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LTC3541-3 High Efficiency Buck + VLDO Regulator
High Efficiency 500mA Buck Plus 300mA VLDO Regulator Auto Start-Up Powers VLDO/Linear Regulator Output Prior to Buck Output Independent 500mA High Efficiency Buck (VIN: 2.7V to 5.5V) 300mA VLDO Regulator with 30mA Standalone Mode No External Schottky Diodes Required FixedBuck Output Voltage: 1.8V VLDO Input Voltage Range (LV : 1.675V to 5.5V) IN FixedVLDO Output Voltage: 1.575V SelectableFixedFrequency,Pulse-SkipOperation orBurstMode(R)Operation Short-CircuitProtected CurrentModeOperationforExcellentLineandLoad TransientResponse ShutdownCurrent:<3A ConstantFrequencyOperation:2.25MHz LowDropoutBuckOperation:100%DutyCycle Small,ThermallyEnhanced,10-Lead(3mmx3mm) DFNPackage
The LTC(R)3541-3 combines a synchronous buck DC/ DCconverterwithaverylowdropoutlinearregulator TM (VLDO regulator)andinternalfeedbackresistornetworks toprovidetwooutputvoltagesfromasingleinputvoltage withminimumexternalcomponents.Whenconfigured fordualoutputoperation,theLTC3541-3'sautostart-up featurewillbringthe1.575VVLDO/linearregulatoroutput intoregulationinacontrolledmannerpriortoenablingthe 1.8Vbuckoutputwithouttheneedforexternalpincontrol. The300mAVLDO/linearregulatoroutputwillsourceonly 30mAuntilthebuckoutputachievesregulation.Theinput voltagerangeisideallysuitedforapplicationspowered fromaLi-Ionbatteryand5Vor3.3Vrails. Thesynchronousbuckconverterprovidesahighefficiency output,typically90%.Itcanprovideupto500mAofoutput currentwhileswitchingat2.25MHz,allowingtheuseof smallsurfacemountinductorsandcapacitors.Amode selectpinallowsBurstModeoperationtobeenabledfor higherefficiencyatlightloadcurrents,ordisabledforlower noise,constantfrequencyoperation. The VLDO regulator provides a low noise, low voltage outputcapableofprovidingupto300mAofoutputcurrent usingonlya2.2Fceramiccapacitor.Theinputsupply voltageoftheVLDOregulator(LVIN)maycomefromthe buckregulatororaseparatesupply.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. VLDO is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5481178, 6611131, 6304066, 6498466, 6580258.
APPLICATIO S

DigitalCameras CellularPhones PCCards WirelessandDSLModems OtherPortablePowerSystems
LTC3541-3 Typical Application
VIN 3V TO 5V SW 2.2H ENVLDO EFFICIENCY (%)
VIN MODE LTC3541-3 ENBUCK GND VOUT LVIN
VOUT1 1.8V 10F
LVOUT PGND
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TYPICAL APPLICATIO
Buck (Burst) Efficiency vs Load Current
100 90 80 70 60 50 40 30 20 10 0 1 10 100 LOAD CURRENT (mA) 0.0001 1000
35413 TA01b
VIN = 3V EFFICIENCY
2.2F
35413 TA01a
VOUT2 1.575V
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1 0.1 POWER LOSS (W) POWER LOSS 0.01 0.001
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FEATURES
DESCRIPTIO
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LTC3541-3
(Note 1)
SupplyVoltages: VIN,LVIN.................................................. -0.3Vto6V LVIN-VIN..........................................................<0.3V PinVoltages: ENVLDO,ENBUCK,MODE,SW......-0.3VtoVIN+0.3V LinearRegulatorIOUT(MAX)(100ms)(Note9)......100mA OperatingAmbientTemperatureRange (Note2)................................................... -40Cto85C . JunctionTemperature(Notes5,10)...................... 125C StorageTemperatureRange.................. -65Cto125C .
TOP VIEW VIN ENBUcK VOUT Nc LVOUT 1 2 3 4 5 11 10 SW 9 ENVLDO 8 MODE 7 GND 6 LVIN
DD PAcKAGE 10-LEAD (3mm x 3mm) PLASTIc DFN TJMAX=125C,JA=43C/W EXPOSEDPAD(PIN11)ISPGND,MUSTBESOLDEREDTOPCB
ORDERPARTNUMBER LTC3541EDD-3
DDPARTMARKING LCHR
Order OptionsTapeandReel:Add#TR LeadFree:Add#PBFLeadFreeTapeandReel:Add#TRPBF LeadFreePartMarking:http://www.linear.com/leadfree/ ConsultLTCMarketingforpartsspecifiedwithwideroperatingtemperatureranges.
ELECTRICAL CHARACTERISTICS
SYMBOL IPK VIN VIN(LINEREG) PARAMETER PeakInductorCurrent InputVoltageRange BuckVINLineRegulation (Note4)
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise specified (Notes 2, 3).
CONDITIONS VIN=4.2V(Note8)

MIN 0.8 2.7
TYP 0.95 0.04 2.2 2.2 0.8 20 0.5 0.25 0.25
MAX 1.25 5.5 0.4
UNITS A V %/V mV/V mV/V mV/V
VIN=2.7Vto5.5V,ENBUCK=VIN, ENVLDO=0V,MODE=VIN(Note6) VIN=3Vto5.5V,LVOUT=1.575V,ENBUCK=VIN, VLDOVINLineRegulation (ReferredtoLVOUT) ENVLDO=VIN,MODE=0V,IOUT(VLDO)=100mA VIN=3Vto5.5V,LVOUT=1.575V,ENBUCK=0V, LinearRegulatorVINLine Regulation(ReferredtoLVOUT) ENVLDO=VIN,IOUT(LDO)=10mA LVIN(LINEREG) LVINLineRegulation LVIN=1.675Vto5.5V,VIN=5.5V,LVOUT=1.575V, (ReferredtoLVOUT) ENBUCK=VIN,ENVLDO=VIN,MODE=VIN, IOUT(VLDO)=100mA VLDODO LVIN-LVOUTDropoutVoltage LVOUT=1.575V,ENBUCK=VIN,ENVLDO=VIN, MODE=VIN,IOUT(VLDO)=50mA(Note9) VLOADREG BuckOutputLoadRegulation ENBUCK=VIN,ENVLDO=0V,MODE=VIN(Note6)
50
VVOUT
VLDOOutputLoadRegulation IOUT(VLDO)=1mA-300mA,LVIN=1.8V,LVOUT=1.575V, ENBUCK=VIN,ENVLDO=VIN,MODE=VIN LinearRegulatorOutputLoad IOUT(LDO)=1mA-30mA,LVOUT=1.575V, ENBUCK=0V,ENVLDO=VIN Regulation ReferenceRegulationVoltage ENBUCK=VIN,ENVLDO=0V,TA=25C (Note6) ENBUCK=VIN,ENVLDO=0V,0CTA85C ENBUCK=VIN,ENVLDO=0V,-40CTA85C
0.5 0.5 1.836 1.840 1.845 1.607 1.610 1.614
1.764 1.760 1.755 1.543 1.540
1.8 1.8 1.8 1.575 1.575 1.575
VLVOUT
ReferenceRegulationVoltage (Note7)
ENBUCK=0V,ENVLDO=VIN,TA=25C ENBUCK=0V,ENVLDO=VIN,0CTA85C ENBUCK=0V,ENVLDO=VIN,-40CTA85C
1.536
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mV % % % V V V V V V
W
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WW
W
ABSOLUTE
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
LTC3541-3 ELECTRICAL CHARACTERISTICS
SYMBOL IS PARAMETER Buck+VLDO BurstModeSleep VINQuiescentCurrent Buck+VLDO BurstModeActive VINQuiescentCurrent Buck+VLDO Pulse-SkipModeActive VINQuiescentCurrent Buck BurstModeSleep VINQuiescentCurrent Buck BurstModeActive VINQuiscentCurrent Buck Pulse-SkipModeActive VINQuiescentCurrent LinearRegulatorVINQuiescent Current VINShutdownQuiescent Current LVINShutdownQuiescent Current OscillatorFrequency RDS(ON)ofP-ChannelMOSFET RDS(ON)ofN-ChannelMOSFET SWLeakage InputPinHighThreshold InputPinLowThreshold InputPinCurrent
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise specified (Notes 2, 3).
