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IR3477
15AHighlyIntegratedSupIRBuckTM
FEATURES
• InputVoltageRange:3Vto27V
• OutputVoltageRange:0.5Vto12V
• Continuous15ALoadCapability
• ConstantOn‐TimeControl
• CompensationLoopnotRequired
• ExcellentEfficiencyatVeryLowOutputCurrents
• ProgrammableSwitchingFrequencyandSoftStart
• ThermallyCompensatedOverCurrentProtection
• PowerGoodOutput
• PrecisionVoltageReference(0.5V,+/‐1%)
• EnableInputwithVoltageMonitoringCapability
• Pre‐biasStartUp
• ThermalShutDown
• Under/OverVoltageFaultProtection
• ForcedContinuousConductionModeOption
• Small,LowProfile5mmx6mmQFNPackage
BASICAPPLICATION
Figure1:IR3477BasicApplicationCircuit
DESCRIPTION
TheIR3477SupIRBuckTMisaneasy‐to‐use,fullyintegrated
andhighlyefficientDC/DCvoltageregulator.Theonboard
constantontimehystereticcontrollerandMOSFETsmake
IR3477aspace‐efficientsolutionthatdeliversupto15Aof
preciselycontrolledoutputvoltage.
Programmableswitchingfrequency,softstart,and
thermallycompensatedovercurrentprotectionallowsfor
averyflexiblesolutionsuitableformanydifferent
applicationsandanidealchoiceforbatterypowered
applications.
Additionalfeaturesincludepre‐biasstartup,veryprecise
0.5Vreference,under/overvoltageshutdown,thermal
protection,powergoodoutput,andenableinputwith
voltagemonitoringcapability.
APPLICATIONS
• NotebookandDesktopComputers
• ConsumerElectronics–STB,LCD,TV,Printers
• 12Vand24VDistributedPowerSystems
• GeneralPurposePOLDC‐DCConverters
• GameConsolesandGraphicsCards
EFFICIENCY
45%
55%
65%
75%
85%
95%
0.01 0.1 1 10 100
Load Current (A)
Efficiency
12VIN
8VIN
19VIN
Figure2:IR3477Efficiency
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IR3477
15AHighlyIntegratedSupIRBuckTM
ORDERINGINFORMATION
IR3477―
MARKINGINFORMATION
Package Tape&ReelQtyPartNumber
M750IR3477MTR1PBF
M4000IR3477MTRPBF
PINDIAGRAM
WC
WC
o
PCBJ
o
JA
/2
/30
-=
=
θ
θ
PBF–LeadFree
TR–TapeandReel
M–PackageType
3477
?YWW?
xxxxx
Site/Date/MarkingCode
LotCode
Pin1Identifier
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IR3477
15AHighlyIntegratedSupIRBuckTM
FUNCTIONALBLOCKDIAGRAM
Figure3:IR3477FunctionalBlockDiagram
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IR3477
15AHighlyIntegratedSupIRBuckTM
TYPICALAPPLICATION
VOUT TP7
TP10
EN
VCC
TP23
+Vsws
TP24
+Vsws
+Vins
R12
open
C26
open
TP8
VOUTS
1
3
4
52
TP21
-Vsws
TP25
-Vin1s
C27
open
TP12
VSWS
1
3
4
52
TP22
+Vsws
VIN
C7
open C8
open C9
330uF C10
47uF
-Vout1s
C11
open
-Vdd2s
-Vdd1s
C1
1uF
R7
1.33K
R8
1.21K
C12
0.1uF
PGOOD
C24
open
ISET
-Vout1s
+Vdd2s
Vout
+Vdd1s
+3.3V
+Vin1s
TP6
PGNDS
TP14
+3.3V
U1
IR3477
3VCBP
8
FCCM
1
SS
6
PGOOD
3
FF 15
GND1
4
FB
5
GND 17
NC1
7
ISET
2
BOOT 14
VIN 13
VCC
10 NC2
9
PGND
11
PHASE 12
EN 16
C4
1uF
VCC
TP4
EN
SW1
EN / FC C M
1
24
3
TP17
PGND
C20
0.1uF
TP26
AGND
C5
open
VSW
C21
1uF
TP11
PGOOD
L1
1.0uH
R6
open
C22
open
TP1
VINS
R4
7.50K
R3
200K
C13
open
C2
22uF
C16
open
+
C3
68uF
TP2
VIN
TP5
PGND
C14
open
C17
open C18
open
TP16
VCC
FB
R5
10K
C15
open
C6
open
TP18
VOLTAGE SENSE
+Vins
1
+Vdd1s
2
+Vdd2s
3
+Vout1s
4
+Vout2s
5-Vout2s 10
-Vdd2s 8
-Vout1s 9
-Vins 6
-Vdd1s 7
-Vout1s
TP9
+Vout1s
R1
10K
FCCM +Vin1s TP20
+Vin1s
+3.3V
C25
1uF
+Vdd1s
-Vdd1s
R13
open
+3.3V TP15
-Vout1s
R14
open
TP19
FB
R11
open
TP13
SS
VSW
+Vdd2s
TP3
FCCM
SS
-Vdd2s
R2
10K
IR3477
C19
open
-Vins
PGND
VOUT
C23
open
Figure4:DemoboardSchematicforVOUT=1.05V,FS=300kHz
DEMOBOARDBILLOFMATERIALS
QTYREFERENCEDESIGNATORVALUEDESCRIPTIONMANUFACTURERPARTNUMBER
