HighVoltageChargePumpsDeliverLowEMI
ByTonyArmstrong
DirectorofProductMarketing
PowerProducts
LinearTechnologyCorporation
(tarmstrong@linear.com)
Background
Switchingregulatorsareapopularchoiceformanypowerconversionsuppliesduetotheirsize,
outputflexibilityandefficiencyadvantages.Conversionefficacyofthesesuppliescannowattain
98%levels,dependingonoperatingconditions.Nevertheless,despitetheseadvantages,they
necessitatecompromisingonotherparameters,oneofthemostdifficultofwhichisnoise.
Butwhatisswitchingregulator“noise?”Tohaveabetterunderstandingofthisterm,let’sstartwith
thefactthatswitchedmodepowersuppliescreatewidebandharmonicenergy.Thisundesirable
energymanifestsitselfintwocomponents,radiatedandconducted,whichareroutinelylabelledin
theindustryas“noise.”However,thisisreallyamisnomer,sinceaswitchingregulator’soutput
“noise”isnotnoiseatall,butcoherenthighfrequencyresiduedirectlyrelatedtotheregulator’s
switching.Thecorrecttermforthisphenomenoniselectromagneticradiation,ormorecommonly,
EMI.Andyes,itcomesinbothradiatedandconductedforms.
Sincequiet,well-regulatedpowersuppliesareimportantforoptimumperformanceinmanycircuit
applications,itisimportanttobeabletomitigateanynoisegeneratedaspartofthisconversion
process.Anobviouswaytoattainthisistosimplyusealinearregulator.However,althoughthey
supplyquietpowersupplyrails,theirconversionefficienciesarepoorathighstep-downratiosand
thiscanleadtothermalissueswiththedesigninhighoutputcurrentapplications.
Correspondingly,switchingregulatorsusuallyhavehigherconversionefficienciesthanlinear
regulators,leadingtosimplerthermaldesignwhenhighoutputcurrentsarerequiredbytheend
application.Itiswellunderstoodthatcomponentselectionandcircuitboardlayoutcanplaya
significantroleindeterminingthesuccessorfailureofvirtuallyallpowersupplies.Theseaspects
settheirfunctionalEMIandthermalbehavior.Fortheun-initiated,switchingpowersupplylayout
mayseemlikea“black”art,butitisinfactabasicaspectofadesignoftenoverlookedintheearly
stagesoftheprocess.SincefunctionalEMIrequirementsalwayshavetobemet,whatisgoodfor
functionalstabilityofthepowersupplyisalsousuallygoodforitsEMIemissions,too.Furthermore,
goodlayoutfromthebeginningdoesnotaddanycosttothedesignandcanactuallyprovidecost
savingsbyeliminatingtheneedforEMIfilters,mechanicalshielding,EMItesttimeandnumerous
boardrevisions.
Moreover,thepotentialproblemsofinterferenceduetonoisecanbeexacerbatedwhenmultiple
DC/DCswitchmoderegulatorsareusedinadesigntogeneratemultiplerailsofiftheyare
paralleledforcurrentsharingandhigheroutputpower.Ifallareoperating(switching)atasimilar
frequency,thecombinedenergygeneratedbymultipleregulatorsinacircuitisthenconcentrated
P373
atonefrequency.Presenceofthisenergycanbecomeaconcern,especiallyiftherestoftheICson
theprintedcircuitboards(PCBs),aswellasothersystemboardsareclosetoeachotherand
susceptibletothisradiatedenergy.Thiscanbeparticularlytroublinginindustrialandautomotive
systemsthataredenselypopulatedandareoftenincloseproximitytoelectricnoisegenerating
sources,suchasmechanicallyswitchedinductiveloads,PWMdrivepoweroutputs,microprocessor
clocksandcontactswitching.Furthermore,ifswitchingatdifferentfrequencies,intermodulation
productscanaliasintosensitivefrequencybands.
SwitchingRegulatorEmissions
Inindustrial,medicalandautomotiveenvironments,switchingregulatorsusuallyreplacelinear
regulatorsinareaswherelowheatdissipationandefficiencyarevalued.Moreover,theswitching
regulatoristypicallythefirstactivecomponentontheinputpowerbusline,andthereforehasa
significantimpactontheEMIperformanceoftheentireproductdesign.
Conductedemissionsrideonthewiresandtracesthatconnectuptoaproduct.Sincethenoiseis
localizedtoaspecificterminalorconnectorinthedesign,compliancewithconductedemission
requirementscanoftenbeassuredrelativelyearlyinthedevelopmentprocesswithagoodlayout
orfilterdesignasalreadystated.