CONDITIONS LVIN=1.8V,LVOUT=1.575V,ENBUCK=VIN, ENVLDO=VIN,MODE=0V,IOUT(VLDO)=10A, VVOUT=2.03V LVIN=1.8V,LVOUT=1.575V,ENBUCK=VIN, ENVLDO=VIN,MODE=VIL,IOUT(VLDO)=10A, VVOUT=1.575V LVIN=1.8V,LVOUT=1.575V,ENBUCK=VIN, ENVLDO=VIN,MODE=VIN,IOUT(VLDO)=10A, VVOUT=1.575V VVOUT=2.03V,IOUT(BUCK)=0A,ENBUCK=VIN, ENVLDO=0V,MODE=0V VVOUT=1.575V,IOUT(BUCK)=0A,ENBUCK=VIN, ENVLDO=0V,MODE=0V VVOUT=1.575V,IOUT(BUCK)=0A,ENBUCK=VIN, ENVLDO=0V,MODE=VIN LVOUT=1.575V,ENBUCK=VIL,ENVLDO=VIN, IOUT(VLDO)=10A ENBUCK=0V,ENVLDO=0V LVIN=3.6V,ENBUCK=0V,ENVLDO=0V
MIN
TYP 85 315 300 55 300 285 50 2.5 0.1
MAX
UNITS A A A A A A A A A
fOSC RPFET RNFET ILSW VIH VIL IMODE, IENBUCK, IENVLDO
1.8
2.25 0.25 0.35 0.01
2.7
MHz
ISW=100mA ISW=-100mA Enable=0V,VSW=0Vor6V,VIN=6V MODE,ENBUCK,ENVLDO MODE,ENBUCK,ENVLDO

1 0.3
A V V A
0.9 0.01 1
Note 1:StressesbeyondthoselistedunderAbsoluteMaximumRatings maycausepermanentdamagetothedevice.ExposuretoanyAbsolute MaximumRatingconditionforextendedperiodsmayaffectdevice reliabilityandlifetime. Note 2:TheLTC3541-3isguaranteedtomeetperformancespecifications from0Cto85C.VLDO/linearregulatoroutputistestedandspecified underpulseloadconditionssuchthatTJTA,andare100%production testedat25C.Specificationsoverthe-40Cto85Coperating temperaturerangeareassuredbydesign,characterizationandcorrelation withstatisticalprocesscontrols. Note 3:MinimumoperatingLVINvoltagerequiredforVLDOregulationis: LVINLVOUT+VDROPOUT Note 4:MinimumoperatingVINvoltagerequiredforVLDOandlinear regulatorregulationis: VINLVOUT+1.4V Note 5:TJiscalculatedfromtheambienttemperature,TA,andpower dissipation,PD,accordingtothefollowingformula: TJ=TA+(PD*43C/W)
Note 6:TheLTC3541-3istestedinaproprietarytestmodethatconnects VBUCKFBtotheoutputoftheerroramplifier.Forthereferenceregulation andlineregulationtests,theoutputoftheerroramplifierissettothe midpoint.Fortheloadregulationtest,theoutputoftheerroramplifieris driventominimumandmaximumofthesignalrange. Note 7:Measurementmadeinclosedlooplinearregulatorconfiguration withLVOUT=1.575V,ILOAD=10A. Note 8:Measurementmadeinaproprietarytestmodewithslope compensationdisabled. Note 9:Measurementassuredbydesign,characterizationandstatistical processcontrol. Note 10:ThisICincludesovertemperatureprotectionthatisintended toprotectthedeviceduringmomentaryoverloadconditions.Junction temperaturewillexceed125Cwhenovertemperatureprotectionisactive. Continuousoperationabovethespecifiedmaximumoperatingjunction temperaturemayimpairdevicereliability.
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LTC3541-3 TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage for Buck (Pulse Skip)
95 90 85 EFFICIENCY (%) IOUT = 100mA EFFICIENCY (%) 80 75 70 65 60 55 50 2 3 4 INPUT VOLTAGE (V)
35413 G01
IOUT = 500mA
EFFICIENCY (%)
IOUT = 30mA
5
Efficiency vs Load Current for Buck (Burst)
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 LOAD CURRENT (mA) 1000
35413 G04
VIN = 3.6V
VIN = 2.7V VIN = 4.2V
DROPOUT VOLTAGE (mV)
BIAS CURRENT (A)
Output (Auto-Start-Up Sequence) vs Time
2.50 2.45 VOUT 1V/DIV FREQUENCY (MHz) LVOUT 1V/DIV VIN 2V/DIV 2.40
2.30 2.25 2.20 2.15 2.10 2.05 2.00 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125
FREQUENCY (MHz)
IVOUT = 200mA ILVOUT = 30mA
2ms/DIV
UW
6
35413 G07
Efficiency vs Input Voltage for Buck (Burst)
100 95 90 85 80 75 70 65 60 55 50 2 3 4 5 INPUT VOLTAGE (V) 6
35413 G02
Efficiency vs Load Current for Buck (Pulse Skip)
100 90 VIN = 2.7V VIN = 3.6V VIN = 4.2V
IOUT = 500mA
80 70 60 50 40 30 20 10 0 0.1 1
IOUT = 100mA
IOUT = 30mA
10 100 LOAD CURRENT (mA)
1000
35413 G03
VLDO Dropout Voltage vs Load Current
100 250 200
Buck (Burst) Plus VLDO Bias Current vs VLDO Load Current
VIN = 3.6V ILOAD_BUCK = 0 IBIAS = IVIN + ILVIN - ILOAD
80
VIN = 3V VIN = 3.6V
60 VIN = 4.2V 40
150
100
20 0
50
0
50
100 150 200 LOAD CURRENT (mA)
250
300
0 0.1
1
10 100 LOAD CURRENT (mA)
1000
35413 G06
35413 G05
Oscillator Frequency vs Temperature
VIN = 3.6V 2.5
Oscillator Frequency vs Supply Voltage
VIN = 3.6V
2.4
2.35
2.3
2.2
2.1
2.0
3
5 4 SUPPLY VOLTAGE (V)
6
35413 G09
35413 G08
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LTC3541-3 TYPICAL PERFOR A CE CHARACTERISTICS
VLDO/Linear Regulator vs Temperature
0.410 0.408 0.406 REFERENCE (V) 0.404 0.402 0.400 0.398 0.396 0.394 0.392 0.390 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125 REFERENCE (V) VIN = 3.6V 0.820 0.816 0.812 0.808 0.804 0.800 0.796 0.792 0.788 0.784 0.780 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125
RDS(ON) ()
Buck (Burst) and VLDO Output
LVOUT 10mV/DIV AC COUPLED VOUT 10mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 2s/DIV LVOUT = 1.5V VOUT = 1.875V ILOAD = 50mA Burst Mode OPERATION
35413 G21
VLDO Load Step from 1mA to 300mA
LVOUT 20mV/DIV AC COUPLED
LVOUT 20mV/DIV AC COUPLED
ILOAD 250mA/DIV VIN = 3.6V 400s/DIV VOUT = 1.575V ILOAD = 1mA TO 300mA
35413 G13
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Buck Reference vs Temperature
VIN = 3.6V 0.700 0.600 0.500 0.400 0.300 0.200 0.100
RDS(0N) vs Temperature
SYNCH SWITCH
MAIN SWITCH
0 -50 -25
VIN = 2.5V VIN = 3.6V VIN = 5.5V 100 125
50 25 75 0 TEMPERATURE (C)
35413 G19
35413 G20
35413 G10
Buck (Pulse Skip) Load Step from 1mA to 500mA
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 40s/DIV VOUT = 1.8V ILOAD = 1mA TO 500mA
35413 G11
Buck (Burst) Load Step from 1mA to 500mA
