LM2596SIMPLESWITCHERPowerConverter150kHz3AStep-DownVoltageRegulatorDecember1997
LM2596
SIMPLESWITCHER®PowerConverter150kHz3AStep-DownVoltageRegulator
GeneralDescription
TheLM2596seriesofregulatorsaremonolithicintegratedcircuitsthatprovidealltheactivefunctionsforastep-down(buck)switchingregulator,capableofdrivinga3Aloadwithexcellentlineandloadregulation.Thesedevicesareavail-ableinfixedoutputvoltagesof3.3V,5V,12V,andanadjust-ableoutputversion.
Requiringaminimumnumberofexternalcomponents,theseregulatorsaresimpletouseandincludeinternalfrequencycompensation†,andafixed-frequencyoscillator.
TheLM2596seriesoperatesataswitchingfrequencyof150kHzthusallowingsmallersizedfiltercomponentsthanwhatwouldbeneededwithlowerfrequencyswitchingregu-lators.Availableinastandard5-leadTO-220packagewithseveraldifferentleadbendoptions,anda5-leadTO-263surfacemountpackage.
Astandardseriesofinductorsareavailablefromseveraldif-ferentmanufacturersoptimizedforusewiththeLM2596se-ries.Thisfeaturegreatlysimplifiesthedesignofswitch-modepowersupplies.
Otherfeaturesincludeaguaranteed±4%toleranceonout-putvoltageunderspecifiedinputvoltageandoutputloadconditions,and±15%ontheoscillatorfrequency.Externalshutdownisincluded,featuringtypically80µAstandbycur-rent.Selfprotectionfeaturesincludeatwostagefrequencyreducingcurrentlimitfortheoutputswitchandanovertem-peratureshutdownforcompleteprotectionunderfaultcondi-tions.
Features
n3.3V,5V,12V,andadjustableoutputversions
nAdjustableversionoutputvoltagerange,1.2Vto37V±4%maxoverlineandloadconditionsnAvailableinTO-220andTO-263packagesnGuaranteed3AoutputloadcurrentnInputvoltagerangeupto40V
nRequiresonly4externalcomponents
nExcellentlineandloadregulationspecificationsn150kHzfixedfrequencyinternaloscillatornTTLshutdowncapability
nLowpowerstandbymode,IQtypically80µAnHighefficiency
nUsesreadilyavailablestandardinductors
nThermalshutdownandcurrentlimitprotection
Applications
nSimplehigh-efficiencystep-down(buck)regulatornOn-cardswitchingregulatorsnPositivetonegativeconverter
Note:†PatentNumber5,382,918.
TypicalApplication
(FixedOutputVoltageVersions)
DS012583-1
SIMPLESWITCHER®andSwitchersMadeSimple®areregisteredtrademarksofNationalSemiconductorCorporation.
©1999NationalSemiconductorCorporationDS012583www.national.com
ConnectionDiagramsandOrderingInformation
BentandStaggeredLeads,ThroughHole
Package
5-LeadTO-220(T)
SurfaceMountPackage5-LeadTO-263(S)
DS012583-2
DS012583-3
OrderNumberLM2596T-3.3,LM2596T-5.0,
LM2596T-12orLM2596T-ADJSeeNSPackageNumberT05D
OrderNumberLM2596S-3.3,LM2596S-5.0,
LM2596S-12orLM2596S-ADJSeeNSPackageNumberTS5B
www.national.com2
AbsoluteMaximumRatings(Note1)
IfMilitary/Aerospacespecifieddevicesarerequired,pleasecontacttheNationalSemiconductorSalesOffice/Distributorsforavailabilityandspecifications.MaximumSupplyVoltageON/OFFPinInputVoltageFeedbackPinVoltageOutputVoltagetoGround(SteadyState)PowerDissipation
StorageTemperatureRangeESDSusceptibility
45V
−0.3≤V≤+25V−0.3≤V≤+25V−1V
Internallylimited−65˚Cto+150˚C
HumanBodyModel(Note2)LeadTemperatureSPackage
VaporPhase(60sec.)Infrared(10sec.)
TPackage(Soldering,10sec.)MaximumJunctionTemperature
2kV
+215˚C+245˚C+260˚C+150˚C
OperatingConditions
TemperatureRangeSupplyVoltage
−40˚C≤TJ≤+125˚C
4.5Vto40V
LM2596-3.3
ElectricalCharacteristics
SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingTempera-tureRange
LM2596-3.3
Symbol
Parameter
Conditions
Typ(Note3)3.3
3.168/3.1353.432/3.465
η
Efficiency
VIN=12V,ILOAD=3A
73
Limit(Note4)
Units(Limits)
SYSTEMPARAMETERS(Note5)TestCircuitFigure1VOUTOutputVoltage
4.75V≤VIN≤40V,0.2A≤ILOAD≤3A
VV(min)V(max)%
LM2596-5.0
ElectricalCharacteristics
SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingTempera-tureRange
LM2596-5.0
Symbol
Parameter
Conditions
Typ(Note3)5.0
4.800/4.7505.200/5.250
η
Efficiency
VIN=12V,ILOAD=3A
80
Limit(Note4)
Units(Limits)
SYSTEMPARAMETERS(Note5)TestCircuitFigure1VOUTOutputVoltage
7V≤VIN≤40V,0.2A≤ILOAD≤3A
VV(min)V(max)%
LM2596-12
ElectricalCharacteristics
SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingTempera-tureRange
LM2596-12
Symbol
Parameter
Conditions
Typ(Note3)12.0
11.52/11.4012.48/12.60
η
Efficiency
VIN=12V,ILOAD=3A
90
Limit(Note4)
Units(Limits)
SYSTEMPARAMETERS(Note5)TestCircuitFigure1VOUTOutputVoltage
15V≤VIN≤40V,0.2A≤ILOAD≤3A
VV(min)V(max)%
3www.national.com
LM2596-ADJ
ElectricalCharacteristics
SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingTempera-tureRange
LM2596-ADJ
Symbol
Parameter
Conditions
Typ(Note3)1.230
1.193/1.1801.267/1.280
η
Efficiency
VIN=12V,VOUT=3V,ILOAD=3A
73
Limit(Note4)
Units(Limits)
SYSTEMPARAMETERS(Note5)TestCircuitFigure1VFBFeedbackVoltage
4.5V≤VIN≤40V,0.2A≤ILOAD≤3AVOUTprogrammedfor3V.CircuitofFigure1VV(min)V(max)%
AllOutputVoltageVersionsElectricalCharacteristics
SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingTempera-tureRange.Unlessotherwisespecified,VIN=12Vforthe3.3V,5V,andAdjustableversionandVIN=24Vforthe12Vver-sion.ILOAD=500mA
LM2596-XX
Symbol
Parameter
Conditions
Typ(Note3)10
50/100
fOOscillatorFrequency
(Note6)
150
127/110173/173
VSATDCICLSaturationVoltageMaxDutyCycle(ON)MinDutyCycle(OFF)CurrentLimit
IOUT=3A(Notes7,8)(Note8)(Note9)
PeakCurrent(Notes7,8)
1.16
1.4/1.5
10004.5
3.6/3.46.9/7.5
ILOutputLeakageCurrent
Output=0V(Notes7,9)Output=−1V(Note10)(Note9)
ON/OFFpin=5V(OFF)
(Note10)
50
2
30
IQISTBYθJCθJAθJAθJAθJAON/OFFCONTROLTestCircuitFigure1ON/OFFPinLogicInput
VIHVILThresholdVoltage
Low(RegulatorON)High(RegulatorOFF)
1.3
0.62.0
VV(max)V(min)
QuiescentCurrentStandbyQuiescentCurrent
ThermalResistance
5
10
80
200/250
TO-220orTO-263Package,JunctiontoCaseTO-220Package,JunctiontoAmbient(Note11)TO-263Package,JunctiontoAmbient(Note12)TO-263Package,JunctiontoAmbient(Note13)TO-263Package,JunctiontoAmbient(Note14)
250503020
AA(min)A(max)µA(max)mAmA(max)mAmA(max)µAµA(max)˚C/W˚C/W˚C/W˚C/W˚C/W
Limit(Note4)
Units(Limits)
DEVICEPARAMETERSIbFeedbackBiasCurrent
AdjustableVersionOnly,VFB=1.3V
nAnA(max)kHzkHz(min)kHz(max)
VV(max)%
www.national.com4
AllOutputVoltageVersions
ElectricalCharacteristics(Continued)
SpecificationswithstandardtypefaceareforTJ=25˚C,andthosewithboldfacetypeapplyoverfullOperatingTempera-tureRange.Unlessotherwisespecified,VIN=12Vforthe3.3V,5V,andAdjustableversionandVIN=24Vforthe12Vver-sion.ILOAD=500mA
LM2596-XX
Symbol
Parameter
Conditions
Typ(Note3)
5
15
ILVLOGIC=0.5V(RegulatorON)
0.02
5Limit(Note4)
Units(Limits)
ON/OFFCONTROLTestCircuitFigure1IHON/OFFPinInputCurrent
VLOGIC=2.5V(RegulatorOFF)
µAµA(max)µAµA(max)
Note1:AbsoluteMaximumRatingsindicatelimitsbeyondwhichdamagetothedevicemayoccur.OperatingRatingsindicateconditionsforwhichthedeviceisin-tendedtobefunctional,butdonotguaranteespecificperformancelimits.Forguaranteedspecificationsandtestconditions,seetheElectricalCharacteristics.Note2:Thehumanbodymodelisa100pFcapacitordischargedthrougha1.5kresistorintoeachpin.Note3:Typicalnumbersareat25˚Candrepresentthemostlikelynorm.
Note4:Alllimitsguaranteedatroomtemperature(standardtypeface)andattemperatureextremes(boldtypeface).Allroomtemperaturelimitsare100%produc-tiontested.AlllimitsattemperatureextremesareguaranteedviacorrelationusingstandardStatisticalQualityControl(SQC)methods.AlllimitsareusedtocalculateAverageOutgoingQualityLevel(AOQL).
Note5:Externalcomponentssuchasthecatchdiode,inductor,inputandoutputcapacitors,andvoltageprogrammingresistorscanaffectswitchingregulatorsys-temperformance.WhentheLM2596isusedasshownintheFigure1testcircuit,systemperformancewillbeasshowninsystemparameterssectionofElectricalCharacteristics.