4C1,C21,C25,C41.00uFcapacitor,X7R,1.00uF,25V,0.1,0603MurataGRM188R71E105KA12D
1C1047uFcapacitor,47uF,6.3V,805TDKC2012X5R0J476M
2C12,C200.100uFcapacitor,X7R,0.100uF,50V,0.1,603TDKC1608X7R1H104K
1C222.0uFcapacitor,X5R,22.0uF,25V,20%,1206TDKC3216X5R1E226M
1C368uFcapacitor,electrolytic,86uF,25V,0.2,SMDPanasonicEEV‐FK1E680P
1C9330uFcapacitor,330uF,2.5V,SMDSanyo2R5TPE330M9
1L11.0uHinductor,ferrite,1.0uH,20A,2.7mOhm,SMTCyntecPIMB103E‐1R0MS‐39
3R1,R2,R510.0Kresistor,thickfilm,10.0K,1/10W,0.01,603KOARK73H1J1002F
1R3200Kresistor,thickfilm,200K,1/10W,0.01,603KOARK73H1JLTD2003F
1R47.50Kresistor,thickfilm,7.50K,1/10W,0.01,603KOARK73H1JLTD7501F
1R71.33Kresistor,thickfilm,1.33K,1/10W,0.01,603KOARK73H1J1331F
1R81.21Kresistor,thickfilm,1.21K,1/10W,0.01,603KOARK73H1JLTD1211F
1SW1SPSTswitch,DIP,SPST,2position,SMTC&KComponentsSD02H0SK
1U1IR34775mmx6mmQFNIRFIR3477MTRPBF
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IR3477
15AHighlyIntegratedSupIRBuckTM
PINDESCRIPTIONS
PIN # PIN NAME I/O LEVEL PIN DESCRIPTION
1FCCM3.3V
ForcedContinuousConductionMode(CCM).Groundthispintoenablediode
emulationmodeordiscontinuousconductionmode(DCM).Pullthispinto3.3V
tooperateinCCMunderallloadconditions.
2ISETConnectingresistortoPHASEpinsetsovercurrenttrippoint.
3PGOOD5VPowergoodopendrainoutput–pullupwitharesistorto3.3V
4,17GNDReferenceBiasreturnandsignalreference.
5FB3.3VInvertinginputtoPWMcomparator,OVP/PGOODsense.
6SS3.3V
Softstart/shutdown.Thispinprovidesuserprogrammablesoft‐startfunction.
ConnectanexternalcapacitorfromthispintoGNDtosetthestartuptimeofthe
outputvoltage.Theconvertercanbeshutdownbypullingthispinbelow0.3V.
7NC‐ ‐
83VCBP3.3VForinternalLDO.Bypasswitha1.0µFcapacitortoGND.
9NC‐ ‐
10VCC5VVCCinput.Gatedrivesupply.Aminimumof1.0µFceramiccapacitorisrequired.
11PGNDReferencePowerreturn.
12PHASEVINPhasenode(orswitchingnode)ofMOSFEThalfbridge.
13VINVINInputvoltageforthesystem.
14BOOTVIN+VCCBootstrappedgatedrivesupply–connectacapacitortoPHASE.
15FFVINInputvoltagefeedforward–setson‐timewitharesistortoVIN.
16EN5VEnablepintoturnonandoffthedevice.Usetwoexternalresistorstosetthe
turnonthreshold(seeElectricalSpecifications)forinputvoltagemonitoring.
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IR3477
15AHighlyIntegratedSupIRBuckTM
ABSOLUTEMAXIMUMRATINGS
Stressesbeyondthoselistedunder“AbsoluteMaximumRatings”maycausepermanentdamagetothedevice.Theseare
stressratingsonlyandfunctionaloperationofthedeviceattheseoranyotherconditionsbeyondthoseindicatedinthe
operationalsectionsofthespecificationsarenotimplied.
VIN,FF ‐0.3Vto30V
VCC,PGOOD,EN‐0.3Vto8V
BOOT ‐0.3Vto38V
PHASE‐0.3Vto30V(DC),‐5V(100ns)
BOOTtoPHASE ‐0.3Vto8V
ISET‐0.3Vto30V,30mA
PGNDtoGND ‐0.3Vto+0.3V
Allotherpins‐0.3Vto3.9V
StorageTemperatureRange ‐65°Cto150°C
JunctionTemperatureRange‐40°Cto150°C
ESDClassificationJEDECClass1C
MoistureSensitivityLevelJEDECLevel2@260°C(Note2)
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IR3477
15AHighlyIntegratedSupIRBuckTM
ELECTRICALSPECIFICATIONS
RECOMMENDEDOPERATINGCONDITIONSFORRELIABLEOPERATIONWITHMARGIN
SYMBOLMINMAXUNITS
RecommendedVINRangeVIN327*
V
RecommendedVCCRangeVCC4.55.5
RecommendedOutputVoltageRangeVOUT0.512
RecommendedOutputCurrentRangeIOUT015A
RecommendedSwitchingFrequencyFSN/A750kHz
RecommendedOperatingJunctionTemperatureTJ‐40125°C
*PHASEpinmustnotexceed30V.
ELECTRICALCHARACTERISTICS
Unlessotherwisespecified,thesespecificationsapplyoverVIN=12V,4.5V<VCC<5.5V,0°C≤TJ≤125°C.