VIN = 3.6V 40s/DIV VOUT = 1.8V ILOAD = 1mA TO 500mA
35413 G12
VLDO Load Step from 100mA to 300mA
ILOAD 250mA/DIV
VIN = 3.6V 400s/DIV VOUT = 1.575V ILOAD = 100mA TO 300mA
35413 G14
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LTC3541-3 TYPICAL PERFOR A CE CHARACTERISTICS
Linear Regulator to VLDO Transient Step, Load = 1mA
LVOUT 10mV/DIV AC COUPLED ILOAD 5mA/DIV
VIN = 3.6V VOUT = 1.575V ILOAD = 1mA
VLDO to Linear Regulator Transient Step, Load = 1mA
LVOUT 10mV/DIV AC COUPLED ILOAD 5mA/DIV
VIN = 3.6V VOUT = 1.575V ILOAD = 1mA
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Linear Regulator to VLDO Transient Step, Load = 30mA
LVOUT 10mV/DIV AC COUPLED ILOAD 25mA/DIV
40s/DIV
35413 G15
VIN = 3.6V VOUT = 1.575V ILOAD = 30mA
40s/DIV
35413 G16
VLDO to Linear Regulator Transient Step, Load = 30mA
LVOUT 10mV/DIV AC COUPLED ILOAD 25mA/DIV
40s/DIV
35413 G17
VIN = 3.6V VOUT = 1.575V ILOAD = 30mA
40s/DIV
35413 G18
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LTC3541-3 PI FU CTIO S
VIN (Pin 1):MainSupplyPin.Thispinmustbeclosely decoupledtoGNDwitha10Forgreatercapacitor. ENBUCK (Pin 2):BuckEnablePin.Thispinenablesthe buckregulatorwhendriventoalogichigh. VOUT (Pin 3):BuckRegulatorOutputPin.Thispinreceives thebuckregulator'soutputvoltage. NC (Pin 4):NotConnected.Thispinmustnotbeconnected orcapacitivelyloaded. LVOUT (Pin 5):VLDO/LinearRegulatorOutputPin.This pinprovidestheregulatedoutputvoltagefromtheVLDO orlinearregulator. LVIN (Pin 6): VLDO/Linear Regulator Input Supply Pin. ThispinprovidestheinputsupplyvoltagefortheVLDO powerFET. GND (Pin 7):AnalogGroundPin. MODE (Pin 8):BuckModeSelectionPin.Thispinenables buck Pulse-Skip operation when driven to a logic high andenablesbuckBurstModeoperationwhendrivento alogiclow. ENVLDO (Pin 9): VLDO/Linear Regulator Enable Pin. Whendriventoalogichigh,thispinenablesthelinear regulatorwhentheENBUCKpinisdriventoalogiclow, andenablestheVLDOregulatorwhentheENBUCKpinis driventoalogichigh.
1 0 1
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SW (Pin 10): Switch Node Pin. This pin connects the internalmainandsynchronouspowerMOSFETswitches totheexternalinductorforthebuckregulator. Exposed Pad (Pin 11): Ground Pin. This pin must be solderedtothePCBtoprovidebothelectricalcontactto groundandgoodthermalcontacttothePCB. Note:Table1detailsthetruthtableforthecontrolpins oftheLTC3541-3.
Table 1. LTC3541-3 Control Truth Table
PIN NAME ENBUCK 0 0 1 ENVLDO 0 1 0 MODE X X 0 LTC3541-3PoweredDown BuckPoweredDown,VLDOPowered Down,LinearRegulatorEnabled BuckEnabled,VLDOPoweredDown, LinearRegulatorPoweredDown, BurstModeOperation BuckEnabled,VLDOPoweredDown, LinearRegulatorPoweredDown, Pulse-SkipMode BuckEnabled,VLDOEnabled,Linear RegulatorPoweredDown,BurstMode Operation BuckEnabled,VLDOEnabled,Linear RegulatorPoweredDown,Pulse-Skip Mode OPERATIONAL DESCRIPTION
1
1
0
1
1
1
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LTC3541-3 W
2.2H 10 1 VIN SW VOUT(BUCK) = 1.8V IOUT(BUCK) = 500mA 10F VOUT PGND LVIN 3 6 VLDO/LINEAR REG REF VIN LVIN LVOUT = 1.575V IOUT = 300mA (LDO) IOUT = 30mA (LINEAR REG) LVOUT GND 5 2.2F
FU CTIO AL BLOCK DIAGRA
VIN(MIN) LVOUT + 1.4V
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500mA BUCK VIN SW REF FB GND
REF ENBUCK ENVLDO MODE CONTROL LOGIC
+ -
LFB CNTRL
2 9 8
GND 7
PGND 11
35413 F01
Figure 1. LTC3541-3 Functional Block Diagram
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LTC3541-3
TheLTC3541-3containsahighefficiencysynchronous buckconverter,averylowdropoutregulator(VLDO)and a linear regulator. It can be used to provide up to two outputvoltagesfromasingleinputvoltagemakingthe LTC3541-3idealforapplicationswithlimitedboardspace. Thecombinationandconfigurationofthesemajorblocks withintheLTC3541-3isdeterminedbywayofthecontrol pinsENBUCKandENVLDOasdefinedinTable1. WiththeENBUCKpindriventoalogichighandENVLDO driven to a logic low, the LTC3541-3 enables the buck convertertoefficientlyreducethevoltageprovidedatthe VINinputpintoanoutputvoltageof1.8Vasdeterminedby aninternalfeedbackresistornetwork.Thebuckregulator canbeconfiguredforPulse-SkiporBurstModeoperationbydrivingtheMODEpintoalogichighorlogiclow respectively.Thebuckregulatoriscapableofproviding a maximum output current of 500mA, which must be takenintoconsiderationwhenusingthebuckregulator toprovidethepowerforboththeVLDOregulatorandfor externalloads. WiththeENBUCKpindriventoalogiclowandENVLDO driventoalogichigh,theLTC3541-3enablesthelinear regulator,providingalownoiseregulatedoutputvoltageof 1.575VattheLVOUTpinwhiledrawingminimalquiescent currentfromtheVINinputpin.Thisfeatureallowsoutput voltageLVOUTtobebroughtintoregulationwithoutthe presenceoftheLVINvoltage. WiththeENBUCKandENVLDOpinsbothdriventoalogic high,theLTC3541-3enablesthehighefficiencybuckconverterandVLDO,providingdualoutputoperationfroma singleinputvoltage.Whenconfiguredinthismanner,the LTC3541-3'sautostart-upsequencingfeaturewillinitally bringtheVLDO/linearregulatoroutput(1.575V)intoregulationinacontrolledmannerusingthelinearregulatorprior toenablingthebuckoutput(1.8V)withouttheneedfor externalpincontrol.TheLTC3541-3automaticallytransitionstheVLDO/linearregulatoroutput(1.575V)fromthe linearregulatortotheVLDOregulatorwithin20msofbuck soft-startinitiation.Adetaileddiscussionofthetransitions betweentheVLDOregulatorandlinearregulatorcanbe foundintheVLDO/LinearRegulatorLoopsection.