Note6:Theswitchingfrequencyisreducedwhenthesecondstagecurrentlimitisactivated.Theamountofreductionisdeterminedbytheseverityofcurrentover-load.
Note7:Nodiode,inductororcapacitorconnectedtooutputpin.
Note8:Feedbackpinremovedfromoutputandconnectedto0VtoforcetheoutputtransistorswitchON.
Note9:Feedbackpinremovedfromoutputandconnectedto12Vforthe3.3V,5V,andtheADJ.version,and15Vforthe12Vversion,toforcetheoutputtransistorswitchOFF.
Note10:VIN=40V.
Note11:Junctiontoambientthermalresistance(noexternalheatsink)fortheTO-220packagemountedvertically,withtheleadssolderedtoaprintedcircuitboardwith(1oz.)copperareaofapproximately1in2.
Note12:JunctiontoambientthermalresistancewiththeTO-263packagetabsolderedtoasingleprintedcircuitboardwith0.5in2of(1oz.)copperarea.Note13:JunctiontoambientthermalresistancewiththeTO-263packagetabsolderedtoasinglesidedprintedcircuitboardwith2.5in2of(1oz.)copperarea.Note14:JunctiontoambientthermalresistancewiththeTO-263packagetabsolderedtoadoublesidedprintedcircuitboardwith3in2of(1oz.)copperareaontheLM2596Ssideoftheboard,andapproximately16in2ofcopperontheothersideofthep-cboard.SeeApplicationInformationinthisdatasheetandthethermalmodelinSwitchersMadeSimple™version4.3software.
TypicalPerformanceCharacteristics
NormalizedOutputVoltage
LineRegulation
(CircuitofFigure1)
Efficiency
DS012583-5
DS012583-4
DS012583-6
5www.national.com
TypicalPerformanceCharacteristics
SwitchSaturationVoltage
(CircuitofFigure1)(Continued)
SwitchCurrentLimitDropoutVoltage
DS012583-8
DS012583-7
DS012583-9
Operating
QuiescentCurrent
Shutdown
QuiescentCurrent
MinimumOperatingSupplyVoltage
DS012583-10
DS012583-11
DS012583-12
ON/OFFThresholdVoltage
ON/OFFPin
Current(Sinking)
SwitchingFrequency
DS012583-15
DS012583-13
DS012583-14
www.national.com6
TypicalPerformanceCharacteristics
FeedbackPinBiasCurrent
(CircuitofFigure1)(Continued)
DS012583-16
TypicalPerformanceCharacteristics
ContinuousModeSwitchingWaveformsVIN=20V,VOUT=5V,ILOAD=2A
L=32µH,COUT=220µF,COUTESR=50mΩ
DiscontinuousModeSwitchingWaveformsVIN=20V,VOUT=5V,ILOAD=500mA
L=10µH,COUT=330µF,COUTESR=45mΩ
DS012583-17
A:OutputPinVoltage,10V/div.B:InductorCurrent1A/div.
C:OutputRippleVoltage,50mV/div.
DS012583-18
HorizontalTimeBase:2µs/div.
A:OutputPinVoltage,10V/div.B:InductorCurrent0.5A/div.
C:OutputRippleVoltage,100mV/div.
HorizontalTimeBase:2µs/div.
LoadTransientResponseforContinuousModeVIN=20V,VOUT=5V,ILOAD=500mAto2AL=32µH,COUT=220µF,COUTESR=50mΩ
LoadTransientResponseforDiscontinuousModeVIN=20V,VOUT=5V,ILOAD=500mAto2AL=10µH,COUT=330µF,COUTESR=45mΩ
DS012583-20
DS012583-19
A:OutputVoltage,100mV/div.(AC)B:500mAto2ALoadPulse
A:OutputVoltage,100mV/div.(AC)B:500mAto2ALoadPulse
HorizontalTimeBase:200µs/div.
HorizontalTimeBase:100µs/div.
7www.national.com
TestCircuitandLayoutGuidelines
FixedOutputVoltageVersions
DS012583-22
CIN—470µF,50V,AluminumElectrolyticNichicon“PLSeries”COUT—220µF,25VAluminumElectrolytic,Nichicon“PLSeries”D1—5A,40VSchottkyRectifier,1N5825L1—68µH,L38
AdjustableOutputVoltageVersions
DS012583-23
whereVREF=1.23V
SelectR1tobeapproximately1kΩ,usea1%resistorforbeststability.CIN—470µF,50V,AluminumElectrolyticNichicon“PLSeries”COUT—220µF,35VAluminumElectrolytic,Nichicon“PLSeries”D1—5A,40VSchottkyRectifier,1N5825L1—68µH,L38R1—1kΩ,1%
CFF—SeeApplicationInformationSection
FIGURE1.StandardTestCircuitsandLayoutGuides
Asinanyswitchingregulator,layoutisveryimportant.Rap-idlyswitchingcurrentsassociatedwithwiringinductancecangeneratevoltagetransientswhichcancauseproblems.Forminimalinductanceandgroundloops,thewiresindicatedbyheavylinesshouldbewideprintedcircuittracesandshouldbekeptasshortaspossible.Forbestresults,ex-ternalcomponentsshouldbelocatedasclosetotheswitcherlCaspossibleusinggroundplaneconstructionorsinglepointgrounding.
Ifopencoreinductorsareused,specialcaremustbetakenastothelocationandpositioningofthistypeofinduc-tor.Allowingtheinductorfluxtointersectsensitivefeedback,lCgroundpathandCOUTwiringcancauseproblems.Whenusingtheadjustableversion,specialcaremustbetakenastothelocationofthefeedbackresistorsandtheas-sociatedwiring.PhysicallylocatebothresistorsneartheIC,androutethewiringawayfromtheinductor,especiallyanopencoretypeofinductor.(Seeapplicationsectionformoreinformation.)
www.national.com8
LM2596SeriesBuckRegulatorDesignProcedure(FixedOutput)
PROCEDURE(FixedOutputVoltageVersion)Given:
VOUT=RegulatedOutputVoltage(3.3V,5Vor12V)VIN(max)=MaximumDCInputVoltageILOAD(max)=MaximumLoadCurrent1.InductorSelection(L1)
A.SelectthecorrectinductorvalueselectionguidefromFig-uresFigure4,Figure5,orFigure6.(Outputvoltagesof3.3V,5V,or12Vrespectively.)Forallothervoltages,seethede-signprocedurefortheadjustableversion.
B.Fromtheinductorvalueselectionguide,identifytheinduc-tanceregionintersectedbytheMaximumInputVoltagelineandtheMaximumLoadCurrentline.Eachregionisidentifiedbyaninductancevalueandaninductorcode(LXX).C.Selectanappropriateinductorfromthefourmanufactur-er’spartnumberslistedinFigure8.
2.OutputCapacitorSelection(COUT)
A.Inthemajorityofapplications,lowESR(EquivalentSeriesResistance)electrolyticcapacitorsbetween82µFand820µFandlowESRsolidtantalumcapacitorsbetween10µFand470µFprovidethebestresults.ThiscapacitorshouldbelocatedclosetotheICusingshortcapacitorleadsandshortcoppertraces.Donotusecapacitorslargerthan820µF.
Foradditionalinformation,seesectiononoutputcapaci-torsinapplicationinformationsection.
B.Tosimplifythecapacitorselectionprocedure,refertothequickdesigncomponentselectiontableshowninFigure2.Thistablecontainsdifferentinputvoltages,outputvoltages,andloadcurrents,andlistsvariousinductorsandoutputca-pacitorsthatwillprovidethebestdesignsolutions.
C.Thecapacitorvoltageratingforelectrolyticcapacitorsshouldbeatleast1.5timesgreaterthantheoutputvoltage,andoftenmuchhighervoltageratingsareneededtosatisfythelowESRrequirementsforlowoutputripplevoltage.D.Forcomputeraideddesignsoftware,seeSwitchersMadeSimple™version4.3orlater.
EXAMPLE(FixedOutputVoltageVersion)
Given:VOUT=5V
VIN(max)=12VILOAD(max)=3A
1.InductorSelection(L1)
A.Usetheinductorselectionguideforthe5VversionshowninFigure5.
B.FromtheinductorvalueselectionguideshowninFigure5,theinductanceregionintersectedbythe12Vhorizontallineandthe3Averticallineis33µH,andtheinductorcodeisL40.
C.Theinductancevaluerequiredis33µH.FromthetableinFigure8,gototheL40lineandchooseaninductorpartnum-berfromanyofthefourmanufacturersshown.(Inmostin-stance,boththroughholeandsurfacemountinductorsareavailable.)
2.OutputCapacitorSelection(COUT)
A.Seesectiononoutputcapacitorsinapplicationinfor-mationsection.
B.FromthequickdesigncomponentselectiontableshowninFigure2,locatethe5Voutputvoltagesection.Intheloadcurrentcolumn,choosetheloadcurrentlinethatisclosesttothecurrentneededinyourapplication,forthisexample,usethe3Aline.Inthemaximuminputvoltagecolumn,selectthelinethatcoverstheinputvoltageneededinyourapplication,inthisexample,usethe15Vline.Continuingonthislinearerecommendedinductorsandcapacitorsthatwillprovidethebestoverallperformance.
Thecapacitorlistcontainsboththroughholeelectrolyticandsurfacemounttantalumcapacitorsfromfourdifferentcapaci-tormanufacturers.Itisrecommendedthatboththemanufac-turersandthemanufacturer’sseriesthatarelistedinthetablebeused.
InthisexamplealuminumelectrolyticcapacitorsfromseveraldifferentmanufacturersareavailablewiththerangeofESRnumbersneeded.
330µF35VPanasonicHFQSeries330µF35VNichiconPLSeriesC.Fora5Voutput,acapacitorvoltageratingatleast7.5Vormoreisneeded.ButevenalowESR,switchinggrade,220µF10Valuminumelectrolyticcapacitorwouldexhibitap-proximately225mΩofESR(seethecurveinFigure14fortheESRvsvoltagerating).ThisamountofESRwouldresultinrelativelyhighoutputripplevoltage.Toreducetherippleto1%oftheoutputvoltage,orless,acapacitorwithahighervalueorwithahighervoltagerating(lowerESR)shouldbeselected.A16Vor25Vcapacitorwillreducetheripplevolt-agebyapproximatelyhalf.
