesetwoapproaches

poseatrade-offindifferentareas:Thelow-sideresistoradds

undesirableextraneousresistanceinthegroundpath,andcircuitry

associatedwiththehigh-sideresistormustcopewithrelatively

largecommon-modesignals.Moreover,iftheopampinFigure1has

itsGND£lnreferredtothepositivesideofRSENSE,thenitscommon-

modeinputrangemustextendbelowzero,thatis,toGND-(RSENSEx

TLOAD).

Figure1.Principleofthelow-sidecurrentmonitor.

Figure2.Exampleofahigh-sidecurrentmonitor.

However,don*tletthesimplicityofalow-sidemeasuringcircuit

causeyoutooverlooktheadvantagesofahigh-sideapproach.Various

faultscanbypassthelow-sidemonitor,therebysubjectingtheload

todangerousandundetectedstress(Figure3).Notethatloads

connectedthroughpathAaremonitored,butanaccidentalconnection

thoughpathBbypassesthemonitor.Ahigh-sidemonitorconnected

directlytothepowersource,ontheotherhand,candetectany

downstreamfailureandtriggerappropriatecorrectiveaction.High-

sidemonitorsarealsowellsuitedforautomotiveapplicationsin

whichthechassisservesasthegroundpotential.

Figure3.PathBcancarrydangerouslyhighcurrentsiftheloadis

connectedaccidentallytoground.

TraditionalHigh-SideMonitor

Unti1recently,themanyimplementationsofthesetwoapproacheswere

basedondiscretecomponentsorsemidiscretecircuitry.Intheir

simplestform,suchhigh-sidemonitorsrequireaprecisionopampcind

ahandfulofprecisionresistors.Onecommonapproachforhigh-side

measurementshasbeentheuseoftheclassicdifferentialamplifier,

whichisemployedasagainamplifierandalevelshifterfromthe

highsidetoground(Figure4).Thoughwidelyused,thisdiscrete

circuithasthreemajordisadvantages:

•Inputresistance(equaltoRI)isrelativelow.

•Inputsusuallyexhibitalargedifferenceininputresistance.

•Resistorsmustbeverywellmatchedtoobtainanacceptable

common-moderejectionratio(CMRR):A0.01%deviationinany

resistorvaluelowerstheCMRRto86dB,a0.1%deviationlowers

itto66dB,anda1%deviationlowersitto46dB.

High-sidecurrentmonitoring,thestateoftheartincurrent

measurement,isforcingthedevelopmentofanumberofnewintegrated

circuitsforthatpurpose.Low-sidemeasuring,ontheotherhand,has

notdrivenmanynew,interestingICs.

Figure4.Thedifferentialamplifierisabasiccomponentinhigh-

sidecurrentmeasurements.

IntegratedDifferentialAmplifier

Theuseofdifferentialamplifiersinhigh-sidecurrentmeasurement

recentlybecamemoreconvenientthankstotheintroductionof

numerousICscontainingnotonlytheprecisionamplifierbutwell-

mcitchedresistorsaswell.ThesedevicesofferCMRRsontheorderof

105dB.AnexampleistheMAX4198/MAX4199(Figure5).Availablcin8-

pinMMAXpackages,theseICsachieveatypicalCMRRof1lOdBandgain

errorbetterthan0.01%.

Figure5.Anintegrateddifferentialamplifier(MAX4198/MAX4199)

exhibitsveryhighCMRR.

DedicatedHigh-SideMonitors

Anotherapproachtohigh-sidecurrentmeasurementisrepresentedby

ICsthatcontainallfunctionsnecessarytoperformthemeasurement.

Theysensehigh-sidecurrentsinthepresenceofcommon-modevoltages

ashighas32Vandprovideaground-referencedcurrent-orvoltage-

sourceoutputthatisproportionaltothecurrentofinterest.Power

management,batterycharging,cindotherapplicationsthatmust

accuratelymeasureorcontrolcurrentcanbenefitfromthese

dedicatedcurrent-senseamplifiers.

High-sidecurrent-senseamplifiersfromMaximemployacurrent­

sensingresistorplacedbetweenthepositiveterminalofthepower

supplyandthesupplyinputofthemonitoredcircuit.This

arrangementavoidsextraneousresistanceinthegroundplane,greatly

simplifiesthelayout,andgenerallyimprovestheoverallcircuit

performance.Thevarietyofuni-andbidirectionalcurrent-senseICs

fromMaximincludesbidirectionaldeviceswithorwithoutinternal

senseresistors.Thebidirectionalamplifierscompriseasignpinfor

indicatingcurrentdirection.