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Buck Regulator Control Loop TheLTC3541-3internalbuckregulatorusesaconstant frequency,currentmode,step-downarchitecture.Boththe main(top,P-channelMOSFET)andsynchronous(bottom, N-channelMOSFET)switchesareinternal.Duringnormal operation,theinternalmainswitchisturnedonatthebeginningofeachclockcycleprovidedtheinternalfeedback voltagetothebuckislessthanthereferencevoltage.The currentintotheinductorprovidedtotheloadincreases untilthecurrentlimitisreached.Oncethecurrentlimitis reachedthemainswitchturnsoffandtheenergystored in the inductor flows through the bottom synchronous switchintotheloaduntilthenextclockcycle. Thepeakinductorcurrentisdeterminedbycomparingthe buckfeedbacksignaltoaninternal0.8Vreference.When the load current increases, the output of the buck and hencethebuckfeedbacksignaldecrease.Thisdecrease causesthepeakinductorcurrenttoincreaseuntiltheaverageinductorcurrentmatchestheloadcurrent.Whilethe mainswitchisoff,thesynchronousswitchisturnedon untileithertheinductorcurrentstartstoreversedirection orthebeginningofanewclockcycle. WhentheMODEpinisdriventoalogiclow,theLTC3541-3 buckregulatoroperatesinBurstModeoperationforhigh efficiency.Inthismode,themainswitchoperatesbased upon load demand. In Burst Mode operation the peak inductorcurrentissettoafixedvalue,whereeachburst eventcanlastfromafewclockcyclesatlightloadsto nearly continuous cycling at moderate loads. Between bursteventsthemainswitchandanyunneededcircuitry areturnedoff,reducingthequiescentcurrent.Inthissleep state,theloadisbeingsuppliedsolelyfromtheoutput capacitor.Astheoutputvoltagedroops,aninternalerror amplifier'soutputrisesuntilawakethresholdisreached causingthemainswitchtoagainturnon.Thisprocess repeatsataratethatisdependantupontheloadcurrent demand.
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OPERATIO
LTC3541-3
WhentheMODEpinisdriventoalogichightheLTC3541-3 operatesinPulse-Skipmodeforlowoutputvoltageripple. In this mode, the LTC3541-3 continues to switch at a constantfrequencydowntoverylowcurrents,whereit willbeginskippingpulsesusedtocontrolthemain(top) switchtomaintaintheproperaverageinductorcurrent. If the input supply voltage is decreased to a value approachingtheoutputvoltage,thedutycycleofthebuck isincreasedtowardmaximumon-timeand100%duty cycle.Theoutputvoltagewillthenbedeterminedbythe input voltage minus the voltage drop across the main switchandtheinductor. VLDO/Linear Regulator Loop IntheLTC3541-3,theVLDOandlinearregulatorloops consist of an amplifier and N-channel MOSFET output stagesthatservotheoutputtomaintainaregulatoroutput voltage,LVOUT.LoopconfigurationsenablingtheVLDOor thelinearregulatorarestablewithanoutputcapacitance aslowas2.2Fandashighas100F.BoththeVLDO regulatorandthelinearregulatorsarecapableofoperating withaninputvoltage,VIN,aslowas3V. TheVLDOregulatorisdesignedtoprovideupto300mA ofoutputcurrentataverylowLVINtoLVOUTvoltage.This allowsaclean,secondary,analogsupplyvoltagetobe providedwithaminimumdropinefficiency.TheVLDO regulatorisprovidedwiththermalprotectionthatisdesignedtodisabletheVLDOfunctionwhentheoutput,pass transistor'sjunctiontemperaturereachesapproximately 160C. In addition to thermal protection, short-circuit detectionisprovidedtodisabletheVLDOfunctionwhen ashort-circuitconditionissensed.Thiscircuitisdesigned suchthatanoutputcurrentofapproximately1Acanbe providedbeforethiscircuitwilltrigger.Asdetailedinthe ElectricalCharacteristics,theVLDOregulatorwillbeout ofregulationwhenthiseventoccurs.Boththethermal and short-circuit faults, when detected, are treated as catastrophicfaultconditions.TheLTC3541-3willbereset uponthedetectionofeitherevent.
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TheN-channelMOSFETincorporatedintheVLDOregulator hasitsdrainconnectedtotheLVINpinasshowninFigure 1.Toensurereliableoperation,theLVINvoltagemustbe stablebeforetheVLDOregulatorisenabled.Forthecase wherethevoltageonLVINissuppliedbythebuckregulator,theinternalpowersupplysequencinglogicassures voltagesareappliedintheappropriatemanner.Forthe casewherethebuckisenabledbeforetheVLDOregulatorandanexternalsupplyisusedtopowertheLVINpin, thevoltageonLVINpinmustbestable1msbeforethe ENVLDOpinisbroughtfromalowtoahigh.Further,the externallysuppliedLVINmustbereducedinconjunction withVINwheneverVINispulledloworremoved. Thelinearregulatorisdesignedtoprovidealoweroutput current(30mA)thanthatavailablefromtheVLDOregulator.Thelinearregulator'soutput,passtransistorhasits draintiedtotheVINrail.Thisallowsthelinearregulator tobeturnedonpriorto,andindependentof,thebuck regulatorwhichordinarilydrivestheVLDOregulator.The linearregulatorisprovidedwiththermalprotectionthat isdesignedtodisablethelinearregulatorfunctionwhen theoutputpasstransistor'sjunctiontemperaturereaches approximately160C.Inadditiontothermalprotection, short-circuit detection is provided to disable the linear regulatorfunctionwhenashort-circuitconditionissensed. This circuit is designed such that an output current of approximately120mAcanbeprovidedbeforethiscircuit willtrigger.AsdetailedintheElectricalCharacteristics, the linear regulator will be out of regulation when this eventoccurs.Boththethermalandshort-circuitfaultsare treatedascatastrophicfaultconditions.TheLTC3541-3 willberesetuponthedetectionofeitherevent. TheN-channelMOSFETincorporatedinthelinearregulator hasitsdrainconnectedtotheVINpinasshowninFigure 1.ThesizeoftheseMOSFETsandtheirassociatedpower bussingisdesignedtoaccommodate30mAofDCcurrent. Currentsabovethisvaluecanbesupportedforshortperiods asstipulatedintheAbsoluteMaximumRatings.