9www.national.com
LM2596SeriesBuckRegulatorDesignProcedure(FixedOutput)
PROCEDURE(FixedOutputVoltageVersion)3.CatchDiodeSelection(D1)
A.Thecatchdiodecurrentratingmustbeatleast1.3timesgreaterthanthemaximumloadcurrent.Also,ifthepowersupplydesignmustwithstandacontinuousoutputshort,thediodeshouldhaveacurrentratingequaltothemaximumcurrentlimitoftheLM2596.Themoststressfulconditionforthisdiodeisanoverloadorshortedoutputcondition.
B.Thereversevoltageratingofthediodeshouldbeatleast1.25timesthemaximuminputvoltage.
C.Thisdiodemustbefast(shortreverserecoverytime)andmustbelocatedclosetotheLM2596usingshortleadsandshortprintedcircuittraces.Becauseoftheirfastswitchingspeedandlowforwardvoltagedrop,Schottkydiodesprovidethebestperformanceandefficiency,andshouldbethefirstchoice,especiallyinlowoutputvoltageapplications.Ultra-fastrecovery,orHigh-Efficiencyrectifiersalsoprovidegoodresults.Ultra-fastrecoverydiodestypicallyhavere-verserecoverytimesof50nsorless.Rectifierssuchasthe1N5400seriesaremuchtooslowandshouldnotbeused.4.InputCapacitor(CIN)
AlowESRaluminumortantalumbypasscapacitorisneededbetweentheinputpinandgroundpintopreventlargevolt-agetransientsfromappearingattheinput.ThiscapacitorshouldbelocatedclosetotheICusingshortleads.Inaddi-tion,theRMScurrentratingoftheinputcapacitorshouldbeselectedtobeatleast1⁄2theDCloadcurrent.Thecapacitormanufacturersdatasheetmustbecheckedtoassurethatthiscurrentratingisnotexceeded.ThecurveshowninFig-ure13showstypicalRMScurrentratingsforseveraldifferentaluminumelectrolyticcapacitorvalues.
Foranaluminumelectrolytic,thecapacitorvoltageratingshouldbeapproximately1.5timesthemaximuminputvolt-age.Cautionmustbeexercisedifsolidtantalumcapacitorsareused(seeApplicationInformationoninputcapacitor).Thetantalumcapacitorvoltageratingshouldbe2timesthemaximuminputvoltageanditisrecommendedthattheybesurgecurrenttestedbythemanufacturer.
Usecautionwhenusingceramiccapacitorsforinputbypass-ing,becauseitmaycausesevereringingattheVINpin.Foradditionalinformation,seesectiononinputcapaci-torsinApplicationInformationsection.
(Continued)
EXAMPLE(FixedOutputVoltageVersion)
3.CatchDiodeSelection(D1)
A.RefertothetableshowninFigure11.Inthisexample,a5A,20V,1N5823Schottkydiodewillprovidethebestperfor-mance,andwillnotbeoverstressedevenforashortedout-put.
4.InputCapacitor(CIN)
TheimportantparametersfortheInputcapacitorarethein-putvoltageratingandtheRMScurrentrating.Withanominalinputvoltageof12V,analuminumelectrolyticcapacitorwithavoltageratinggreaterthan18V(1.5xVIN)wouldbeneeded.Thenexthighercapacitorvoltageratingis25V.TheRMScurrentratingrequirementfortheinputcapacitorinabuckregulatorisapproximately1⁄2theDCloadcurrent.Inthisexample,witha3Aload,acapacitorwithaRMScurrentratingofatleast1.5Aisneeded.ThecurvesshowninFigure13canbeusedtoselectanappropriateinputcapacitor.Fromthecurves,locatethe35Vlineandnotewhichcapaci-torvalueshaveRMScurrentratingsgreaterthan1.5A.A680µF/35Vcapacitorcouldbeused.
Forathroughholedesign,a680µF/35Velectrolyticcapaci-tor(PanasonicHFQseriesorNichiconPLseriesorequiva-lent)wouldbeadequate.othertypesorothermanufacturerscapacitorscanbeusedprovidedtheRMSripplecurrentrat-ingsareadequate.
Forsurfacemountdesigns,solidtantalumcapacitorscanbeused,butcautionmustbeexercisedwithregardtotheca-pacitorsurgecurrentrating(seeApplicationInformationoninputcapacitorsinthisdatasheet).TheTPSseriesavailablefromAVX,andthe593DseriesfromSpraguearebothsurgecurrenttested.
www.national.com10
LM2596SeriesBuckRegulatorDesignProcedure(FixedOutput)
Conditions
OutputVoltage(V)3.3
LoadCurrent(A)3
MaxInputVoltage(V)
5710406
2
5
3
104081015409
2
12
3
20401518304015
2
2040
2222223322334722223347226868223368683368150
L41L41L41L40L33L32L39L41L41L40L39L33L38L38L41L40L44L44L32L38L42
Inductor
Inductance
(µH)
Inductor(#)
PanasonicHFQSeries(µF/V)470/25560/35680/35560/35470/25330/35330/35470/25560/25330/35330/35470/25180/35180/35470/25330/25180/25180/35330/25180/2582/25
OutputCapacitor
ThroughHoleElectrolytic
NichiconPLSeries(µF/V)560/16560/35680/35470/35470/35330/35270/50560/16560/25330/35270/35560/16180/35180/35470/25330/25180/25180/35330/25180/2582/25
(Continued)
SurfaceMountTantalumAVXTPSSeries(µF/V)330/6.3330/6.3330/6.3330/6.3330/6.3330/6.3220/10220/10220/10220/10220/10220/10100/10100/10100/16100/16100/16100/16100/16100/1668/20
Sprague595DSeries(µF/V)390/6.3390/6.3390/6.3390/6.3390/6.3390/6.3330/10330/10330/10330/10330/10330/10270/10270/10180/16180/16120/20120/20180/16120/2068/25
FIGURE2.LM2596FixedVoltageQuickDesignComponentSelectionTable
11www.national.com
LM2596SeriesBuckRegulatorDesignProcedure(AdjustableOutput)
PROCEDURE(AdjustableOutputVoltageVersion)Given:
VOUT=RegulatedOutputVoltageVIN(max)=MaximumInputVoltageILOAD(max)=MaximumLoadCurrent
F=SwitchingFrequency(Fixedatanominal150kHz).1.ProgrammingOutputVoltage(SelectingR1andR2,asshowninFigure1)
Usethefollowingformulatoselecttheappropriateresistorvalues.
EXAMPLE(AdjustableOutputVoltageVersion)Given:
VOUT=20VVIN(max)=28VILOAD(max)=3A
F=SwitchingFrequency(Fixedatanominal150kHz).1.ProgrammingOutputVoltage(SelectingR1andR2,asshowninFigure1)
SelectR1tobe1kΩ,1%.SolveforR2.
SelectavalueforR1between240Ωand1.5kΩ.Thelowerresistorvaluesminimizenoisepickupinthesensitivefeed-backpin.(Forthelowesttemperaturecoefficientandthebeststabilitywithtime,use1%metalfilmresistors.)
R2=1k(16.26−1)=15.26k,closest1%valueis15.4kΩ.R2=15.4kΩ.
2.InductorSelection(L1)
A.CalculatetheinductorVolt•microsecondconstantE•T(V•µs),fromthefollowingformula:
2.InductorSelection(L1)
A.CalculatetheinductorVolt•microsecondconstant(E•T),
whereVSAT=internalswitchsaturationvoltage=1.16VandVD=diodeforwardvoltagedrop=0.5V
B.UsetheE•TvaluefromthepreviousformulaandmatchitwiththeE•TnumberontheverticalaxisoftheInductorValueSelectionGuideshowninFigure7.
C.onthehorizontalaxis,selectthemaximumloadcurrent.D.IdentifytheinductanceregionintersectedbytheE•TvalueandtheMaximumLoadCurrentvalue.Eachregionisidentifiedbyaninductancevalueandaninductorcode(LXX).
E.Selectanappropriateinductorfromthefourmanufactur-er’spartnumberslistedinFigure8.
B.E•T=34.2(V•µs)C.ILOAD(max)=3A
D.FromtheinductorvalueselectionguideshowninFigure7,theinductanceregionintersectedbythe34(V•µs)horizon-tallineandthe3Averticallineis47µH,andtheinductorcodeisL39.
E.FromthetableinFigure8,locatelineL39,andselectaninductorpartnumberfromthelistofmanufacturerspartnum-bers.
www.national.com12
LM2596SeriesBuckRegulatorDesignProcedure(AdjustableOutput)
(Continued)
PROCEDURE(AdjustableOutputVoltageVersion)3.OutputCapacitorSelection(COUT)
A.Inthemajorityofapplications,lowESRelectrolyticorsolidtantalumcapacitorsbetween82µFand820µFprovidethebestresults.ThiscapacitorshouldbelocatedclosetotheICusingshortcapacitorleadsandshortcoppertraces.Donotusecapacitorslargerthan820µF.Foradditionalinforma-tion,seesectiononoutputcapacitorsinapplicationin-formationsection.
B.Tosimplifythecapacitorselectionprocedure,refertothequickdesigntableshowninFigure3.Thistablecontainsdif-ferentoutputvoltages,andlistsvariousoutputcapacitorsthatwillprovidethebestdesignsolutions.
C.Thecapacitorvoltageratingshouldbeatleast1.5timesgreaterthantheoutputvoltage,andoftenmuchhighervolt-ageratingsareneededtosatisfythelowESRrequirementsneededforlowoutputripplevoltage.
EXAMPLE(AdjustableOutputVoltageVersion)3.OutputCapacitorSeIection(COUT)
A.SeesectiononCOUTinApplicationInformationsection.B.FromthequickdesigntableshowninFigure3,locatetheoutputvoltagecolumn.Fromthatcolumn,locatetheoutputvoltageclosesttotheoutputvoltageinyourapplication.Inthisexample,selectthe24Vline.Undertheoutputcapacitorsection,selectacapacitorfromthelistofthroughholeelec-trolyticorsurfacemounttantalumtypesfromfourdifferentcapacitormanufacturers.Itisrecommendedthatboththemanufacturersandthemanufacturersseriesthatarelistedinthetablebeused.
Inthisexample,throughholealuminumelectrolyticcapaci-torsfromseveraldifferentmanufacturersareavailable.
220µF/35VPanasonicHFQSeries150µF/35VNichiconPLSeries
C.Fora20Voutput,acapacitorratingofatleast30Vormoreisneeded.Inthisexample,eithera35Vor50Vcapaci-torwouldwork.A35Vratingwaschosen,althougha50Vrat-ingcouldalsobeusedifaloweroutputripplevoltageisneeded.