Radiatedemissionsareadifferentmatteraltogether.Everythingontheboardthatcarriescurrent
radiatesanelectromagneticfield.Everytraceontheboardisanantennaandeverycopperplaneis
aresonator.Anything,otherthanapuresinewaveorDCvoltage,generatesnoisealloverthe
signalspectrum.Evenwithcarefuldesign,apowersupplydesignerneverreallyknowshowbadthe
radiatedemissionsaregoingtobeuntilthesystemistested.Andradiatedemissionstestingcannot
beformallyperformeduntilthedesignisessentiallycomplete.
FiltersareoftenusedtoreduceEMIbyattenuatingthestrengthatacertainfrequencyorovera
rangeoffrequencies.Aportionofthisenergythattravelsthroughspace(radiated)isattenuatedby
addingmetallicandmagneticshields.ThepartthatridesonPCBtraces(conducted)istamedby
addingferritebeadsandotherfilters.EMIcannotbeeliminatedbutcanbeattenuatedtoalevel
thatisacceptablebyothercommunication,signalprocessinganddigitalcomponents.Moreover,
severalregulatorybodiesenforcestandardstoensurecomplianceinbothindustrialand
automotivesystems.
Moderninputfiltercomponentsinsurfacemounttechnologyhavebetterperformancethan
through-holeparts.However,thisimprovementisoutpacedbytheincreaseddemandscreated
bytoday'shighfrequencyswitchingregulators.Thelowminimumonandofftimesrequiredat
higheroperatingfrequenciesresultinhigherharmoniccontentduetothefasterswitchtransitions,
therebyincreasingradiatednoise.However,thesehighswitchedgeratesareneededtogethigher
conversionefficiencies.Aswitchedcapacitorchargepumpdoesnotexhibitthisbehaviorsinceit
operatesatmuchlowerswitchingfrequenciesandmostimportantlycantolerateslowerswitching
transitionswithoutdegradationinefficiency.
SavvyPCBdesignerswillmakethehotloopssmallanduseshieldinggroundlayersasclosetothe
activelayeraspossible.Nevertheless,devicepin-outs,packageconstruction,thermaldesign
requirementsandpackagesizesneededforadequateenergystorageindecouplingcomponents
dictateaminimumhotloopsize.Tofurthercomplicatematters,intypicalplanarprintedcircuit
boards,themagneticortransformerstylecouplingbetweentracesabove30MHzwilldiminishall
filtereffortssincethehighertheharmonicfrequenciesare,themoreeffectiveunwantedmagnetic
couplingbecomes.
AnAlternativeSolutiontoEMIIssues
ThetriedandtruesolutiontoEMIissuesistouseashieldingboxforthecompletecircuit,andeven
then,ashielddoesnotpreventcouplingtosensitivecircuitsinsidethebox.Ofcourse,thisadds
cost,increasesrequiredboardspace,makesthermalmanagementandtestingmoredifficultand
introducesadditionalassemblyexpense.Anotherfrequentlyusedmethodistoslowdownthe
switchingedges.Thishastheundesiredeffectofreducingtheefficiency,increasingminimumon,
offtimes,andtheirassociateddeadtimesandcompromisesthepotentialcurrentcontrolloop
speed.
Afewyearsago,LinearTechnologyintroducedourLT8614SilentSwitcher®regulator,which
deliversthedesiredeffectsofashieldedboxwithoutusingone,whilealsoeliminatingmanyofthe
abovementioneddrawbacks.Nevertheless,insomenoiseapplications,powersupplydesigners
simplydonotliketouseinductor-basedregulatorsduetotheirassociatedEMIemissions.Atthe
sametime,theuseofalinearregulator(akaLDO)maybeprecludedduetoitsrelativelylow
conversionefficiencyandneedforheatsinking.Asaresult,theyturntoacommonalternative
knownasachargepump.
Chargepumpshavebeenaroundfordecades,andtheyprovideDC/DCvoltageconversion,usinga
switchnetworktochargeanddischargetwoormorecapacitors.Thebasicchargepumpswitch
networktogglesbetweenchargeanddischargestatesofthecapacitors.AsshowninFigure1,C1
the“flyingcapacitor”shuttlescharge,andC2the“reservoircapacitor”holdschargeandfiltersthe
outputvoltage.Additional“flyingcapacitors”andswitcharraysenablemultiplegains.
Figure 1. Simplified Charge Pump Block Diagram of a Voltage Inverter
WhenswitchesS1andS3areon,orclosed,andswitchesS2andS4areoff,oropen,theinput
powersupplychargesC1.Duringthenextcycle,S1andS3areoff,S2andS4areon,andcharge
transferstoC2,generatingVOUT=-(V+).