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNIT
ControlLoop
ReferenceAccuracyVREFVFB=0.5V0.4950.50.505V
On‐TimeAccuracyRFF=180K,TJ=65°C280300320ns
Min.OffTime 500580 ns
Soft‐StartCurrentEN=High81012µA
DCMComparatorOffsetMeasureatVPHASE‐4.5‐2.50mV
FeedbackInputCurrentVFB=0.5V,TA=25°C,Note1 0.010.2µA
SupplyCurrent
VCCSupplyCurrent(standby) EN=Low,NoSwitching23µA
VCCSupplyCurrent(dynamic) EN=High,FS=300kHz9.7mA
FFShutdownCurrentEN=Low,RFF=180K2µA
ForcedContinuousConductionMode(FCCM)
FCCMStartThreshold 2V
FCCMStopThreshold 0.6V
GateDrive
DeadtimeMonitorbodydiode
conductiononPHASEpin,
Note1
530ns
BootstrapPFET
ForwardVoltageI(BOOT)=10mA300mV
UpperMOSFET
StaticDrain‐to‐SourceOn‐ResistanceVCC=5V,ID=12A,TJ=25°C 14.520mΩ
LowerMOSFET
StaticDrain‐to‐SourceOn‐ResistanceVCC=5V,ID=12A,TJ=25°C67.5mΩ
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IR3477
15AHighlyIntegratedSupIRBuckTM
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNIT
FaultProtection
ISETPinOutputCurrentOnthebasisof25°C171921µA
ISETPinOutputCurrent
TemperatureCoefficient
Onthebasisof25°C,Note14400ppm/
°C
UnderVoltageThresholdFallingVFB&Monitor
PGOOD
0.370.40.43V
UnderVoltageHysteresisRisingVFB,Note17.5mV
OverVoltageThresholdRisingVFB&MonitorPGOOD0.5860.6250.655V
OverVoltageHysteresisFallingVFB,Note17.5mV
VCCTurn‐onThreshold‐40°Cto125°C3.94.24.5 V
VCCTurn‐offThreshold 3.63.94.2V
VCCThresholdHysteresis 300mV
ENRisingThreshold‐40°Cto125°C1.11.251.45V
ENHysteresis 400mV
ENInputCurrentEN=3.3V15µA
PGOODPullDownResistance 2550Ω
PGOODDelayThresholdVSS 1V
ThermalShutdownThreshold Note1125140 °C
ThermalShutdownThreshold
Hysteresis
Note120°C
Note:
1. Guaranteedbydesignbutnottestedinproduction
2. Upgradetoindustrial/MSL2levelappliesfromdatecodes1227(markingexplainedonapplicationnoteAN1132page2).
Productswithpriordatecodeof1227arequalifiedwithMSL3forConsumerMarket.
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IR3477
15AHighlyIntegratedSupIRBuckTM
TYPICALOPERATINGDATA
TestedwithdemoboardshowninFigure4,VIN=12V,VCC=5V,VOUT=1.05V,Fs=300kHz,TA=25oC,noairflow,
unlessotherwisespecified.
45%
55%
65%
75%
85%
95%
0.01 0.1 1 10 100
Load Current (A)
Efficiency
12VIN
8VIN
19VIN
Figure5:Efficiencyvs.LoadCurrentforVOUT=1.05V
0
50
100
150
200
250
300
350
400
03691215
Load Current (A)
Switching Frequency (kHz)
Figure7:SwitchingFrequencyvs.LoadCurrent
1.038
1.040
1.042
1.044
1.046
1.048
1.050
0 3 6 9 12 15
Load Current (A)
Output Voltage (V)
12VIN
8VIN
19VIN
Figure9:LoadRegulation
40%
50%
60%
70%
80%
90%
100%
0.01 0.1 1 10 100
Load Current (A)
Efficiency
V
OUT = 1.05V; L = 1µH, 3.0mΩ
V
OUT = 1.5V; L = 1.5µH, 3.4mΩ
V
OUT = 3.3V; L = 2.2µH, 4.6mΩ
Figure6:Efficiencyvs.LoadCurrentforVIN=12V
0
200
400
600
800
1000
1200
1400
200 250 300 350 400 450 500 550 600 650 700 750
Switching Frequency (kHz)
RFF (kOhm)
5.0 Vout 4.5
4.0 3.5
3.0 2.5
2.0 1.5
1.0 0.5
Figure8:RFFvs.SwitchingFrequency
1.038
1.040
1.042
1.044
1.046
1.048
1.050
8 9 10 11 12 13 14 15 16 17 18 19
Input Voltage (V)
Output Voltage (V)
Figure10:LineRegulationatIOUT=15A
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IR3477
15AHighlyIntegratedSupIRBuckTM
TYPICALOPERATINGDATA
TestedwithdemoboardshowninFigure4,VIN=12V,VCC=5V,VOUT=1.05V,Fs=300kHz,TA=25oC,noairflow,unless
otherwisespecified.