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OPERATIO
LTC3541-3
TransitioningfromlinearregulatormodetoVLDOmode, accomplishedbybringingENBUCKfromalogiclowtoa logichighwhileENVLDOisalogichighorasoccursduring theautostart-upsequence,isdesignedtobeasseamlessandtransientfreeaspossible.Theprecisetransient responseofLVOUTduetothistransitionisafunctionof COUTandtheloadcurrent.WaveformsgivenintheTypical PerformanceCharacteristicssectionshowtypicaltransient responsesusingtheminimumCOUTof2.2Fandload currents of 1mA and 30mA respectively. Generally, the amplitudeofanytransientspresentwilldecreaseasCOUT isincreased.Toensurereliableoperationandadherence to the load regulation limits presented in the Electrical Characterstics table, the load current must not exceed thelinearregulatorIOUTlimitof30mAwithin20msafter ENBUCKhastransitionedtoalogichigh.The300mAIOUT limitofVLDOappliesthereafter.Further,forconfigurations thatdonotusetheLTC3541-3'sbuckregulatortoprovide theVLDOinputvoltage(LVIN),theusermustensurea stableLVINvoltageispresentnolessthan1mspriorto ENBUCKtransitioningtoalogichigh.
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In a similar manner, transitioning from VLDO mode to linearregulatormode,accomplishedbybringingENBUCK fromahighlowtoalogiclowwhileENVLDOisalogic high,isdesignedtobeasseamlessandtransientfreeas possible.Again,theprecisetransientresponseofLVOUT duetothistransitionisafunctionofCOUTandtheload current. Waveforms given in the Typical Performance Characteristicssectionshowtypicaltransientresponses usingtheminimumCOUTof2.2Fandloadcurrentsof 1mAand30mArespectively.Generally,theamplitudeof anytransientspresentwilldecreaseasCOUTisincreased. Toensurereliableoperationandadherencetotheload regulationlimitspresentedintheElectricalCharacterstics table,theloadcurrentmustnotexceedthelinearregulator IOUTlimitof30mA1mspriortoENBUCKtransitioningto alogiclowandthereafer.Further,forconfigurationsthat donotusetheLTC3541-3'sbuckregulatortoprovidethe VLDOinputvoltage(LVIN),theusermustcontinuetoensure astableLVINvoltagenolessthan1msafterENBUCKhas transitionedtoalogiclow.
35413fc
OPERATIO
LTC3541-3
ThebasicLTC3541-3applicationcircuitisshownonthe firstpageofthisdatasheet.Externalcomponentselection isdrivenbytheloadrequirementandrequirestheselection ofL,followedbyCIN,COUT,andfeedbackresistorvalues forthebuckandtheselectionoftheoutputcapacitorand feedbackvaluesfortheVLDOandlinearregulator. BUCK REGULATOR Inductor Selection Formostapplications,theappropriateinductorvaluewillbe 2.2H.Itsvalueischosenlargelybasedonthedesiredripple currentandburstrippleperformance.Generally,largevalue inductorsreduceripplecurrent,andconversely,smallvalue inductorsproducehigherripplecurrent.HigherVINorVOUT mayalsoincreasetheripplecurrentasshowninEquation 1.Areasonablestartingpointforsettingripplecurrentis IL=200mA(40%of500mA). IL = V 1 VOUT 1- OUT VIN ( f )(L ) (1)
TheDCcurrentratingoftheinductorshouldbeatleast equaltothemaximumloadcurrentplushalftheripple currenttopreventcoresaturation.Thus,a600mArated inductorshouldbeenoughformostapplications(500mA +100mA).Forbetterefficiency,choosealowDCresistanceinductor. Inductor Core Selection Different core materials and shapes will change the size/current and price/current relationship of an inductor.Toroidorshieldedpotcoresinferriteorpermalloy materialsaresmallanddon'tradiatemuchenergy,but generallycostmorethanpowderedironcoreinductors withsimilarelectricalcharacteristics.Thechoiceofwhich styleinductortouseoftendependsmoreonthepricevs sizerequirementandanyradiatedfield/EMIrequirements ratherthanwhattheLTC3541-3requirestooperate.Table2 showssometypicalsurfacemountinductorsthatwork wellinLTC3541-3applications.