However,untilrecently,chargepumpshavehadlimitedinputandoutputvoltageranges,whichhas
limitedtheiruseinindustrialandautomotiveapplicationswhereinputsupto40Vor60Vare
commonplace.However,thisisnowchangedwiththeintroductionofhighvoltagechargepumps
fromLinearTechnology.
HighVoltageChargePumps
TheLTC3245isabuck-boostregulatorthatdispenseswiththetraditionalinductorandusesa
switched-capacitorchargepumpinstead.Itsinputvoltagerangeis2.7Vto38V,anditcanbeused
withoutafeedbackdividertoproduceoneoftwofixedoutputvoltages,3.3Vor5V,or
programmedviaafeedbackdividertoanyoutputvoltagefrom2.5Vto5.5V.Maximumoutput
currentis250mA.TheLTC3245iscapableofregulatingavoltageaboveorbelowtheinputvoltage,
allowingittosatisfyautomotivecoldcrankrequirements,forexample.SeeFigure2foritscomplete
schematic.
Figure2.LTC3245SchematicDeliveringaFixed5VOutputfroma2.7Vto38VInput
Thischargepumpiscapableofachievingefficienciesof80%whendelivering5Vat100mAfroma
12Vsource,almosttwotimeshigherthanthatofalinearregulator,makingitpossibletoavoidthe
spaceandcostrequirementsofanLDOwithaheatsink.Itgeneratesalmostthreetimeslower
powerlossatfullloadversusanLDO.SeeFigure3fortheLTC3245efficiencyandpowerlosscurves.
Figure3.LTC3245Efficiency/PowerLossCurvefor12VInputto5VOut
TheLTC3245alsohasexcellentradiatedandconductedEMIperformance,asshowinFigures4a
and4b.ThesemeasurementsweretakeninamicrochamberinaccordancewithCISPR22and
CISPR25.Ascanbeseen,whenproperlydecoupled,theLTC3245doesnotpresentanyissuewhen
strivingtomeetgovernmentregulationsforradiatedandconductedemissions.
Figure4.Radiated(a)andConducted(b)EmissionsoftheLTC3245
Inmanyindustrial,medicalandautomotiveapplications,adual-polaritysupplyisoftenneededto
operateelectronicssuchasopamps,driversandsensors.However,thereisrarelyadual-polarity
supplyavailableatthepointofload.Itwasbecauseofthisneed,andthelackofasimple-to-use
solution,thatLinearTechnologydevelopedtheLTC3260.
TheLTC3260isaninvertingchargepumpDC/DCconverterwithduallownoiseLDOregulator
followersthatcanproducepositiveandnegativesuppliesfromasingle4.5Vto32Vinputpower
source,asillustratedintheFigure5completeschematic.Itcanswitchbetweenhighefficiency
BurstMode®operationandlownoiseconstantcurrentfrequencymode,allowingthedesignerto
makeanoptimumtrade-offfortheapplication.
TheLTC3260cansupplyupto100mAfromtheinvertedinputvoltageatitschargepumpoutput,
VOUT.ThisVOUTalsoservesastheinputsupplytoanegativeLDOregulator,LDO-.Thechargepump
frequencycanbeadjustedbetween50kHzand500kHzwithasingleresistor.EachoftheLTC3260’s
LDOsiscapableofsupportingupto50mAloads.And,eachLDOhasadropoutvoltageof300mVat
50mA,andanoutputresistordividernetworkallowsthesettingoftheoutputvoltage.Whenboth
regulatorsaredisabled,theshutdownquiescentcurrentisonly2µA.
Figure5.A12VInputSupplyto±5VOutputs
Conclusion
ItiswellknownthatEMIconsiderationsforindustrial,medicalandautomotiveenvironments
requirecarefulattentionduringtheinitialdesignprocessinordertoensurethattheywillpassEMI
testingoncethesystemiscompleted.Untilnow,therehasbeennosurefirewaytoguaranteethat
thiscouldeasilybeattainedwiththerightpowerICselectionforallbutverylowpowersystems.
RecentintroductionsoflowEMIregulatorsandDC/DCconvertersfromLinearTechnologyinclude
anextensivelineoflinearregulators,aswellastheLT86xxSilentSwitcherbuckconverters.Now
ourgrowingfamilyofhighvoltagechargepumpsofferathirdchoice.Theseproductsprovidemuch
higherefficiencyandlowerpowerlosswhencomparedtolinearregulatorsanddonotrequirethe
compensation,layout,magneticsandEMIissuesassociatedwithswitchingregulators.