Figure11:Startup
Figure13:DCM(IOUT=0.1A)
Figure15:OverCurrentProtection
(testedbyshortingVOUTtoPGND)
Figure12:Shutdown
Figure14:CCM(IOUT=12A)
Figure16:OverVoltageProtection
(testedbyshortingFBtoVOUT)
EN
PGOOD
SS
VOUT
EN
PGOOD
SS
V
OUT
5V/div 5V/div 1V/div 500mV/div 5ms/div 5V/div 5V/div 1V/div 500mV/div 500µs/div
V
OUT
PHASE
iL
20mV/div 5V/div 5A/div 10µs/div 20mV/div 5V/div 10A/div 2µs/div
5V/div 1V/div 1V/div 10A/div 2ms/div 5V/div 1V/div 500mV/div 2A/div 50µs/div
V
OUT
PHASE
iL
PGOOD
SS
VOUT
iL
PGOOD
FB
V
OUT
iL
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IR3477
15AHighlyIntegratedSupIRBuckTM
TYPICALOPERATINGDATA
TestedwithdemoboardshowninFigure4,VIN=12V,VCC=5V,VOUT=1.05V,Fs=300kHz,TA=25oC,noairflow,unless
otherwisespecified.
Figure17:LoadTransient0‐8A
Figure19:DCM/FCCMTransition
Figure21:ThermalImageatVIN=12V,IOUT=15A
(IR3477:98oC,Inductor:69oC,PCB:49oC)
Figure18:LoadTransient4‐12A
Figure20:FCCM/DCMTransition
Figure22:ThermalImageatVIN=19V,IOUT=15A
(IR3477:111oC,Inductor:75oC,PCB:55oC)
V
OUT
PHASE
iL
50mV/div 10V/div 5A/div 20µs/div
V
OUT
PHASE
iL
FCCM
PHASE
VOUT
iL
FCCM
PHASE
V
OUT
iL
50mV/div 10V/div 5A/div 20µs/div
5V/div 10V/div 500mV/div 5A/div 10µs/div 2V/div 10V/div 500mV/div 5A/div 5µs/div
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IR3477
15AHighlyIntegratedSupIRBuckTM
THEORYOFOPERATION
PWMCOMPARATOR
ThePWMcomparatorinitiatesaSETsignal(PWMpulse)
whentheFBpinfallsbelowthereference(VREF)orthe
softstart(SS)voltage.
ON‐TIMEGENERATOR
ThePWMon‐timedurationisprogrammedwithan
externalresistor(RFF)fromtheinputsupply(VIN)totheFF
pin.ThesimplifiedequationforRFFisshowninequation1.
TheFFpinisheldtoaninternalreferenceafterENgoes
HIGH.AcopyofthecurrentinRFFchargesatiming
capacitor,whichsetstheon‐timeduration,asshownin
equation2.
CONTROLLOGIC
Thecontrollogicmonitorsinputpowersources,sequences
theconverterthroughthesoft‐startandprotectivemodes,
andinitiatesaninternalRUNsignalwhenallconditionsare
met.
VCCand3VCBPpinsarecontinuouslymonitored,andthe
IR3477willbedisabledifthevoltageofeitherpindrops
belowthefallingthresholds.EN_DELAYwillbecomeHIGH
whenVCCand3VCBPareinthenormaloperatingrange
andtheENpin=HIGH.
SOFTSTART
WithEN=HIGH,aninternal10µAcurrentsourcecharges
theexternalcapacitor(CSS)ontheSSpintosettheoutput
voltageslewrateduringthesoftstartinterval.Thesoft
starttime(tSS)canbecalculatedfromequation3.
ThefeedbackvoltagetrackstheSSpinuntilSSreachesthe
0.5Vreferencevoltage(Vref),thenfeedbackisregulated
toVref.CSSwillcontinuetobecharged,andwhenSSpin
reachesVSS(seeElectricalSpecification),SS_DELAYgoes
HIGH.WithEN_DELAY=LOW,thecapacitorvoltageandSS
pinisheldtotheFBpinvoltage.Anormalstartup
sequenceisshowninFigure23.
PGOOD
ThePGOODpinisopendrainanditneedstobeexternally
pulledhigh.Highstateindicatesthatoutputisin
regulation.ThePGOODlogicmonitorsEN_DELAY,
SS_DELAY,andunder/overvoltagefaultsignals.PGOODis
releasedonlywhenEN_DELAYandSS_DELAY=HIGHand
outputvoltageiswithintheOVandUVthresholds.
PRE‐BIASSTARTUP
IR3477isabletostartupintopre‐chargedoutput,which
preventsoscillationanddisturbancesoftheoutput
voltage.
Withconstanton‐timecontrol,theoutputvoltageis
comparedwiththesoftstartvoltage(SS)orVref,
dependingonwhichoneislower,andwillnotstart
switchingunlesstheoutputvoltagedropsbelowthe
reference.Thisschemepreventsdischargeofapre‐biased
outputvoltage.
SHUTDOWN
TheIR3477willshutdownifVCCisbelowitsUVLOlimit.
TheIR3477canbeshutdownbypullingtheENpinbelow
itslowerthreshold.Alternatively,theoutputcanbe
shutdownbypullingthesoftstartpinbelow0.3V.
Figure23:NormalStartup
(1)
F201
V
R
SW
OUT
FF ⋅⋅
=pFV
(2)
V
201R
T
IN
FF
ON pFV ⋅⋅
=
(3)
A10
5.0
μ
VC
tSS
SS
⋅
=
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IR3477
15AHighlyIntegratedSupIRBuckTM
UNDER/OVERVOLTAGEMONITOR
TheIR3477monitorsthevoltageattheFBnodethrougha
350nsfilter.IftheFBvoltageisbelowtheundervoltage
threshold,UV#issettoLOWholdingPGOODtobeLOW.If
theFBvoltageisabovetheovervoltagethreshold,OV#is
settoLOW,theshutdownsignal(SD)issettoHIGH,
MOSFETgatesareturnedoff,andPGOODsignalispulled
low.TogglingVCCorENwillallowthenextstartup.Figure
24and25showPGOODstatuschangewhenUV/OVis
detected.Theovervoltageandundervoltagethresholds
canbefoundintheElectricalSpecificationsection.