Othermanufacturersorothertypesofcapacitorsmayalsobeused,providedthecapacitorspecifications(especiallythe100kHzESR)closelymatchthetypeslistedinthetable.Re-fertothecapacitormanufacturersdatasheetforthisinforma-tion.
4.FeedforwardCapacitor(CFF)(SeeFigure1)
Foroutputvoltagesgreaterthanapproximately10V,anaddi-tionalcapacitorisrequired.Thecompensationcapacitoristypicallybetween100pFand33nF,andiswiredinparallelwiththeoutputvoltagesettingresistor,R2.Itprovidesaddi-tionalstabilityforhighoutputvoltages,lowinput-outputvolt-ages,and/orverylowESRoutputcapacitors,suchassolidtantalumcapacitors.
4.FeedforwardCapacitor(CFF)
ThetableshowninFigure3containsfeedforwardcapacitorvaluesforvariousoutputvoltages.Inthisexample,a560pFcapacitorisneeded.
Thiscapacitortypecanbeceramic,plastic,silvermica,etc.(Becauseoftheunstablecharacteristicsofceramiccapaci-torsmadewithZ5Umaterial,theyarenotrecommended.)
13www.national.com
LM2596SeriesBuckRegulatorDesignProcedure(AdjustableOutput)
(Continued)
PROCEDURE(AdjustableOutputVoltageVersion)5.CatchDiodeSelection(D1)
A.Thecatchdiodecurrentratingmustbeatleast1.3timesgreaterthanthemaximumloadcurrent.Also,ifthepowersupplydesignmustwithstandacontinuousoutputshort,thediodeshouldhaveacurrentratingequaltothemaximumcurrentlimitoftheLM2596.Themoststressfulconditionforthisdiodeisanoverloadorshortedoutputcondition.B.Thereversevoltageratingofthediodeshouldbeatleast1.25timesthemaximuminputvoltage.
C.Thisdiodemustbefast(shortreverserecoverytime)andmustbelocatedclosetotheLM2596usingshortleadsandshortprintedcircuittraces.Becauseoftheirfastswitchingspeedandlowforwardvoltagedrop,Schottkydiodesprovidethebestperformanceandefficiency,andshouldbethefirstchoice,especiallyinlowoutputvoltageapplications.Ultra-fastrecovery,orHigh-Efficiencyrectifiersarealsoagoodchoice,butsometypeswithanabruptturn-offcharac-teristicmaycauseinstabilityorEMlproblems.Ultra-fastre-coverydiodestypicallyhavereverserecoverytimesof50nsorless.Rectifierssuchasthe1N4001seriesaremuchtooslowandshouldnotbeused.
6.InputCapacitor(CIN)
AlowESRaluminumortantalumbypasscapacitorisneededbetweentheinputpinandgroundtopreventlargevoltagetransientsfromappearingattheinput.Inaddition,theRMScurrentratingoftheinputcapacitorshouldbeselectedtobeatleast1⁄2theDCloadcurrent.Thecapacitormanufacturersdatasheetmustbecheckedtoassurethatthiscurrentratingisnotexceeded.ThecurveshowninFigure13showstypicalRMScurrentratingsforseveraldifferentaluminumelectro-lyticcapacitorvalues.
ThiscapacitorshouldbelocatedclosetotheICusingshortleadsandthevoltageratingshouldbeapproximately1.5timesthemaximuminputvoltage.
Ifsolidtantaluminputcapacitorsareused,itisrecomendedthattheybesurgecurrenttestedbythemanufacturer.
Usecautionwhenusingahighdielectricconstantceramiccapacitorforinputbypassing,becauseitmaycausesevereringingattheVINpin.
Foradditionalinformation,seesectiononinputcapaci-torsinapplicationinformationsection.
6.InputCapacitor(CIN)
TheimportantparametersfortheInputcapacitorarethein-putvoltageratingandtheRMScurrentrating.Withanominalinputvoltageof28V,analuminumelectrolyticaluminumelec-trolyticcapacitorwithavoltageratinggreaterthan42V(1.5xVIN)wouldbeneeded.Sincethethenexthighercapacitorvoltageratingis50V,a50Vcapacitorshouldbeused.Thecapacitorvoltageratingof(1.5xVIN)isaconservativeguide-line,andcanbemodifiedsomewhatifdesired.
TheRMScurrentratingrequirementfortheinputcapacitorofabuckregulatorisapproximately1⁄2theDCloadcurrent.Inthisexample,witha3Aload,acapacitorwithaRMScurrentratingofatleast1.5Aisneeded.
ThecurvesshowninFigure13canbeusedtoselectanap-propriateinputcapacitor.Fromthecurves,locatethe50VlineandnotewhichcapacitorvalueshaveRMScurrentrat-ingsgreaterthan1.5A.Eithera470µFor680µF,50Vca-pacitorcouldbeused.
Forathroughholedesign,a680µF/50Velectrolyticcapaci-tor(PanasonicHFQseriesorNichiconPLseriesorequiva-lent)wouldbeadequate.OthertypesorothermanufacturerscapacitorscanbeusedprovidedtheRMSripplecurrentrat-ingsareadequate.
Forsurfacemountdesigns,solidtantalumcapacitorscanbeused,butcautionmustbeexercisedwithregardtotheca-pacitorsurgecurrentrting(seeApplicationInformationorin-putcapacitorsinthisdatasheet).TheTPSseriesavailablefromAVX,andthe593DseriesfromSpraguearebothsurgecurrenttested.
EXAMPLE(AdjustableOutputVoltageVersion)5.CatchDiodeSelection(D1)
A.RefertothetableshowninFigure11.Schottkydiodesprovidethebestperformance,andinthisexamplea5A,40V,1N5825Schottkydiodewouldbeagoodchoice.The5Adi-oderatingismorethanadequateandwillnotbeover-stressedevenforashortedoutput.
Tofurthersimplifythebuckregulatordesignprocedure,Na-tionalSemiconductorismakingavailablecomputerdesignsoftwaretobeusedwiththeSimpleSwitcherlineotswitch-ingregulators.SwitchersMadeSimple(version4.3orlater)isavailableona31⁄2\"disketteforIBMcompatiblecomputers.www.national.com14
LM2596SeriesBuckRegulatorDesignProcedure(AdjustableOutput)
OutputVoltage(V)
246912152428
ThroughHoleOutputCapacitor
PanasonicHFQSeries(µF/V)820/35560/35470/25330/25330/25220/35220/35100/50
NichiconPLSeries(µF/V)820/35470/35470/25330/25330/25220/35150/35100/50
33nF10nF3.3nF1.5nF1nF680pF560pF390pFFeedforwardCapacitor
SurfaceMountOutputCapacitor
AVXTPSSeries(µF/V)330/6.3330/6.3220/10100/16100/1668/2033/2510/35
Sprague595DSeries(µF/V)470/4390/6.3330/10180/16180/16120/2033/2515/50
33nF10nF3.3nF1.5nF1nF680pF220pF220pFFeedforwardCapacitor
FIGURE3.OutputCapacitorandFeedforwardCapacitorSelectionTable
LM2596SeriesBuckRegulatorDesignProcedure
INDUCTORVALUESELECTIONGUIDES(ForContinuousModeOperation)
DS012583-24
DS012583-26
FIGURE4.LM2596-3.3
FIGURE6.LM2596-12
DS012583-25DS012583-27
FIGURE5.LM2596-5.0FIGURE7.LM2596-ADJ
15www.national.com
LM2596SeriesBuckRegulatorDesignProcedure
Inductance
(µH)L15L21L22L23L24L25L26L27L28L29L30L31L32L33L34L35L36L37L38L39L40L41L42L43L44
22684733221533022015010068473322152201501006847332215010068
Cur-rent(A)0.990.991.171.401.702.100.801.001.201.471.782.202.503.103.401.702.102.503.103.503.503.502.703.403.40
Schott
ThroughHole67148350671440706714408067144090671483706714838067144100671441106714412067144130671441406714415067144160671483906714840067144170671441806714419067144200671442106714422067144230671484106714424067144250
SurfaceMount671484606714445067144460671444706714848067148490671444806714449067144500671445106714452067144530671445406714850067148790
—————6714829067148300
———
HoleRL-1284-22-43RL-5471-5RL-5471-6RL-5471-7RL-1283-22-43RL-1283-15-43RL-5471-1RL-5471-2RL-5471-3RL-5471-4RL-5471-5RL-5471-6RL-5471-7RL-1283-22-43RL-1283-15-43RL-5473-1RL-5473-4RL-5472-1RL-5472-2RL-5472-3RL-5472-4RL-5472-5RL-5473-4RL-5473-2RL-5473-3
Renco
Through
SurfaceMountRL1500-22RL1500-68
———————————————————————
(Continued)
PulseEngineeringThroughHolePE-53815PE-53821PE-53822PE-53823PE-53824PE-53825PE-53826PE-53827PE-53828PE-53829PE-53830PE-53831PE-53932PE-53933PE-53934PE-53935PE-54036PE-54037PE-54038PE-54039PE-54040PE-54041PE-54042PE-54043PE-54044
SurfaceMountPE-53815-SPE-53821-SPE-53822-SPE-53823-SPE-53825-SPE-53824-S
CoilcraftSurfaceMountDO3308-223DO3316-683DO3316-473DO3316-333DO3316-223DO3316-153
PE-53826-SDO5022P-334PE-53827-SDO5022P-224PE-53828-SDO5022P-154PE-53829-SDO5022P-104PE-53830-SDO5022P-683PE-53831-SPE-53932-SPE-53933-SPE-53934-SPE-53935-SPE-54036-SPE-54037-SPE-54038-SPE-54039-SPE-54040-SPE-54041-SPE-54042-S
DO5022P-473DO5022P-333DO5022P-223DO5022P-153
——————————
FIGURE8.InductorManufacturersPartNumbers
CoilcraftInc.
CoilcraftInc.,EuropePulseEngineeringInc.PulseEngineeringInc.,Europe
RencoElectronicsInc.SchottCorp.