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Table 2. Representative Surface Mount Inductors
PART NUMBER Sumida CDRH3D23 VALUE (H) 1.0 1.5 2.2 3.3 2.2 3.3 1.0 1.5 2.2 3.3 1.0 2.2 DCR MAX DC ( MAX) CURRENT (A) 0.025 0.029 0.038 0.048 0.116 0.174 0.058 0.068 0.104 0.138 0.060 0.097 2 1.65 1.3 1.1 0.950 0.770 2.7 2.2 1.8 1.3 1.00 0.79 SIZE W x L x H (mm3) 3.9x3.9x2.4 Sumida CMD4D06 Coilcraft ME3220 3.5x4.3x0.8 2.5x3.2x2.0 Murata LQH3C 2.5x3.2x2.0
APPLICATIO S I FOR ATIO W U U
CIN and COUT Selection Incontinuousmode,thesourcecurrentofthetopMOSFET isasquarewaveofdutycycleVOUT/VIN.Topreventlarge voltagetransients,alowESRinputcapacitorsizedforthe maximumRMScurrentmustbeused.ThemaximumRMS capacitorcurrentisgivenby: cIN required IRMS IOMAX VOUT ( VIN - VOUT ) VIN
1/2 2
This formula has a maximum at VIN = 2VOUT, where IRMS=IOUT/2.Thissimple,worst-caseconditioniscommonly used for design. Note that the capacitor manufacturer'sripplecurrentratingsareoftenbasedon2000 hoursoflife.Thismakesitadvisabletofurtherderatethe capacitororchooseacapacitorratedatahighertemperaturethanrequired.Alwaysconsultthemanufacturerwith anyquestionregardingpropercapacitorchoice. TheselectionofCOUTforthebuckregulatorisdrivenby thedesiredbucklooptransientresponse,requiredeffective seriesresistance(ESR)andburstrippleperformance. TheLTC3541-3minimizestherequirednumberofexternal componentsbyprovidinginternalloopcompensationfor thebuckregulatorloop.Loopstability,transientresponse andburstrippleperformancecanbetailoredbychoice ofoutputcapacitance.Formanyapplications,desirable stability,transientresponseandrippleperformancecan
35413fc
LTC3541-3
be obtained by choosing an output capacitor value of 10Fto22F. Typically,oncetheESRrequirementforCOUThasbeen met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple VOUT is determinedby: 1 VOUT IL ESR + 8 fcOUT wheref=operatingfrequency,COUT=outputcapacitance andIL=ripplecurrentintheinductor.Forafixedoutput voltage,theoutputrippleishighestatmaximuminput voltagesinceILincreaseswithinputvoltage. Aluminumelectrolyticanddrytantalumcapacitorsareboth availableinsurfacemountconfigurations.Inthecaseof tantalum,itiscriticalthatthecapacitorsaresurgetested foruseinswitchingpowersupplies.Anexcellentchoiceis theAVXTPSseriesofsurfacemounttantalum.Theseare speciallyconstructedandtestedforlowESRsotheygive thelowestESRforagivenvolume.Othercapacitortypes includeSanyoPOSCAP,KemetT510andT495series,and Sprague593Dand595Dseries.Consultthemanufacturer forotherspecificrecommendations. Using Ceramic Input and Output Capacitors Highvalue,lowcostceramiccapacitorsarenowbecoming availableinsmallercasesizes.Theirhighripplecurrent, high voltage rating, and low ESR make them ideal for switchingregulatorapplications.SincetheLTC3541-3's controlloopdoesnotdependontheoutputcapacitor'sESR forstableoperation,ceramiccapacitorscanbeusedfreely toachieveverylowoutputrippleandsmallcircuitsize. However, care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitorisusedattheinputandthepowerissupplied
byawalladapterthroughlongwires,aloadstepatthe outputcaninduceringingattheinput,VIN.Atbest,this ringingcancoupletotheoutputandbemistakenasloop instability.Atworst,asuddeninrushofcurrentthrough
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thelongwirescanpotentiallycauseavoltagespikeatVIN, largeenoughtodamagethepart. Whenchoosingtheinputandoutputceramiccapacitors, choose the X5R or X7R dielectric formulations. These dielectricshavethebesttemperatureandvoltagecharacteristicsofalltheceramicsforagivenvalueandsize.
Checking Transient Response Theregulatorloopresponsecanbecheckedbylooking attheloadtransientresponse.Switchingregulatorstake severalcyclestorespondtoastepinloadcurrent.When aloadstepoccurs,VOUTimmediatelyshiftsbyanamount equalto(ILOAD*ESR),whereESRistheeffectiveseries resistanceofCOUT.ILOADalsobeginstochargeordischargeCOUT,whichgeneratesafeedbackerrorsignal.The regulatorloopthenactstoreturnVOUTtoitssteady-state value.DuringthisrecoverytimeVOUTcanbemonitored for overshoot or ringing that would indicate a stability problem.Foradetailedexplanationofswitchingcontrol looptheoryseeApplicationNote76. Asecond,moreseveretransientiscausedbyswitching inloadswithlarge(>1F)supplybypasscapacitors.The dischargedbypasscapacitorsareeffectivelyputinparallelwithCOUT,causingarapiddropinVOUT.Noregulator candeliverenoughcurrenttopreventthisproblemifthe loadswitchresistanceislowanditisdrivenquickly.The onlysolutionistolimittherisetimeoftheswitchdrive so that the load rise time is limited to approximately (25*CLOAD).Thus,a10Fcapacitorchargingto3.3V would require a 250s rise time, limiting the charging currenttoabout130mA. VLDO/LINEAR REGULATOR Output Capacitance and Transient Response TheLTC3541-3isdesignedtobestablewithawiderange ofceramicoutputcapacitors.TheESRoftheoutputcapacitoraffectsstability,mostnotablywithsmallcapacitors.A minimumoutputcapacitorof2.2FwithanESRof0.05
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APPLICATIO S I FOR ATIO W U U
LTC3541-3
orlessisrecommendedtoensurestability.TheLTC3541-3 VLDOisamicropowerdeviceandoutputtransientresponse will be a function of output capacitance. Larger values ofoutputcapacitancedecreasethepeakdeviationsand provideimprovedtransientresponseforlargerloadcurrent changes.Notethatbypasscapacitorsusedtodecouple individual components powered by the LTC3541-3 will increasetheeffectiveoutputcapacitorvalue.HighESR tantalum and electrolytic capacitors may be used, but alowESRceramiccapacitormustbeinparallelatthe output.ThereisnominimumESRormaximumcapacitor sizerequirement.
20 0 cHANGE IN VALUE (%) -20 -40 -60 -80 -100
BOTH cAPAcITORS ARE 1F, 10V, 0603 cASE SIZE X5R
Y5V
0
2
6 4 Dc BIAS VOLTAGE (V)
8
10
35413 F06
Figure 6. Change in Capacitor vs Bias Voltage
20 0 cHANGE IN VALUE (%) X5R -20 Y5V -40 -60 -80
BOTH cAPAcITORS ARE 1F, 10V, 0603 cASE SIZE -100 -50 0 25 50 -25 TEMPERATURE (c)
75
35413 F07
Figure 7. Change in Capacitor vs Temperature