*typicalfilterdelay
Figure24:Under/OverVoltageMonitor
*typicalfilterdelay
Figure25:OverVoltageProtection
OVERCURRENTMONITOR
Theover‐currentcircuitrymonitorstheoutputcurrent
duringeachswitchingcycle.Thevoltageacrossthelower
MOSFET,VPHASE,ismonitoredforovercurrentandzero
crossing.TheOCPcircuitevaluatesVPHASEforanover
currentconditiontypically270nsafterthelowerMOSFET
isgatedon.Thisdelayfunctionstofilteroutswitching
noise.Theminimumlowergateintervalallowstimeto
sampleVPHASE.
Theovercurrenttrippointisprogrammedwitharesistor
fromtheISETpintoPHASEpin,asshowninequation4.
Whenovercurrentisdetected,theMOSFETgatesaretri‐
stateandSSvoltageispulledto0V.Thisinitiatesanew
softstartcycle.IfthereisatotaloffourOCevents,the
IR3477willdisableswitching.TogglingVCCorENwillallow
thenextstartup.
Figure26:OverCurrentProtection
UNDERVOLTAGELOCK‐OUT
TheIR3477hasVCCandENundervoltagelock‐out(UVLO)
protection.WheneitherVCCorENisbelowtheirUVLO
threshold,IR3477isdisabled.IR3477willrestartwhen
bothVCCandENareabovetheirUVLOthresholds.
OVERTEMPERATUREPROTECTION
WhentheIR3477exceedsitsovertemperaturethreshold,
theMOSFETgatesaretri‐stateandPGOODispulledlow.
Switchingresumesoncetemperaturedropsbelowthe
overtemperaturehysteresislevel.
(4)
19
IR
ROC DSON
ISET A
μ
⋅
=
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IR3477
15AHighlyIntegratedSupIRBuckTM
GATEDRIVELOGIC
Thegatedrivelogicfeaturesadaptivedeadtime,
diodeemulation,andaminimumlowergateinterval.
Anadaptivedeadtimepreventsthesimultaneous
conductionoftheupperandlowerMOSFETs.Thelower
gatevoltagemustbebelowapproximately1VafterPWM
goesHIGHbeforetheupperMOSFETcanbegatedon.
Also,thedifferentialvoltagebetweentheuppergateand
PHASEmustbebelowapproximately1VafterPWMgoes
LOWbeforethelowerMOSFETcanbegatedon.
TheupperMOSFETisgatedonaftertheadaptivedelay
forPWM=HIGHandthelowerMOSFETisgatedonafter
theadaptivedelayforPWM=LOW.WhenFCCM=LOW,
thelowerMOSFETisdriven‘off’whentheZCROSSsignal
indicatesthattheinductorcurrentisabouttoreverse
direction.TheZCROSScomparatormonitorsthePHASE
voltagetodeterminewhentoturnoffthelowerMOSFET.
ThelowerMOSFETstays‘off’untilthenextPWMfalling
edge.Whenthelowerpeakoftheinductorcurrentis
abovezero,IR3477operatesincontinuousconduction
mode.Thecontinuousconductionmodecanalsobe
selectedforallloadcurrentlevelsbypullingFCCMto
HIGH.
WhenevertheupperMOSFETisturned‘off’,itstays
‘off’fortheMinOffTimedenotedintheElectrical
Specifications.Thisminimumdurationallowstimeto
rechargethebootstrapcapacitorandallowstheover
currentmonitortosamplethePHASEvoltage.
COMPONENTSELECTION
Selectionofcomponentsfortheconverterisaniterative
processwhichinvolvesmeetingthespecificationsand
tradeoffsbetweenperformanceandcost.Thefollowing
sectionswillguideonethroughtheprocess.
InductorSelection
Inductorselectioninvolvesmeetingthesteadystate
outputripplerequirement,minimizingtheswitchingloss
oftheupperMOSFET,meetingtransientresponse
specificationsandminimizingtheoutputcapacitance.
TheoutputvoltageincludesaDCvoltageandasmallAC
ripplecomponentduetothelowpassfilterwhichhas
incompleteattenuationoftheswitchingharmonics.
Neglectingtheinductanceinserieswiththeoutput
capacitor,themagnitudeoftheACvoltagerippleis
determinedbythetotalinductorripplecurrentflowing
throughthetotalequivalentseriesresistance(ESR)ofthe
outputcapacitorbank.
Onecanuseequation5tofindtherequiredinductance.
ΔIisdefinedasshowninFigure27.Themainadvantage
ofsmallinductanceisincreasedinductorcurrentslewrate
duringaloadtransient,whichleadstoasmalleroutput
capacitancerequirementasdiscussedintheOutput
CapacitorSelectionsection.Thedrawbackofusingsmaller
inductancesisincreasedswitchingpowerlossintheupper
MOSFET,whichreducesthesystemefficiencyand
increasesthethermaldissipation.