PhoneFAXPhoneFAXPhoneFAXPhoneFAXPhoneFAXPhoneFAX
(800)322-2645(708)639-1469+111236730595+441236730627(619)674-8100(619)674-8262+3539324107+3539324459(800)645-5828(516)586-5562(612)475-1173(612)475-1786
FIGURE9.InductorManufacturersPhoneNumbers
www.national.com16
LM2596SeriesBuckRegulatorDesignProcedure
NichiconCorp.PanasonicAVXCorp.Sprague/Vishay
PhoneFAXPhoneFAXPhoneFAXPhoneFAX
(Continued)
(708)843-7500(708)843-2798(714)373-7857(714)373-7102(803)448-9411(803)448-1943(207)324-4140(207)324-7223
FIGURE10.CapacitorManufacturersPhoneNumbers
VR
SurfaceMountSchottky
3ADiodes
ThroughHoleSchot-tky1N5820SR302MBR3201N5821MBR33031DQ031N5822SR304MBR340
MURS32030WF10
31DQ04SR305MBR35031DQ05
FIGURE11.DiodeSelectionTable
50WQ05
MUR320UltraFastRecoveryAllofthesediodesareratedtoatleast50V.
SurfaceMountSchot-tky
UltraFastRecovery
4A–6ADiodes
ThroughHoleSchot-tkySR5021N5823SB520SR5031N5824SB530
50WQ04
MURS62050WF10
SB55050SQ080SR5041N5825SB540
MUR620HER601UltraFastRecoveryAllofthesediodesareratedtoatleast50V.
UltraFastRecoveryAllofthesediodesareratedtoatleast50V.
20V
SK32
30V
30WQ03SK33
Allofthesediodesareratedtoatleast50V.
50WQ03
40VSK34MBRS34030WQ04
50VorMoreSK35MBRS36030WQ05
17www.national.com
BlockDiagram
DS012583-21
FIGURE12.
ApplicationInformation
PINFUNCTIONS
+VIN—ThisisthepositiveinputsupplyfortheICswitchingregulator.Asuitableinputbypasscapacitormustbepresentatthispintominimizevoltagetransientsandtosupplytheswitchingcurrentsneededbytheregulator.Ground—Circuitground.
Output—Internalswitch.Thevoltageatthispinswitchesbetween(+VIN−VSAT)andapproximately−0.5V,withadutycycleofapproximatelyVOUT/VIN.Tominimizecouplingtosensitivecircuitry,thePCboardcopperareaconnectedtothispinshouldbekepttoaminimum.
Feedback—Sensestheregulatedoutputvoltagetocom-pletethefeedbackloop.
ON/OFF—Allowstheswitchingregulatorcircuittobeshutdownusinglogiclevelsignalsthusdroppingthetotalinputsupplycurrenttoapproximately80µA.Pullingthispinbelowathresholdvoltageofapproximately1.3Vturnstheregulatoron,andpullingthispinabove1.3V(uptoamaximumof25V)shutstheregulatordown.Ifthisshutdownfeatureisnotneeded,theON/OFFpincanbewiredtothegroundpinoritcanbeleftopen,ineithercasetheregulatorwillbeintheONcondition.
EXTERNALCOMPONENTS
INPUTCAPACITOR
CIN—AlowESRaluminumortantalumbypasscapacitorisneededbetweentheinputpinandgroundpin.Itmustbelo-catedneartheregulatorusingshortleads.Thiscapacitorpreventslargevoltagetransientsfromappearingatthein-put,andprovidestheinstantaneouscurrentneededeachtimetheswitchturnson.
TheimportantparametersfortheInputcapacitorarethevoltageratingandtheRMScurrentrating.BecauseoftherelativelyhighRMScurrentsflowinginabuckregulator’sin-putcapacitor,thiscapacitorshouldbechosenforitsRMScurrentratingratherthanitscapacitanceorvoltageratings,
www.national.com
18
althoughthecapacitancevalueandvoltageratingaredi-rectlyrelatedtotheRMScurrentrating.
TheRMScurrentratingofacapacitorcouldbeviewedasacapacitor’spowerrating.TheRMScurrentflowingthroughthecapacitorsinternalESRproducespowerwhichcausestheinternaltemperatureofthecapacitortorise.TheRMScurrentratingofacapacitorisdeterminedbytheamountofcurrentrequiredtoraisetheinternaltemperatureapproxi-mately10˚Caboveanambienttemperatureof105˚C.Theabilityofthecapacitortodissipatethisheattothesurround-ingairwilldeterminetheamountofcurrentthecapacitorcansafelysustain.CapacitorsthatarephysicallylargeandhavealargesurfaceareawilltypicallyhavehigherRMScurrentratings.Foragivencapacitorvalue,ahighervoltageelectro-lyticcapacitorwillbephysicallylargerthanalowervoltagecapacitor,andthusbeabletodissipatemoreheattothesur-roundingair,andthereforewillhaveahigherRMScurrentrating.
TheconsequencesofoperatinganelectrolyticcapacitorabovetheRMScurrentratingisashortenedoperatinglife.Thehighertemperaturespeedsuptheevaporationoftheca-pacitor’selectrolyte,resultingineventualfailure.
Selectinganinputcapacitorrequiresconsultingthemanu-facturersdatasheetformaximumallowableRMSripplecur-rent.Foramaximumambienttemperatureof40˚C,agen-eralguidelinewouldbetoselectacapacitorwitharipplecurrentratingofapproximately50%oftheDCloadcurrent.Forambienttemperaturesupto70˚C,acurrentratingof75%oftheDCloadcurrentwouldbeagoodchoiceforaconservativedesign.Thecapacitorvoltageratingmustbeatleast1.25timesgreaterthanthemaximuminputvoltage,andoftenamuchhighervoltagecapacitorisneededtosat-isfytheRMScurrentrequirements.
AgraphshowninFigure13showstherelationshipbetweenanelectrolyticcapacitorvalue,itsvoltagerating,andtheRMScurrentitisratedfor.ThesecurveswereobtainedfromtheNichicon“PL”seriesoflowESR,highreliabilityelectro-
ApplicationInformation
(Continued)
lyticcapacitorsdesignedforswitchingregulatorapplications.Othercapacitormanufacturersoffersimilartypesofcapaci-tors,butalwayscheckthecapacitordatasheet.
“Standard”electrolyticcapacitorstypicallyhavemuchhigherESRnumbers,lowerRMScurrentratingsandtypicallyhaveashorteroperatinglifetime.
Becauseoftheirsmallsizeandexcellentperformance,sur-facemountsolidtantalumcapacitorsareoftenusedforinputbypassing,butseveralprecautionsmustbeobserved.Asmallpercentageofsolidtantalumcapacitorscanshortiftheinrushcurrentratingisexceeded.Thiscanhappenatturnonwhentheinputvoltageissuddenlyapplied,andofcourse,higherinputvoltagesproducehigherinrushcurrents.Sev-eralcapacitormanufacturersdoa100%surgecurrenttest-ingontheirproductstominimizethispotentialproblem.Ifhighturnoncurrentsareexpected,itmaybenecessarytolimitthiscurrentbyaddingeithersomeresistanceorinduc-tancebeforethetantalumcapacitor,orselectahighervolt-agecapacitor.Aswithaluminumelectrolyticcapacitors,theRMSripplecurrentratingmustbesizedtotheloadcurrent.FEEDFORWARDCAPACITOR
(AdjustableOutputVoltageVersion)
CFF—AFeedforwardCapacitorCFF,shownacrossR2inFigure1isusedwhentheouputvoltageisgreaterthan10VorwhenCOUThasaverylowESR.Thiscapacitoraddsleadcompensationtothefeedbackloopandincreasesthephasemarginforbetterloopstability.ForCFFselection,seethede-signproceduresection.
allowableoutputripplevoltage,typically1%to2%oftheout-putvoltage.Butiftheselectedcapacitor’sESRisextremelylow,thereisapossibilityofanunstablefeedbackloop,re-sultinginanoscillationattheoutput.Usingthecapacitorslistedinthetables,orsimilartypes,willprovidedesignsolu-tionsunderallconditions.
Ifverylowoutputripplevoltage(lessthan15mV)isre-quired,refertothesectiononOutputVoltageRippleandTransientsforapostripplefilter.
Analuminumelectrolyticcapacitor’sESRvalueisrelatedtothecapacitancevalueanditsvoltagerating.Inmostcases,highervoltageelectrolyticcapacitorshavelowerESRvalues(seeFigure14).Often,capacitorswithmuchhighervoltageratingsmaybeneededtoprovidethelowESRvaluesre-quiredforlowoutputripplevoltage.
Theoutputcapacitorformanydifferentswitcherdesignsof-tencanbesatisfiedwithonlythreeorfourdifferentcapacitorvaluesandseveraldifferentvoltageratings.SeethequickdesigncomponentselectiontablesinFigure2and4fortypi-calcapacitorvalues,voltageratings,andmanufacturersca-pacitortypes.
Electrolyticcapacitorsarenotrecommendedfortempera-turesbelow−25˚C.TheESRrisesdramaticallyatcoldtem-peraturesandtypicallyrises3X@−25˚Candasmuchas10Xat−40˚C.SeecurveshowninFigure15.
SolidtantalumcapacitorshaveamuchbetterESRspecforcoldtemperaturesandarerecommendedfortemperaturesbelow−25˚C.
DS012583-29
FIGURE14.CapacitorESRvsCapacitorVoltageRating
(TypicalLowESRElectrolyticCapacitor)
DS012583-28
FIGURE13.RMSCurrentRatingsforLowESR
ElectrolyticCapacitors(Typical)OUTPUTCAPACITOR
COUT—Anoutputcapacitorisrequiredtofiltertheoutputandprovideregulatorloopstability.LowimpedanceorlowESRElectrolyticorsolidtantalumcapacitorsdesignedforswitchingregulatorapplicationsmustbeused.Whenselect-inganoutputcapacitor,theimportantcapacitorparametersare;the100kHzEquivalentSeriesResistance(ESR),theRMSripplecurrentrating,voltagerating,andcapacitancevalue.Fortheoutputcapacitor,theESRvalueisthemostimportantparameter.
TheoutputcapacitorrequiresanESRvaluethathasanup-perandlowerlimit.Forlowoutputripplevoltage,alowESRvalueisneeded.Thisvalueisdeterminedbythemaximum
19
CATCHDIODE
Buckregulatorsrequireadiodetoprovideareturnpathfortheinductorcurrentwhentheswitchturnsoff.ThismustbeafastdiodeandmustbelocatedclosetotheLM2596usingshortleadsandshortprintedcircuittraces.