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Extraconsiderationmustbegiventotheuseofceramic capacitors.Ceramiccapacitorsaremanufacturedwitha varietyofdielectrics,eachwithdifferentbehavioracross temperature and applied voltage. The most common dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U andY5Vdielectricsaregoodforprovidinghighcapacitancesinasmallpackage,butexhibitlargevoltageand temperature coefficients as shown in Figures 6 and 7. Whenusedwitha2Vregulator,a1FY5Vcapacitorcan loseasmuchas75%ofitsinitialcapacitanceoverthe operatingtemperaturerange.TheX5RandX7Rdielectrics resultinmorestablecharacteristicsandareusuallymore suitableforuseastheoutputcapacitor.TheX7Rtypehas betterstabilityacrosstemperature,whiletheX5Risless expensiveandisavailableinhighervalues.Inallcases, theoutputcapacitanceshouldneverdropbelow1For instabilityordegradedperformancemayoccur. EFFICIENCY CONSIDERATIONS Generally,theefficiencyofaregulatorisequaltotheoutputpowerdividedbytheinputpowertimes100%.Itis oftenusefultoanalyzeindividuallosstermstodetermine whichtermsarelimitingefficiencyandwhatifanychange wouldyieldthegreatestimprovement.Efficiencycanbe expressedas: Efficiency=100%-(L1+L2+L3+...) whereL1,L2,etc.aretheindividuallosstermsasapercentageofinputpower. Althoughalldissipativeelementsinthecircuitproduce losses,threemainsourcestypicallyaccountforthemajorityofthelossesintheLTC3541-3circuits:VINquiescent current,I2RlossesandlossacrossVLDOoutputdevice. WhenoperatingwithboththebuckandVLDOregulator active (ENBUCK and ENVLDO equal to logic high), VIN quiescentcurrentlossandlossacrosstheVLDOoutput devicedominatetheefficiencylossatlowloadcurrents, whereastheI2RlossandlossacrosstheVLDOoutput devicedominatetheefficiencylossatmediumtohighload currents.Atlowloadcurrentswiththepartoperatingwith thelinearregulator(ENBUCKequaltologiclow,ENVLDO
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APPLICATIO S I FOR ATIO W U U
LTC3541-3
equaltologichigh),efficiencyistypicallydominatedby thelossacrossthelinearregulatoroutputdeviceandVIN quiescentcurrent.Inatypicalefficiencyplot,theefficiency curveatverylowloadcurrentscanbemisleadingsince theactualpowerlostisoflittleconsequence. 1.TheVINquiescentcurrentlossinthebuckisduetotwo components:theDCbiascurrentasgivenintheElectrical Characteristicsandtheinternalmainswitchandsynchronousswitchgatechargecurrents.Thegatechargecurrent resultsfromswitchingthegatecapacitanceoftheinternal powerswitches.Eachtimethegateisswitchedfromhigh tolowtohighagain,apacketofcharge,dQ,movesfrom VINtoground.TheresultingdQ/dtisthecurrentoutof VINthatistypicallylargerthantheDCbiascurrentand proportionaltofrequency.BoththeDCbiasandgatecharge lossesareproportionaltoVINandthustheireffectswill bemorepronouncedathighersupplyvoltages. 2.I2Rlossesarecalculatedfromtheresistancesofthe internalswitches,RSW,andexternalinductorRL.Incontinuousmode,theaverageoutputcurrentflowingthrough inductorLis"chopped"betweenthemainswitchandthe synchronousswitch.Thus,theseriesresistancelooking into the SW pin is a function of both top and bottom MOSFETRDS(ON)andthedutycycle(DC)asfollows: RSW=(RDS(ON)TOP)(DC)+(RDS(ON)BOT)(1-DC) TheRDS(ON)forboththetopandbottomMOSFETscan beobtainedfromtheTypicalPerformanceCharacteristics curves. Thus, to obtain I2R losses, simply add RSW to RLandmultiplytheresultbythesquareoftheaverage outputcurrent. 3.LossesintheVLDO/linearregulatorareduetotheDCbias currentsasgivenintheElectricalCharacteristicsandtothe (VIN-VOUT)voltagedropacrosstheinternaloutputdevice transistor. Other losses when the buck and VLDO regulator are in operation (ENBUCK and ENVLDO equal logic high), includingCINandCOUTESRdissipativelossesandinductorcorelosses,generallyaccountforlessthan2%total additionalloss.
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THERMAL CONSIDERATIONS The LTC3541-3 requires the package backplane metal (GNDpin)tobewellsolderedtothePCboard.Thisgives the DFN package exceptional thermal properties. The power handling capability of the device will be limited by the maximum rated junction temperature of 125C. TheLTC3541-3hasinternalthermallimitingdesignedto protectthedeviceduringmomentaryoverloadconditions. Forcontinuousnormalconditions,themaximumjunction temperatureratingof125Cmustnotbeexceeded.Itis importanttogivecarefulconsiderationtoallsourcesof thermalresistancefromjunctiontoambient.Additional heatsourcesmountednearbymustalsobeconsidered. Forsurfacemountdevices,heatsinkingisaccomplished byusingtheheat-spreadingcapabilitiesofthePCboard anditscoppertraces.Copperboardstiffenersandplated throughholescanalsobeusedtospreadtheheatgeneratedbypowerdevices. ToavoidtheLTC3541-3exceedingthemaximumjunction temperature,somethermalanalysisisrequired.Thegoal ofthethermalanalysisistodeterminewhetherthepower dissipatedexceedsthemaximumjunctiontemperatureof thepart.Thetemperatureriseisgivenby: TR=PD*JA wherePDisthepowerdissipatedbytheregulatorandJA isthethermalresistancefromthejunctionofthedieto theambienttemperature. Thejunctiontemperature,TJ,isgivenby: TJ=TA+TR whereTAistheambienttemperature. Asanexample,considertheLTC3541-3ataninputvoltageVINof3V,anLVINvoltageof1.8Vprovidedbythebuck regulator,anLVOUTvoltageof1.575V,aloadcurrentof 300mAfortheVLDOregulator,aloadcurrentof200mA forthebuck(totalloadforbuck=500mA),andanambient temperatureof85C.Fromthetypicalperformancegraph ofswitchresistance,theRDS(ON)oftheP-channelswitchat
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APPLICATIO S I FOR ATIO W U U
LTC3541-3
85Cisapproximately0.25.TheRDS(ON)oftheN-channel switchisapproximately0.4. Therefore,powerdissipated bythepartisapproximately: PD=(ILOADBUCK)2*RSW(ON)+(ILOADVLDO)* (LVIN-LVOUT)=145mW Forthe3mmx3mmDFNpackage,theJAis43C/W. Thus,thejunctiontemperatureoftheregulatoris: TJ=85C+(0.145)(43)=91C whichiswellbelowthemaximumjunctiontemperature of125C. Notethatathighersupplyvoltages,thejunctiontemperatureislowerduetoreducedswitchresistanceRDS(ON). PC BOARD LAYOUT CHECKLIST Whenlayingouttheprintedcircuitboard,thefollowing checklistshouldbeusedtoensureproperoperationofthe LTC3541-3.Checkthefollowinginyourlayout: 1.Thepowertraces,consistingoftheGNDtrace,theSW traceandtheVINtraceshouldbekeptshort,directand wide. 2.Doesthe(+)plateofCINconnecttoVINascloselyas possible?ThiscapacitorprovidestheACcurrenttothe internalpowerMOSFETs. 3.Keeptheswitchingnode,SW,awayfromthesensitive LFBnode. 4. Keep the (-) plates of CIN and COUT as close as possible.
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DESIGN EXAMPLE Asadesignexample,assumetheLTC3541-3isusedin asinglelithium-ionbatterypoweredcellularphoneapplication.TheVINwillbeoperatingfromamaximumof 4.2Vdowntoabout3V.Theloadcurrentrequirementis amaximumof0.5Aforthebuckoutputbutmostofthe timeitwillbeinstandbymode,requiringonly2mA.Efficiencyatbothlowandhighloadcurrentsisimportant. Theoutputvoltageforthebuckis1.8V.Therequirementfor theoutputvoltageoftheVLDOregulatoris1.575Vwhile providingupto0.3Aofcurrent.Withthisinformationwe cancalculateLusingEquation2: L= V 1 VOUT 1- OUT VIN ( f )( IL ) (2) SubstitutingVOUT=1.8V,VIN=3.6V(typ),IL=200mA andf=2.25MHzinEquation3gives: L= 1.8 V 1.8 V 1- = 2 H 2.25MHz(200mA) 3.6 V (3) A2.2Hinductorworkswellforthisapplication.Forbest efficiencychoosea600mAorgreaterinductorwithless than0.2seriesresistance. CINwillrequireanRMScurrentratingofatleast0.25A = ILOAD(MAX)/2 at temperature. COUT for the buck is chosentohaveavalueof22FandanESRoflessthan 0.25.Inmostcases,aceramiccapacitorwillsatisfy thisrequirement. COUTfortheVLDOregulatorischosenas2.2F.