Figure27:TypicalInputCurrentWaveform
InputCapacitorSelection
Themainfunctionoftheinputcapacitorbankistoprovide
theinputripplecurrentandfastslewratecurrentduring
theloadcurrentstepup.Theinputcapacitorbankmust
haveadequateripplecurrentcarryingcapabilitytohandle
thetotalRMScurrent.Figure27showsatypicalinput
current.Equation6showstheRMSinputcurrent.
TheRMSinputcurrentcontainstheDCloadcurrentand
theinductorripplecurrent.Asshowninequation5,the
inductorripplecurrentisunrelatedtotheloadcurrent.
ThemaximumRMSinputcurrentoccursatthemaximum
outputcurrent.Themaximumpowerdissipationinthe
inputcapacitorequalsthesquareofthemaximumRMS
inputcurrenttimestheinputcapacitor’stotalESR.
Thevoltageratingoftheinputcapacitorneedstobe
greaterthanthemaximuminputvoltagebecauseofhigh
frequencyringingatthephasenode.Thetypical
percentageis25%.
()
(6)
I
ΔI
3
1
1FsTI
dttf
Ts
1
I
2
OUT
ON
OUT
Ts
0
2
IN_RMS
⎟
⎠
⎞
⎜
⎝
⎛
⋅+⋅⋅⋅=
⋅⋅= ∫
(
)
(5)
L2
VVT
ΔIOUTINON
⋅
⋅
=−
March27,2013|V2.5|PD97604
15
IR3477
15AHighlyIntegratedSupIRBuckTM
OutputCapacitorSelection
Selectionoftheoutputcapacitorrequiresmeeting
voltageovershootrequirementsduringloadremoval,and
meetingsteadystateoutputripplevoltagerequirements.
Theoutputcapacitoristhemostexpensiveconverter
componentandincreasestheoverallsystemcost.
Theoutputcapacitordecouplingintheconvertertypically
includesthelowfrequencycapacitor,suchasSpecialty
PolymerAluminum,andmidfrequencyceramiccapacitors.
Thefirstpurposeofoutputcapacitorsistoprovidecurrent
whentheloaddemandexceedstheinductorcurrent,
asshowninFigure28.Equation7showsthecharge
requirementforacertainloadstep.Theadvantage
providedbytheIR3477ataloadstepisthereduceddelay
comparedtoafixedfrequencycontrolmethod.Ifthe
loadincreasesrightafterthePWMsignalgoeslow,the
longestdelaywillbeequaltotheminimumlowergate
on‐timeasshownintheElectricalSpecificationssection.
TheIR3477alsoreducestheinductorcurrentslewtime,
thetimeittakesfortheinductorcurrenttoreachequality
withtheoutputcurrent,byincreasingtheswitching
frequencyupto1/(TON+MinOffTime).Thisresultsin
reducedrecoverytime.
Figure28:ChargeRequirementduringLoadStep
Theoutputvoltagedrop,VDROP,initiallydependsonthe
characteristicoftheoutputcapacitor.VDROPisthesumof
theequivalentseriesinductance(ESL)oftheoutput
capacitortimestherateofchangeoftheoutputcurrent
andtheESRtimesthechangeoftheoutputcurrent.
VESRisusuallymuchgreaterthanVESL.TheIR3477
requiresatotalESRsuchthattheripplevoltageatthe
FBpinisgreaterthan7mV.
Thesecondpurposeoftheoutputcapacitoristominimize
theovershootoftheoutputvoltagewhentheload
decreasesasshowninFigure29.Byusingthelawof
energybeforeandaftertheloadremoval,equation8
showstheoutputcapacitancerequirementforaload
stepdown.
Figure29:TypicalOutputVoltageResponseWaveform
BootCapacitorSelection
Thebootcapacitorstartsthecyclefullychargedtoa
voltageofVB(0).Cgequals0.58nFinIR3477.Choosea
sufficientlysmallΔVsuchthatVB(0)‐ΔVexceedsthe
maximumgatethresholdvoltagetoturnontheupper
MOSFET.
Chooseabootcapacitorvaluelargerthanthecalculated
CBOOTinequation9.Equation9isbasedonchargebalance
atCCMoperation.Usuallythebootcapacitorwillbe
dischargedtoamuchlowervoltagewhenthecircuitis
operatinginDCMmodeatlightload,duetomuchlonger
lowerMOSFETofftimeandthebiascurrentdrawnbythe
IC.Bootcapacitanceneedstobeincreasedifinsufficient
turn‐onoftheupperMOSFETisobservedatlightload,
typicallylargerthan0.1µFisneeded.Thevoltageratingof
thispartneedstobelargerthanVB(0)plusthedesired
deratingvoltage.It’sESRandESLneedstobelowinorder
toallowittodeliverthelargecurrentanddi/dt’swhich
driveMOSFETsmostefficiently.Insupportofthese
requirementsaceramiccapacitorshouldbechosen.