Becauseoftheirveryfastswitchingspeedandlowforwardvoltagedrop,Schottkydiodesprovidethebestperformance,especiallyinlowoutputvoltageapplications(5Vandlower).Ultra-fastrecovery,orHigh-Efficiencyrectifiersarealsoagoodchoice,butsometypeswithanabruptturnoffcharac-teristicmaycauseinstabilityorEMIproblems.Ultra-fastre-coverydiodestypicallyhavereverserecoverytimesof50nsorless.Rectifierssuchasthe1N5400seriesaremuchtooslowandshouldnotbeused.
www.national.com
ApplicationInformation
(Continued)
DS012583-31
DS012583-30
FIGURE15.CapacitorESRChangevsTemperatureINDUCTORSELECTION
Allswitchingregulatorshavetwobasicmodesofoperation;continuousanddiscontinuous.Thedifferencebetweenthetwotypesrelatestotheinductorcurrent,whetheritisflowingcontinuously,orifitdropstozeroforaperiodoftimeinthenormalswitchingcycle.Eachmodehasdistinctivelydifferentoperatingcharacteristics,whichcanaffecttheregulatorsperformanceandrequirements.Mostswitcherdesignswilloperateinthediscontinuousmodewhentheloadcurrentislow.
TheLM2596(oranyoftheSimpleSwitcherfamily)canbeusedforbothcontinuousordiscontinuousmodesofopera-tion.
Inmanycasesthepreferredmodeofoperationisthecon-tinuousmode.Itoffersgreateroutputpower,lowerpeakswitch,inductoranddiodecurrents,andcanhavelowerout-putripplevoltage.Butitdoesrequirelargerinductorvaluestokeeptheinductorcurrentflowingcontinuously,especiallyatlowoutputloadcurrentsand/orhighinputvoltages.Tosimplifytheinductorselectionprocess,aninductorselec-tionguide(nomograph)wasdesigned(seeFigure4through8).Thisguideassumesthattheregulatorisoperatinginthecontinuousmode,andselectsaninductorthatwillallowapeak-to-peakinductorripplecurrenttobeacertainpercent-ageofthemaximumdesignloadcurrent.Thispeak-to-peakinductorripplecurrentpercentageisnotfixed,butisallowedtochangeasdifferentdesignloadcurrentsareselected.(SeeFigure16.)
FIGURE16.(∆IIND)Peak-to-PeakInductor
RippleCurrent(asaPercentageoftheLoadCurrent)
vsLoadCurrentByallowingthepercentageofinductorripplecurrenttoin-creaseforlowloadcurrents,theinductorvalueandsizecanbekeptrelativelylow.
Whenoperatinginthecontinuousmode,theinductorcurrentwaveformrangesfromatriangulartoasawtoothtypeofwaveform(dependingontheinputvoltage),withtheaveragevalueofthiscurrentwaveformequaltotheDCoutputloadcurrent.
Inductorsareavailableindifferentstylessuchaspotcore,toroid,E-core,bobbincore,etc.,aswellasdifferentcorema-terials,suchasferritesandpowderediron.Theleastexpen-sive,thebobbin,rodorstickcore,consistsofwirewoundonaferritebobbin.Thistypeofconstructionmakesforaninex-pensiveinductor,butsincethemagneticfluxisnotcom-pletelycontainedwithinthecore,itgeneratesmoreElectro-MagneticInterference(EMl).Thismagneticfluxcaninducevoltagesintonearbyprintedcircuittraces,thuscaus-ingproblemswithboththeswitchingregulatoroperationandnearbysensitivecircuitry,andcangiveincorrectscoperead-ingsbecauseofinducedvoltagesinthescopeprobe.AlsoseesectiononOpenCoreInductors.
WhenmultipleswitchingregulatorsarelocatedonthesamePCboard,opencoremagneticscancauseinterferencebe-tweentwoormoreoftheregulatorcircuits,especiallyathighcurrents.AtorroidorE-coreinductor(closedmagneticstruc-ture)shouldbeusedinthesesituations.
TheinductorslistedintheselectionchartincludeferriteE-coreconstructionforSchott,ferritebobbincoreforRencoandCoilcraft,andpowderedirontoroidforPulseEngineer-ing.
Exceedinganinductor’smaximumcurrentratingmaycausetheinductortooverheatbecauseofthecopperwirelosses,orthecoremaysaturate.Iftheinductorbeginstosaturate,theinductancedecreasesrapidlyandtheinductorbeginstolookmainlyresistive(theDCresistanceofthewinding).Thiscancausetheswitchcurrenttoriseveryrapidlyandforcetheswitchintoacycle-by-cyclecurrentlimit,thusreducingtheDCoutputloadcurrent.Thiscanalsoresultinoverheat-ingoftheinductorand/ortheLM2596.Differentinductortypeshavedifferentsaturationcharacteristics,andthisshouldbekeptinmindwhenselectinganinductor.Theinductormanufacturer’sdatasheetsincludecurrentandenergylimitstoavoidinductorsaturation.
www.national.com20
ApplicationInformation
DISCONTINUOUSMODEOPERATION
(Continued)
Theselectionguidechoosesinductorvaluessuitableforcontinuousmodeoperation,butforlowcurrentapplicationsand/orhighinputvoltages,adiscontinuousmodedesignmaybeabetterchoice.Itwoulduseaninductorthatwouldbephysicallysmaller,andwouldneedonlyonehalftoonethirdtheinductancevalueneededforacontinuousmodede-sign.Thepeakswitchandinductorcurrentswillbehigherinadiscontinuousdesign,butattheselowloadcurrents(1Aandbelow),themaximumswitchcurrentwillstillbelessthantheswitchcurrentlimit.
Discontinuousoperationcanhavevoltagewaveformsthatareconsiderabledifferentthanacontinuousdesign.Theout-putpin(switch)waveformcanhavesomedampedsinusoi-dalringingpresent.(SeeTypicalPerformanceCharacteris-ticsphototitledDiscontinuousModeSwitchingWaveforms)Thisringingisnormalfordiscontinuousoperation,andisnotcausedbyfeedbackloopinstabilities.Indiscontinuousop-eration,thereisaperiodoftimewhereneithertheswitchorthediodeareconducting,andtheinductorcurrenthasdroppedtozero.Duringthistime,asmallamountofenergycancirculatebetweentheinductorandtheswitch/diodeparasiticcapacitancecausingthischaracteristicringing.Nor-mallythisringingisnotaproblem,unlesstheamplitudebe-comesgreatenoughtoexceedtheinputvoltage,andeventhen,thereisverylittleenergypresenttocausedamage.Differentinductortypesand/orcorematerialsproducediffer-entamountsofthischaracteristicringing.Ferritecoreinduc-torshaveverylittlecorelossandthereforeproducethemostringing.Thehighercorelossofpowderedironinductorspro-ducelessringing.Ifdesired,aseriesRCcouldbeplacedinparallelwiththeinductortodampentheringing.Thecom-puteraideddesignsoftwareSwitchersMadeSimple(ver-sion4.3)willprovideallcomponentvaluesforcontinuousanddiscontinuousmodesofoperation.
inginoscillationproblems.Ifverylowoutputripplevoltageisneeded(lessthan20mV),apostripplefilterisrecom-mended.(SeeFigure1.)Theinductancerequiredistypicallybetween1µHand5µH,withlowDCresistance,tomaintaingoodloadregulation.AlowESRoutputfiltercapacitorisalsorequiredtoassuregooddynamicloadresponseandripplereduction.TheESRofthiscapacitormaybeaslowasde-sired,becauseitisoutoftheregulatorfeedbackloop.ThephotoshowninFigure17showsatypicaloutputripplevolt-age,withandwithoutapostripplefilter.
Whenobservingoutputripplewithascope,itisessentialthatashort,lowinductancescopeprobegroundconnectionbeused.Mostscopeprobemanufacturersprovideaspecialprobeterminatorwhichissolderedontotheregulatorboard,preferableattheoutputcapacitor.Thisprovidesaveryshortscopegroundthuseliminatingtheproblemsassociatedwiththe3inchgroundleadnormallyprovidedwiththeprobe,andprovidesamuchcleanerandmoreaccuratepictureoftheripplevoltagewaveform.
Thevoltagespikesarecausedbythefastswitchingactionoftheoutputswitchandthediode,andtheparasiticinductanceoftheoutputfiltercapacitor,anditsassociatedwiring.Tominimizethesevoltagespikes,theoutputcapacitorshouldbedesignedforswitchingregulatorapplications,andtheleadlengthsmustbekeptveryshort.Wiringinductance,straycapacitance,aswellasthescopeprobeusedtoevalu-atethesetransients,allcontributetotheamplitudeofthesespikes.
Whenaswitchingregulatorisoperatinginthecontinuousmode,theinductorcurrentwaveformrangesfromatriangu-lartoasawtoothtypeofwaveform(dependingontheinputvoltage).Foragiveninputandoutputvoltage,thepeak-to-peakamplitudeofthisinductorcurrentwaveformre-mainsconstant.Astheloadcurrentincreasesordecreases,theentiresawtoothcurrentwaveformalsorisesandfalls.Theaveragevalue(orthecenter)ofthiscurrentwaveformisequaltotheDCloadcurrent.
Iftheloadcurrentdropstoalowenoughlevel,thebottomofthesawtoothcurrentwaveformwillreachzero,andtheswitcherwillsmoothlychangefromacontinuoustoadiscon-tinuousmodeofoperation.Mostswitcherdesigns(irregard-lesshowlargetheinductorvalueis)willbeforcedtorundis-continuousiftheoutputislightlyloaded.Thisisaperfectlyacceptablemodeofoperation.
DS012583-32
FIGURE17.PostRippleFilterWaveform
OUTPUTVOLTAGERIPPLEANDTRANSIENTS
Theoutputvoltageofaswitchingpowersupplyoperatinginthecontinuousmodewillcontainasawtoothripplevoltageattheswitcherfrequency,andmayalsocontainshortvoltagespikesatthepeaksofthesawtoothwaveform.