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APPLICATIO S I FOR ATIO W U U
LTC3541-3 TYPICAL APPLICATIO S
Dual Output with Minimal External Components Using Auto Start-Up Sequence, Buck in Burst Mode Operation for High Efficiency Down to Low Load Currents
VIN 3V TO 4.2V VOUT 1V/DIV SW 2.2H ENVLDO LVOUT 1V/DIV VIN MODE LTC3541-3 ENBUCK GND VOUT LVIN 10F LVOUT PGND VOUT2 1.575V 300mA
VOUT1 1.8V 200mA
Dual Output with Minimal External Components Using Auto Start-Up Sequence, Buck in Pulse-Skip Mode for Low Noise Operation
VIN 3V TO 4.2V VOUT 1V/DIV SW 2.2H ENVLDO LVOUT 1V/DIV VIN MODE LTC3541-3 ENBUCK GND VOUT LVIN 10F LVOUT PGND VOUT2 1.575V 300mA
VOUT1 1.8V 200mA
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VIN 2V/DIV
2.2F
35413 TA02a
IVOUT = 200mA ILVOUT = 30mA
2ms/DIV
35413TA02b
VIN 2V/DIV
2.2F
35413 TA03a
IVOUT = 200mA ILVOUT = 30mA
2ms/DIV
35413TA03b
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LTC3541-3 TYPICAL APPLICATIO S
Dual Output Using Minimal External Components with VOUT2 Controlled by External Logic Signal, Buck in Burst Mode Operation for High Efficiency Down to Low Load Currents
VIN 3V TO 4.2V VOUT 1V/DIV SW 2.2H ENVLDO LVOUT 1V/DIV VIN 2V/DIV VIN MODE LTC3541-3 ENBUCK GND VOUT LVIN 10F LVOUT PGND VOUT2 1.575V 300mA
VOUT1 1.8V 200mA
Dual Output Using Minimal External Components with VOUT1 Controlled by External Logic Signal, Buck in Burst Mode Operation for High Efficiency Down to Low Load Currents
VIN 3V TO 4.2V
SW 2.2H VOUT1 1.8V 200mA 10F
ENVLDO LVOUT 1V/DIV VIN 2V/DIV
VIN MODE LTC3541-3 ENBUCK GND VOUT LVIN LVOUT PGND VOUT2 1.575V 2.2F 300mA
35413 TA05a
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2.2F
35413 TA04a
IVOUT = 200mA ILVOUT = 300mA
4ms/DIV
35413TA04b
VOUT 1V/DIV
IVOUT = 200mA ILVOUT = 30mA
4ms/DIV
35413TA05b
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LTC3541-3 U
DD Package 10-Lead Plastic DFN (3mm x 3mm)
(Reference LTc DWG # 05-08-1699)
0.675 0.05 PAcKAGE OUTLINE 0.25 0.05 0.50 BSc 2.38 0.05 (2 SIDES) R = 0.115 TYP 6 0.38 0.10 10 3.00 0.10 (4 SIDES) PIN 1 TOP MARK (SEE NOTE 6) 5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES) BOTTOM VIEW--EXPOSED PAD 1 1.65 0.10 (2 SIDES)
(DD10) DFN 1103
PACKAGE DESCRIPTIO
3.50 0.05 1.65 0.05 2.15 0.05 (2 SIDES)
RECOMMENDED SOLDER PAD PITcH AND DIMENSIONS
0.25 0.05 0.50 BSc
0.00 - 0.05
NOTE: 1. DRAWING TO BE MADE A JEDEc PAcKAGE OUTLINE M0-229 VARIATION OF (WEED-2). cHEcK THE LTc WEBSITE DATA SHEET FOR cURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO ScALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PAcKAGE DO NOT INcLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXcEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENcE FOR PIN 1 LOcATION ON THE TOP AND BOTTOM OF PAcKAGE
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Information furnished by Linear Technology corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LTC3541-3 RELATED PARTS
PART NUMBER LT 3023
(R)
DESCRIPTION Dual,2x100mA,LowNoiseMicropowerLDO
COMMENTS VIN:1.8Vto20V,VOUT(MIN)=1.22V,VDO=0.30V,IQ=40A,ISD<1A, VOUT=ADJ,DFN,MSPackages,LowNoise<20VRMS(P-P),Stablewith 1FCeramicCapacitors VIN:1.8Vto20V,VOUT(MIN)=1.22V,VDO=0.30V,IQ=60A,ISD<1A, VOUT=ADJ,DFN,TSSOPPackages,LowNoise<20VRMS(P-P),Stablewith 1FCeramicCapacitors VIN:0.9Vto5.5V,VOUT(MIN)=0.4V,2.7Vto5.5VBiasVoltageRequired, VDO=45mV,IQ=50A,ISD<1A,VOUT=ADJ,DFNPackages,Stablewith 1FCeramicCapacitors 1.5MHzConstantFrequencyCurrentModeOperation,VINfrom2.5Vto 5.5V,VOUTDownto0.6V,DFN,MSPackages 2.25MHzConstantFrequencyCurrentModeOperation,VINfrom2.5Vto 5.5V,VOUTDownto0.6V,DFN,MSPackages 600mA,1.5MHzCurrentModeBuckRegulator,I2CProgrammable VOUTfrom0.85Vto1.55V,two50mALDOs,BackupBatteryInputwith PowerPathControl,QFNPackage VIN:2.7Vto5.5V,VOUT(MIN)Buck=0.8V,VOUT(MIN)VDLO=0.4VOUT(MIN), 14-PinDFNPackage VIN:2.7Vto5.5V,VOUT(MIN)=0.6V,SwitchestoLDOModeat3A, DD8,MS8/EPackages VIN:2.7Vto5.5V,VOUT(MIN)Buck=0.8V,VOUT(MIN)VLDO=0.4V, 3mmx3mm10-PinDFNPackage 95%Efficiency,VIN:2.5Vto5.5V,VOUT(MIN)=0.6V,IQ=40A,ISD<1A, DFNand10-PinMSPackages VINfrom2.65Vto9.8V,ConstantFrequency550kHzOperation
LT3024
Dual,100mA/500mA,LowNoiseMicropowerLDO
LTC3025
300mA,MicropowerVLDOLinearRegulator
LTC3407 LTC3407-2 LTC3445
DualSynchronous600mASynchronousStep-Down DC/DCRegulator DualSynchronous800mASynchronousStep-Down DC/DCRegulator,2.25MHz I2CControllableBuckRegulatorwithTwoLDOsand BackupBatteryInput TripleOutputStep-DownConverter1AOutputBuck, TwoEach300mAVDLOs 600mA(IOUT),HighEfficiency,1.5MHz/2.25MHz SynchronousStep-DownRegulatorwithLDOMode HighEfficiencyBuck+VLDORegulator
LTC3446 LTC3448 LTC3541
LTC3548/LTC3548-1 Dual800mA/400mAIOUT,2.25MHz,Synchronous Step-DownDC/DCConverter LTC3548-2 LTC3700 Step-DownDC/DCControllerwithLDORegulator PowerPathisatrademarkofLinearTechnologyCorporation.
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0 Linear Technology Corporation
1630 Mccarthy Blvd., Milpitas, cA 95035-7417
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
LT 0507 REV C * PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2006

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