()
(7b)
VV
IL
2
1
V
1
C
(7a)t I0.5VC Q
OUTIN
2
STEP
DROP
OUT
STEP
⎥
⎦
⎤
⎢
⎣
⎡⋅
⋅=
Δ
⋅
⋅=⋅=
−
IOUT ISTEP
VOUT VL
VESR
VOS
VDROP
(8)
VV
IL
C 2
OUT
2
OS
2
STEP
OUT −
⋅
=
(9) 1
ΔV
(0)V
CC B
gBOOT ⎟
⎠
⎞
⎜
⎝
⎛−⋅=
t
Load
Current ISTEP
Inductor
Slew
Rate
Output
Charge
Δt
March27,2013|V2.5|PD97604
16
IR3477
15AHighlyIntegratedSupIRBuckTM
DESIGNEXAMPLE
DESIGNCRITERIA
• InputVoltage,VIN=6Vto21V
• OutputVoltage,VOUT=1.25V
• SwitchingFrequency,Fs=400kHz
• InductorRippleCurrent,2ΔI=3A
• MaximumOutputCurrent,IOUT=12A
• OverCurrentTrip,IOC=18A
• CurrentTransientStepSize=5A
• OvershootAllowance,VOS=VOUT+50mV
• UndershootAllowance,VDROP=50mV
FindRFF:
Pickastandardvalue158kΩ,1%resistor.
FindRISET:
Picka7.15kΩ,1%standardresistor.
FindaresistivevoltagedividerforVOUT=1.25V:
R2=1.33kΩ,R1=1.96kΩ,both1%standardresistors.
Choosethesoftstartcapacitor:
Oncethesoftstarttimehaschosen,suchas1000usto
reachtothereferencevoltage,a22nFforCSSisusedto
meet1000us.
Chooseaninductortomeetthedesignspecification:
ChoosetheinductorwiththelowestDCRandACpower
lossaspossibletoincreasetheoverallsystemefficiency.
Forinstance,chooseaPIMB103E‐1R0MS‐39manufactured
byCYNTEC.Theinductanceofthispartis1µHandhas
2.7mΩDCR.Ripplecurrentneedstoberecalculatedusing
thechoseninductor.
Chooseaninputcapacitor:
APanasonic10µF(ECJ3YB1E106M)accommodates6Arms
ofripplecurrentat300kHz.Duetothechemistryof
multilayerceramiccapacitors,thecapacitancevariesover
temperatureandoperatingvoltage,bothACandDC.One
10µFcapacitorisrecommended.Inapracticalsolution,
one1µFcapacitorisrequiredalongwith10µF.The
purposeofthe1µFcapacitoristosuppresstheswitching
noiseanddeliverhighfrequencycurrent.
Chooseanoutputcapacitor:
Tomeettheundershootandovershootspecification,
equations7band8willbeusedtocalculatetheminimum
outputcapacitance.Asaresult,200μFwillbeneededfor
5Aloadremoval.Tomeetthestabilityrequirement,
chooseanoutputcapacitorwithESRlargerthan6mΩ.
Combinethosetworequirements,onecanchooseasetof
outputcapacitorsfrommanufacturessuchasSP‐Cap
(SpecialtyPolymerCapacitor)fromPanasonicorPOSCAP
fromSanyo.A220μF(EEFSL0D221R)fromPanasonicwith
9mΩESRwillmeetbothrequirements.
Ifanallceramicoutputcapacitorsolutionisdesired,the
externalslopeinjectioncircuitcomposedofR6,C13,and
C14isrequiredasexplainedintheStabilityConsiderations
section.Inthisdesignexample,wecanchooseC14=1nF
andC13=100nF.TocalculatethevalueofR6with
PIMB103E‐1R0MS‐39asourinductor:
PickastandardvalueforR6=3.74kΩ.
k 156
400k201
1.25
RFF Ω=
⋅⋅
=HzpFV V
Ω=
Ω
=⋅7.1k
91
18 7.5m
R
ISET AA
μ
V0.5 V
R
R
R
VOUT
12
2
FB =⋅
+
=
()
()
HHzAV VVV
μ
1.0
400k321
1.25-211.25
FΔI2V
VVV
L
sIN
OUTINOUT
=
⋅⋅
⋅
=
⋅
⋅
=
⋅
−
(
)
A
HzHV VVV
I3
400k121
1.25-211.25
2Δ=
⋅⋅
⋅
=
μ
A
A
A
V
V
A9.2
21
5.1
3
1
1
21
1.25
21I
2
IN_RMS =
⎟
⎠
⎞
⎜
⎝
⎛
⋅+⋅⋅=
Ω
=
⋅Ω
=
⋅
=
k
nFm H
CDCR
L
R
7.3
1007.2
1
13
6
μ
March27,2013|V2.5|PD97604
17
IR3477
15AHighlyIntegratedSupIRBuckTM
STABILITYCONSIDERATIONS
Constant‐on‐timecontrolisafast,ripplebasedcontrol
scheme.Unstableoperationcanoccurifcertainconditions
arenotmet.Thesysteminstabilityisusuallycausedby:
SwitchingnoisecoupledtoFBinput:
ThiscausesthePWMcomparatortotriggerprematurely
afterthe500nsminimumon‐timeforlowerMOSFET.
Itwillresultindoubleormultiplepulseseveryswitching
cycleinsteadoftheexpectedsinglepulse.Doublepulsing
cancauseshigheroutputvoltageripple,butinmost
applicationitwillnotaffectoperation.Thiscanusuallybe
preventedbycarefullayoutofthegroundplaneandthe
FBsensingtrace.
SteadystaterippleonFBpinbeingtoosmall:
ThePWMcomparatorinIR3477requiresminimum
7mVp‐pripplevoltagetooperatestably.Notenoughripple
willresultinsimilardoublepulsingissuedescribedabove.
Solvingthismayrequireusingoutputcapacitorswith
higherESR.