Theoutputripplevoltageisafunctionoftheinductorsaw-toothripplecurrentandtheESRoftheoutputcapacitor.Atypicaloutputripplevoltagecanrangefromapproximately0.5%to3%oftheoutputvoltage.Toobtainlowripplevolt-age,theESRoftheoutputcapacitormustbelow,however,cautionmustbeexercisedwhenusingextremelylowESRcapacitorsbecausetheycanaffecttheloopstability,result-21
DS012583-33
FIGURE18.Peak-to-PeakInductorRippleCurrentvsLoadCurrent
Inaswitchingregulatordesign,knowingthevalueofthepeak-to-peakinductorripplecurrent(∆IIND)canbeusefulfordetermininganumberofothercircuitparameters.Param-eterssuchas,peakinductororpeakswitchcurrent,mini-www.national.com
ApplicationInformation
(Continued)
mumloadcurrentbeforethecircuitbecomesdiscontinuous,outputripplevoltageandoutputcapacitorESRcanallbecalculatedfromthepeak-to-peak∆IIND.WhentheinductornomographsshowninFigure4through8areusedtoselectaninductorvalue,thepeak-to-peakinductorripplecurrentcanimmediatelybedetermined.ThecurveshowninFigure18showstherangeof(∆IIND)thatcanbeexpectedfordiffer-entloadcurrents.Thecurvealsoshowshowthepeak-to-peakinductorripplecurrent(∆IIND)changesasyougofromthelowerbordertotheupperborder(foragivenloadcurrent)withinaninductanceregion.Theupperborderrep-resentsahigherinputvoltage,whilethelowerborderrepre-sentsalowerinputvoltage(seeInductorSelectionGuides).Thesecurvesareonlycorrectforcontinuousmodeopera-tion,andonlyiftheinductorselectionguidesareusedtose-lecttheinductorvalue
Considerthefollowingexample:
VOUT=5V,maximumloadcurrentof2.5A
VIN=12V,nominal,varyingbetween10Vand16V.
TheselectionguideinFigure5showsthattheverticallinefora2.5Aloadcurrent,andthehorizontallineforthe12Vin-putvoltageintersectapproximatelymidwaybetweentheup-perandlowerbordersofthe33µHinductanceregion.A33µHinductorwillallowapeak-to-peakinductorcurrent(∆IIND)toflowthatwillbeapercentageofthemaximumloadcurrent.ReferringtoFigure18,followthe2.5Alineapproxi-matelymidwayintotheinductanceregion,andreadthepeak-to-peakinductorripplecurrent(∆IIND)onthelefthandaxis(approximately620mAp-p).
Astheinputvoltageincreasesto16V,itapproachestheup-perborderoftheinductanceregion,andtheinductorripplecurrentincreases.ReferringtothecurveinFigure18,itcanbeseenthatforaloadcurrentof2.5A,thepeak-to-peakin-ductorripplecurrent(∆IIND)is620mAwith12Vin,andcanrangefrom740mAattheupperborder(16Vin)to500mAatthelowerborder(10Vin).
Oncethe∆IINDvalueisknown,thefollowingformulascanbeusedtocalculateadditionalinformationabouttheswitchingregulatorcircuit.
1.PeakInductororpeakswitchcurrent
OPENCOREINDUCTORS
Anotherpossiblesourceofincreasedoutputripplevoltageorunstableoperationisfromanopencoreinductor.Ferritebobbinorstickinductorshavemagneticlinesoffluxflowingthroughtheairfromoneendofthebobbintotheotherend.ThesemagneticlinesoffluxwillinduceavoltageintoanywireorPCboardcoppertracethatcomeswithintheinduc-tor’smagneticfield.Thestrengthofthemagneticfield,theorientationandlocationofthePCcoppertracetothemag-neticfield,andthedistancebetweenthecoppertraceandtheinductor,determinetheamountofvoltagegeneratedinthecoppertrace.AnotherwayoflookingatthisinductivecouplingistoconsiderthePCboardcoppertraceasoneturnofatransformer(secondary)withtheinductorwindingastheprimary.Manymillivoltscanbegeneratedinacoppertracelocatednearanopencoreinductorwhichcancausestabilityproblemsorhighoutputripplevoltageproblems.Ifunstableoperationisseen,andanopencoreinductorisused,it’spossiblethatthelocationoftheinductorwithre-specttootherPCtracesmaybetheproblem.Todetermineifthisistheproblem,temporarilyraisetheinductorawayfromtheboardbyseveralinchesandthencheckcircuitop-eration.Ifthecircuitnowoperatescorrectly,thenthemag-neticfluxfromtheopencoreinductoriscausingtheproblem.SubstitutingaclosedcoreinductorsuchasatorroidorE-corewillcorrecttheproblem,orre-arrangingthePClayoutmaybenecessary.MagneticfluxcuttingtheICdevicegroundtrace,feedbacktrace,orthepositiveornegativetracesoftheoutputcapacitorshouldbeminimized.
Sometimes,locatingatracedirectlybeneathabobbinin-ductorwillprovidegoodresults,provideditisexactlyinthecenteroftheinductor(becausetheinducedvoltagescancelthemselvesout),butifitisoffcenteronedirectionortheother,thenproblemscouldarise.Iffluxproblemsarepresent,eventhedirectionoftheinductorwindingcanmakeadifferenceinsomecircuits.
Thisdiscussiononopencoreinductorsisnottofrightentheuser,buttoalerttheuseronwhatkindofproblemstowatchoutforwhenusingthem.Opencorebobbinor“stick”induc-torsareaninexpensive,simplewayofmakingacompactef-ficientinductor,andtheyareusedbythemillionsinmanydif-ferentapplications.
THERMALCONSIDERATIONS
TheLM2596isavailableintwopackages,a5-pinTO-220(T)anda5-pinsurfacemountTO-263(S).
2.
Minimumloadcurrentbeforethecircuitbecomesdis-continuous
3.4.
OutputRippleVoltage=(∆IIND)x(ESRofCOUT)=0.62Ax0.1Ω=62mVp-p
TheTO-220packageneedsaheatsinkundermostcondi-tions.Thesizeoftheheatsinkdependsontheinputvoltage,theoutputvoltage,theloadcurrentandtheambienttem-perature.ThecurvesinFigure19showtheLM2596Tjunc-tiontemperaturerisesaboveambienttemperaturefora3Aloadanddifferentinputandoutputvoltages.ThedataforthesecurveswastakenwiththeLM2596T(TO-220pack-age)operatingasabuckswitchingregulatorinanambienttemperatureof25˚C(stillair).Thesetemperaturerisenum-bersareallapproximateandtherearemanyfactorsthatcanaffectthesetemperatures.Higherambienttemperaturesre-quiremoreheatsinking.
TheTO-263surfacemountpackagetabisdesignedtobesolderedtothecopperonaprintedcircuitboard.Thecopperandtheboardaretheheatsinkforthispackageandtheotherheatproducingcomponents,suchasthecatchdiodeandinductor.ThePCboardcopperareathatthepackageissolderedtoshouldbeatleast0.4in2,andideallyshouldhave2ormoresquareinchesof2oz.(0.0028)in)copper.Additionalcopperareaimprovesthethermalcharacteristics,
www.national.com22
ApplicationInformation
(Continued)
CircuitDataforTemperatureRiseCurve
TO-220Package(T)
CapacitorsInductorDiodePCboard
ThroughholeelectrolyticThroughhole,Renco
Throughhole,5A40V,Schottky
3squareinchessinglesided2oz.copper(0.0028\")
butwithcopperareasgreaterthanapproximately6in2,onlysmallimprovementsinheatdissipationarerealized.Iffur-therthermalimprovementsareneeded,doublesided,multi-layerPCboardwithlargecopperareasand/orairflowarerecommended.
ThecurvesshowninFigure20showtheLM2596S(TO-263package)junctiontemperatureriseaboveambienttempera-turewitha2Aloadforvariousinputandoutputvoltages.ThisdatawastakenwiththecircuitoperatingasabuckswitchingregulatorwithallcomponentsmountedonaPCboardtosimulatethejunctiontemperatureunderactualoperatingconditions.Thiscurvecanbeusedforaquickcheckfortheapproximatejunctiontemperatureforvariousconditions,butbeawarethattherearemanyfactorsthatcanaffectthejunc-tiontemperature.Whenloadcurrentshigherthan2Aareused,doublesidedormultilayerPCboardswithlargecop-perareasand/orairflowmightbeneeded,especiallyforhighambienttemperaturesandhighoutputvoltages.
Forthebestthermalperformance,widecoppertracesandgenerousamountsofprintedcircuitboardcoppershouldbeusedintheboardlayout.(Oneexceptiontothisistheoutput(switch)pin,whichshouldnothavelargeareasofcopper.)Largeareasofcopperprovidethebesttransferofheat(lowerthermalresistance)tothesurroundingair,andmovingairlowersthethermalresistanceevenfurther.
Packagethermalresistanceandjunctiontemperaturerisenumbersareallapproximate,andtherearemanyfactorsthatwillaffectthesenumbers.Someofthesefactorsincludeboardsize,shape,thickness,position,location,andevenboardtemperature.Otherfactorsare,tracewidth,totalprintedcircuitcopperarea,copperthickness,single-ordouble-sided,multilayerboardandtheamountofsolderontheboard.TheeffectivenessofthePCboardtodissipateheatalsodependsonthesize,quantityandspacingofothercomponentsontheboard,aswellaswhetherthesurround-ingairisstillormoving.Furthermore,someofthesecompo-nentssuchasthecatchdiodewilladdheattothePCboardandtheheatcanvaryastheinputvoltagechanges.Fortheinductor,dependingonthephysicalsize,typeofcoremate-rialandtheDCresistance,itcouldeitheractasaheatsinktakingheatawayfromtheboard,oritcouldaddheattotheboard.
FIGURE19.JunctionTemperatureRise,TO-220
DS012583-35
CircuitDataforTemperatureRiseCurve
TO-263Package(S)
CapacitorsInductorDiodePCboard
Surfacemounttantalum,molded“D”sizeSurfacemount,PulseEngineering,68µHSurfacemount,5A40V,Schottky9squareinchessinglesided2oz.copper(0.0028\")
FIGURE20.JunctionTemperatureRise,TO-263
DS012583-36
FIGURE21.DelayedStartup
DS012583-34
23www.national.com
ApplicationInformation
(Continued)
tional47kΩresistorisneededfromtheON/OFFpintothegroundpintostaywithinthe25VmaximumlimitoftheON/OFFpin.
INVERTINGREGULATOR
ThecircuitinFigure25convertsapositiveinputvoltagetoanegativeoutputvoltagewithacommonground.Thecircuitoperatesbybootstrappingtheregulator’sgroundpintothenegativeoutputvoltage,thengroundingthefeedbackpin,theregulatorsensestheinvertedoutputvoltageandregu-latesit.