Theseuni-andbidirectionalcurrent-senseICsincludemodelswith

adjustablegain,fixedinternalgainsof+20V/V,+50V/V,or+100V/V,

andinternalgainplusasingleordualcomparator.Theycomein

smallpackagestomeetthestrictdemandsofcompactapplications.

(Maximhastheonlyhigh-sidecurrent-senseamplifierinatinySOT23

package.)

Commontoal1high-sideICmonitorsfromMaximisanabilityto

provideground-referencedvoltageorcurrentoutputswithfeworno

additionalcomponents.Theoutputsignalisproportionaltothe

measuredhigh-sidecurrent,whichcanhaveacommon-modevoltageas

highas32V.Figures6through9showsomeavailablearchitectures

fortheintegratedhigh-sidecurrentmonitor.Notethatthe

MAX471/MAX472current-sourceoutputisproportionaltothevoltage

acrossRSENSE.

Equationsforthenewhigh-sidemonitorsindicatethattheeffectof

externalresistorsontheCMRRisnolongeranissue,becauseCMRR

(typically>90dB)isnowdeterminedbytheintegratedamplifieralone.

Integrationofthecurrent-measurefunctioninasingleICprovides

thefollowingadvantages:

•Tighttoleranceonactiveandpassiveintegratedcomponents

•Anexcellenttemperaturecoefficient(TC)

•Smallsize

•Lowpowerconsumption

•Easeofuse

Figure6.Thissimplifiedschematicofabidirectionalhigh-side

currentmonitor(MAX471/MAX472)includesaSIGNoutputforcurrent

direction.

Figure7.Aunidirectionalhigh-sidecurrentmonitor(MAX4372).

Figure8.Anotherunidirectionalhigh-sidecurrentmonitor(MAX4172).

Figure9.Yetanotherarchitecturefortheunidirectionalhigh-side

currentmonitor(MAX4173).

ConsiderationsWhenSelectingRSENSE

Acarefulconsiderationoftheshuntresistor(RSENSE)isan

importantandnecessarypartofdesigninganykindofcurrentmonitor.

ThefollowingcriteriashouldgoverntheselectionofRSENSE:

•VoltageLoss:HighRSENSEvaluescausethepowcr-sourccvoltagc

todegradethroughIRloss.ThelowestRSENSEvaluegivesthe

leastvoltageloss.

•Accuracy:HighRSENSEvaluesletyoumeasurelow-levelcurrents

moreaccurately,becausethevoltageoffsetandinputbias­

currentoffsetsarelesssignificantwithrespecttothesense

voltage.

•EfficiencyandPowerDissipation:Athighcurrentlevels,the

I2RlossinRSENSEcanbesubstantial,sotakethatinto

considerationwhenchoosingtheresistorvalueandpower­

dissipationrating(wattage).Excessiveheatinthesense

resistorcanalsocauseitsvaluetodrift.

•Inductance:IfISENSEhasalargehigh-frequencycomponent,

RSENSEmusthavelowinductcince.Wire-woundresistorshavethe

highestinductance.Metal-fiImresistorsarcsomewhatbetter,

butlow-inductancemetal-filmresistors(availableinvalues

under11/2Q)arerecommended.Unlikethemetal-filmand

wire-woundtypes(spiral-wrappedaroundacore),low-inductance

metal-filmresistorsconsistofastraightbandofmetal.

•Cost:UsingaPC-boardtraceasasenseresistor(Figure10)is

analternativemethodforapplicationswherethecostofRSENSE

isanissue.Youwillneedtoadjustthefull-scalecurrent

valuewithapotentiometerbecauseofinaccuracyinthecopper

resistance.Theresistancetemperaturecoefficientofcopperis

fairlyhigh(approximately0.4%/°C)insystemsthatundergo

widetemperaturevariations.

Figure10.Thishigh-sidecurrentmonitor(MAX4172)employsaPC-

boardtraceforRSENSE.

High-Side-MonitorApplications

ThecircuitinFigure11isavariablelinearcurrentsource.IC1

convertsRIcurrenttoaproportionaloutputvoltage,enablingthe

voltageregulator(IC2)toproducearegulatedoutputcurrent.Toset

aspecific,regulated10UTlevelbetween0mAand500mA,apply5Vto

0Vat1CONTROL(5VsetsIOUT=0mA,and0VsetsTOUT=500mA).Asan

alternative,youcanintroduceaD/Aconverterasshowntoprovide

digitalcontrolofIOUT.For