ESRloopinstability:
Thestabilitycriteriaofconstanton‐timeis:
IfESRistoosmallthatthiscriteriaisviolatedthensub‐
harmonicoscillationwilloccur.Thisissimilartothe
instabilityproblemofpeak‐current‐modecontrolwith
D>0.5.IncreasingESRisthemosteffectivewaytostabilize
thesystem,butthetradeoffisthelargeroutputvoltage
ripple.
Systemwithallceramicoutputcapacitors:
Forapplicationswithallceramicoutputcapacitors,theESR
isusuallytoosmalltomeetthestabilitycriteria.Inthese
applications,externalslopecompensationisnecessaryto
maketheloopstable.Therampinjectioncircuit,composed
ofR6,C13,andC14,showninFigure4isrequired.
TheinductorcurrentripplesensedbyR6andC13isAC
coupledtotheFBpinthroughC14.C14isusuallychosen
between1to10nF,andC13between10to100nF.R6
shouldthenbechosensuchthatL/DCR=C13*R6.
LAYOUTRECOMMENDATIONS
BypassCapacitor:
A1µFhighqualityceramiccapacitorshouldbeplacedon
thesamesideastheIR3477andconnectedtoVCCand
PGNDpinsdirectly.
BootCircuit:
CBOOTshouldbeplacedneartheBOOTandPHASEpinsto
reducetheimpedancewhentheupperMOSFETturnson.
PowerStage:
Figure30showsthecurrentpathsandtheirdirections
fortheonandoffperiods.Theontimepathhaslow
averageDCcurrentandhighACcurrent.Therefore,itis
recommendedtoplacetheinputceramiccapacitor,upper,
andlowerMOSFETinatightloopasshowninFigure30.
Thepurposeofthetightloopfromtheinputceramic
capacitoristosuppressthehighfrequency(10MHzrange)
switchingnoiseandreduceElectromagneticInterference
(EMI).Ifthispathhashighinductance,thecircuitwill
causevoltagespikesandringing,andincreasethe
switchingloss.TheofftimepathhaslowACandhigh
averageDCcurrent.Therefore,itshouldbelaidoutwith
atightloopandwidetraceatbothendsoftheinductor.
Loweringtheloopresistancereducesthepowerloss.The
typicalresistancevalueof1‐ouncecopperthicknessis
0.5mΩpersquareinch.
Figure30:CurrentPathofPowerStage
2
ONOUT TCESR >⋅
Q1
Q2
March27,2013|V2.5|PD97604
18
IR3477
15AHighlyIntegratedSupIRBuckTM
PCBMETALANDCOMPONENTPLACEMENT
• Leadlands(the13ICpins)widthshouldbeequal
tonominalpartleadwidth.Theminimumleadto
leadspacingshouldbe≥0.2mmtominimize
shorting.
• Leadlandlengthshouldbeequaltomaximum
partleadlength+0.3mmoutboardextension.
Theoutboardextensionensuresalargetoefillet
thatcanbeeasilyinspected.
• Padlands(the4bigpads)lengthandwidth
shouldbeequaltomaximumpartpadlengthand
width.However,theminimummetaltometal
spacingshouldbenolessthan;0.17mmfor2oz.
Copperornolessthan0.1mmfor1oz.Copperor
nolessthan0.23mmfor3oz.Copper.
Figure31:MetalandComponentPlacement
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March27,2013|V2.5|PD97604
19
IR3477
15AHighlyIntegratedSupIRBuckTM
SOLDERRESIST
• ItisrecommendedthattheleadlandsareNon
SolderMaskDefined(NSMD).Thesolderresist
shouldbepulledawayfromthemetalleadlands
byaminimumof0.025mmtoensureNSMD
pads.
• ThelandpadshouldbeSolderMaskDefined
(SMD),withaminimumoverlapofthesolder
resistontothecopperof0.05mmto
accommodatesolderresistmisalignment.
• Ensurethatthesolderresistinbetweenthelead
landsandthepadlandis≥0.15mmduetothe
highaspectratioofthesolderresiststrip
separatingtheleadlandsfromthepadland.
Figure32:SolderResist
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March27,2013|V2.5|PD97604
20
IR3477
15AHighlyIntegratedSupIRBuckTM
STENCILDESIGN
• TheStencilaperturesfortheleadlandsshouldbe
approximately80%oftheareaoftheleadlads.
Reducingtheamountofsolderdepositedwill
minimizetheoccurrencesofleadshorts.Iftoo
muchsolderisdepositedonthecenterpadthe
partwillfloatandtheleadlandswillopen.
• Themaximumlengthandwidthofthelandpad
stencilapertureshouldbeequaltothesolder
resistopeningminusanannular0.2mmpullback
inordertodecreasetheriskofshortingthe
centerlandtotheleadlandswhenthepartis
pushedintothesolderpaste.
Figure33:StencilDesign
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March27,2013|V2.5|PD97604
21
IR3477
15AHighlyIntegratedSupIRBuckTM
PACKAGEINFORMATION
Figure34:PackageDimensions
Dataandspecificationssubjecttochangewithoutnotice.
ThisproducthasbeendesignedandqualifiedfortheIndustrialMarket(Note2).
QualificationStandardscanbefoundonIR’sWebsite.
IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, Califor nia 90245, USA Tel: (310) 252-7105
TAC Fax: (310) 252-7903
Visit us at www.irf.com for sales contact information
www.irf.com