DS012583-37
FIGURE22.UndervoltageLockout
forBuckRegulator
DELAYEDSTARTUP
ThecircuitinFigure21usesthetheON/OFFpintoprovideatimedelaybetweenthetimetheinputvoltageisappliedandthetimetheoutputvoltagecomesup(onlythecircuitrypertainingtothedelayedstartupisshown).Astheinputvolt-agerises,thechargingofcapacitorC1pullstheON/OFFpinhigh,keepingtheregulatoroff.Oncetheinputvoltagereachesitsfinalvalueandthecapacitorstopscharging,andresistorR2pullstheON/OFFpinlow,thusallowingthecir-cuittostartswitching.ResistorR1isincludedtolimitthemaximumvoltageappliedtotheON/OFFpin(maximumof25V),reducespowersupplynoisesensitivity,andalsolimitsthecapacitor,C1,dischargecurrent.Whenhighinputripplevoltageexists,avoidlongdelaytime,becausethisripplecanbecoupledintotheON/OFFpinandcauseproblems.Thisdelayedstartupfeatureisusefulinsituationswheretheinputpowersourceislimitedintheamountofcurrentitcandeliver.Itallowstheinputvoltagetorisetoahighervoltagebeforetheregulatorstartsoperating.Buckregulatorsrequirelessinputcurrentathigherinputvoltages.
UNDERVOLTAGELOCKOUT
Someapplicationsrequiretheregulatortoremainoffuntiltheinputvoltagereachesapredeterminedvoltage.Anund-ervoltagelockoutfeatureappliedtoabuckregulatorisshowninFigure22,whileFigure23and24appliesthesamefeaturetoaninvertingcircuit.ThecircuitinFigure23fea-turesaconstantthresholdvoltageforturnonandturnoff(zenervoltageplusapproximatelyonevolt).Ifhysteresisisneeded,thecircuitinFigure24hasaturnONvoltagewhichisdifferentthantheturnOFFvoltage.Theamountofhyster-esisisapproximatelyequaltothevalueoftheoutputvolt-age.Ifzenervoltagesgreaterthan25Vareused,anaddi-
DS012583-38
ThiscircuithasanON/OFFthresholdofapproximately13V.
FIGURE23.UndervoltageLockout
forInvertingRegulator
ThisexampleusestheLM2596-5.0togeneratea−5Vout-put,butotheroutputvoltagesarepossiblebyselectingotheroutputvoltageversions,includingtheadjustableversion.Sincethisregulatortopologycanproduceanoutputvoltagethatiseithergreaterthanorlessthantheinputvoltage,themaximumoutputcurrentgreatlydependsonboththeinputandoutputvoltage.ThecurveshowninFigure26providesaguideastotheamountofoutputloadcurrentpossibleforthedifferentinputandoutputvoltageconditions.
Themaximumvoltageappearingacrosstheregulatoristheabsolutesumoftheinputandoutputvoltage,andthismustbelimitedtoamaximumof40V.Forexample,whenconvert-ing+20Vto−12V,theregulatorwouldsee32Vbetweentheinputpinandgroundpin.TheLM2596hasamaximuminputvoltagespecof40V.
Additionaldiodesarerequiredinthisregulatorconfiguration.DiodeD1isusedtoisolateinputvoltagerippleornoisefromcouplingthroughtheCINcapacitortotheoutput,underlightornoloadconditions.Also,thisdiodeisolationchangesthetopologytoclosleyresembleabuckconfigurationthuspro-vidinggoodclosedloopstability.ASchottkydiodeisrecom-mendedforlowinputvoltages,(becauseofitslowervoltagedrop)butforhigherinputvoltages,afastrecoverydiodecouldbeused.
WithoutdiodeD3,whentheinputvoltageisfirstapplied,thechargingcurrentofCINcanpulltheoutputpositivebysev-eralvoltsforashortperiodoftime.AddingD3preventstheoutputfromgoingpositivebymorethanadiodevoltage.
www.national.com24
ApplicationInformation
(Continued)
DS012583-39
Thiscircuithashysteresis
RegulatorstartsswitchingatVIN=13VRegulatorstopsswitchingatVIN=8V
FIGURE24.UndervoltageLockoutwithHysteresisforInvertingRegulator
DS012583-40
CIN470COUT220—68µF/25VTant.Sprague595DµF/50VElec.PanasonicHFQ
—47µF/20VTant.Sprague595DµF/25VElec.PanasonicHFQ
FIGURE25.Inverting−5VRegulatorwithDelayedStartup
Thistypeofinvertingregulatorcanrequirerelativelylargeamountsofinputcurrentwhenstartingup,evenwithlightloads.InputcurrentsashighastheLM2596currentlimit(ap-prox4.5A)areneededforatleast2msormore,untiltheout-putreachesitsnominaloutputvoltage.Theactualtimede-pendsontheoutputvoltageandthesizeoftheoutputcapacitor.Inputpowersourcesthatarecurrentlimitedorsourcesthatcannotdeliverthesecurrentswithoutgettingloadeddown,maynotworkcorrectly.Becauseoftherela-tivelyhighstartupcurrentsrequiredbytheinvertingtopology,thedelayedstartupfeature(C1,R1andR2)showninFigure25isrecommended.Bydelayingtheregulatorstartup,theinputcapacitorisallowedtochargeuptoahighervoltagebeforetheswitcherbeginsoperating.Aportionofthehighin-putcurrentneededforstartupisnowsuppliedbytheinputcapacitor(CIN).Forseverestartupconditions,theinputca-pacitorcanbemademuchlargerthannormal.INVERTINGREGULATORSHUTDOWNMETHODSTousetheON/OFFpininastandardbuckconfigurationissimple,pullitbelow1.3V(@25˚C,referencedtoground)toturnregulatorON,pullitabove1.3VtoshuttheregulatorOFF.Withtheinvertingconfiguration,somelevelshiftingisrequired,becausethegroundpinoftheregulatorisnolongeratground,butisnowsettingatthenegativeoutputvoltagelevel.TwodifferentshutdownmethodsforinvertingregulatorsareshowninFigure27and28.DS012583-41
FIGURE26.InvertingRegulatorTypicalLoadCurrentBecauseofdifferencesintheoperationoftheinvertingregu-lator,thestandarddesignprocedureisnotusedtoselecttheinductorvalue.Inthemajorityofdesigns,a33µH,3.5Ain-ductoristhebestchoice.Capacitorselectioncanalsobenarroweddowntojustafewvalues.UsingthevaluesshowninFigure25willprovidegoodresultsinthemajorityofinvert-ingdesigns.
25www.national.com
ApplicationInformation
(Continued)
DS012583-42
FIGURE27.InvertingRegulatorGroundReferencedShutdown
DS012583-43
FIGURE28.InvertingRegulatorGroundReferencedShutdownusingOptoDevice
TYPICALTHROUGHHOLEPCBOARDLAYOUT,FIXEDOUTPUT(1XSIZE),DOUBLESIDED
DS012583-44
CIN—470µF,50V,AluminumElectrolyticPanasonic,“HFQSeries”COUT—330µF,35V,AluminumElectrolyticPanasonic,“HFQSeries”D1—5A,40VSchottkyRectifier,1N5825L1—47µH,L39,Renco,ThroughHoleThermalloyHeatSink#7020
www.national.com26
ApplicationInformation
(Continued)
TYPICALTHROUGHHOLEPCBOARDLAYOUT,ADJUSTABLEOUTPUT(1XSIZE),DOUBLESIDED
DS012583-45
CIN—470µF,50V,AluminumElectrolyticPanasonic,“HFQSeries”COUT—220µF,35VAluminumElectrolyticPanasonic,“HFQSeries”D1—5A,40VSchottkyRectifier,1N5825L1—47µH,L39,Renco,ThroughHoleR1—1kΩ,1%
R2—UseformulainDesignProcedureCFF—SeeFigure3.
ThermalloyHeatSink#7020
FIGURE29.PCBoardLayout
27www.national.com
PhysicalDimensions
inches(millimeters)unlessotherwisenoted
5-LeadTO-220(T)
OrderNumberLM2596T-3.3,LM2596T-5.0,
LM2596T-12orLM2596T-ADJNSPackageNumberT05D
www.national.com28
LM2596SIMPLESWITCHERPowerConverter150kHz3AStep-DownVoltageRegulatorPhysicalDimensions
inches(millimeters)unlessotherwisenoted(Continued)
5-LeadTO-263SurfaceMountPackage(S)OrderNumberLM2596S-3.3,LM2596S-5.0,
LM2596S-12orLM2596S-ADJNSPackageNumberTS5B
LIFESUPPORTPOLICY
NATIONAL’SPRODUCTSARENOTAUTHORIZEDFORUSEASCRITICALCOMPONENTSINLIFESUPPORTDEVICESORSYSTEMSWITHOUTTHEEXPRESSWRITTENAPPROVALOFTHEPRESIDENTANDGENERALCOUNSELOFNATIONALSEMICONDUCTORCORPORATION.Asusedherein:1.Lifesupportdevicesorsystemsaredevicesorsystemswhich,(a)areintendedforsurgicalimplantintothebody,or(b)supportorsustainlife,andwhosefailuretoperformwhenproperlyusedinaccordancewithinstructionsforuseprovidedinthelabeling,canbereasonablyexpectedtoresultinasignificantinjurytotheuser.
NationalSemiconductorCorporationAmericas
Tel:1-800-272-9959Fax:1-800-737-7018Email:support@nsc.com
www.national.com
NationalSemiconductorEurope
Fax:+49(0)180-5308586Email:europe.support@nsc.com
DeutschTel:+49(0)180-5308585EnglishTel:+49(0)180-5327832FrançaisTel:+49(0)180-5329358ItalianoTel:+49(0)180-5341680
2.Acriticalcomponentisanycomponentofalifesupportdeviceorsystemwhosefailuretoperformcanbereasonablyexpectedtocausethefailureofthelifesupportdeviceorsystem,ortoaffectitssafetyoreffectiveness.
NationalSemiconductorAsiaPacificCustomerResponseGroupTel:65-2544466Fax:65-2504466
Email:sea.support@nsc.com
NationalSemiconductorJapanLtd.
Tel:81-3-5639-7560Fax:81-3-5639-7507
Nationaldoesnotassumeanyresponsibilityforuseofanycircuitrydescribed,nocircuitpatentlicensesareimpliedandNationalreservestherightatanytimewithoutnoticetochangesaidcircuitryandspecifications.
因篇幅问题不能全部显示，请点此查看更多更全内容