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LTC3541-1 High Efficiency Buck + VLDO Regulator
High Efficiency, 500mA Buck Plus 300mA VLDOTM Regulator Auto Start-Up Powers VLDO/Linear Regulator Output Prior to Buck Regulator Output Independent High Efficiency, 500mA Buck (VIN: 2.7V to 5.5V) 300mA VLDO Regulator with 30mA Standalone Mode No External Schottky Diodes Required Buck Output Voltage Range: 0.8V to 5V VLDO Input Voltage Range (LV ): 0.9V to 5.5V IN VLDO Output Voltage Range VLDO: 0.4V to 4.1V SelectableFixedFrequency,Pulse-SkipOperationor BurstMode(R)Operation Short-CircuitProtected CurrentModeOperationforExcellentLineandLoad TransientResponse ConstantFrequencyOperation:2.25MHz LowDropoutBuckOperation:100%DutyCycle Small,ThermallyEnhanced,10-Lead(3mmx3mm) DFNPackage
TheLTC (R)3541-1combinesasynchronousbuckDC/ DCconverterwithaverylowdropoutlinearregulator (VLDO)toprovideuptotwooutputvoltagesfromasingle inputvoltagewithminimalexternalcomponents.When configuredfordualoutputoperation,theLTC3541-1'sauto start-upfeaturewillbringtheVLDO/linearregulatoroutput intoregulationinacontrolledmannerpriortoenablingthe Buckregulatoroutputwithouttheneedforexternalpin control.BuckoutputpriortoVLDO/linearregulatoroutput sequencingmayalsobeobtainedviaexternalpincontrol. TheinputvoltagerangeisideallysuitedforLi-Ionbatterypoweredapplications,aswellaspoweringsub-3.3Vlogic from5Vor3.3Vrails. Thesynchronousbuckconverterprovidesahighefficiency output,typically90%,capableofprovidingupto500mA ofcontinuousoutputcurrentwhileswitchingat2.25MHz, allowingtheuseofsmallsurfacemountinductorsandcapacitors.Amode-selectpinallowsBurstModeoperation tobeenabledforhigherefficiencyatlightloadcurrents,or disabledforlowernoise,constantfrequencyoperation. The VLDO regulator provides a low noise, low voltage outputcapableofprovidingupto300mAofcontinuous outputcurrentusingonlya2.2Fceramiccapacitor.The inputsupplyvoltageoftheVLDOregulator(LVIN)may comefromthebuckregulatororaseparatesupply.
, 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

PDAs/PalmtopPCs DigitalCameras CellularPhones PCCards WirelessandDSLModems OtherPortablePowerSystems
LTC3541-1 Typical Application
VIN 2.9V TO 5.5V
EFFICIENCY (%)
VIN VOUT1 2.5V 200mA 22pF 243k 2.2H
ENBUCK MODE LTC3541-1 SW GND LVIN LFB
BUCKFB LVOUT PGND 10F 115k
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TYPICAL APPLICATIO
Buck (Burst) Efficiency and Power Loss vs Load Current
100 90 80 EFFICIENCY 0.1 POWER LOSS (W) 1
ENVLDO 150k
70 60 50 40 30 20 10 0 1 VIN = 3.3V VOUT = 2.5V 10 100 LOAD CURRENT (mA) POWER LOSS
412k VOUT2 1.5V 300mA
2.2F
35411 TA01a
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0.01 0.001 0.0001 1000
35411 TA01b
FEATURES
DESCRIPTIO
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LTC3541-1
(Note 1)
SupplyVoltages: VIN,LVIN.................................................. -0.3Vto6V LVIN-VIN..........................................................<0.3V PinVoltages: ENVLDO,ENBUCK,MODE,SW, LFB,BUCKFB.............................-0.3Vto(VIN+0.3V) LinearRegulatorIOUT(MAX)(100ms)(Note9)......100mA OperatingAmbientTemperatureRange (Note2)................................................... -40Cto85C . JunctionTemperature(Note5)............................. 125C StorageTemperatureRange.................. -65Cto125C .
ORdeR INFORmATION
LEAD FREE FINISH LTC3541EDD-1#PBF TAPE AND REEL LTC3541EDD-1#TRPBF PART MARKING* LCWQ PACKAGE DESCRIPTION 10-Lead(3mmx3mm)PlasticDFN TEMPERATURE RANGE -40Cto85C ConsultLTCMarketingforpartsspecifiedwithwideroperatingtemperatureranges. ConsultLTCMarketingforinformationonnon-standardleadbasedfinishparts. *Formoreinformationonleadfreepartmarking,goto:http://www.linear.com/leadfree/ Formoreinformationontapeandreelspecifications,goto:http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
SYMBOL IPK IBUCKFB ILFB VIN VIN(LINEREG) PARAMETER PeakInductorCurrent BUCKFBPinInputCurrent LFBPinInputCurrent InputVoltageRange BuckVINLineRegulation
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise specified (Note 2)
CONDITIONS VIN=4.2V(Note8) VBUCKFB=0.9V VLFB=0.45V (Note4) VIN=2.7Vto5.5V,ENBUCK=VIN, ENVLDO=0V,MODE=VIN(Note6) VIN=2.7Vto5.5V,LVOUT=1.2V,ENBUCK=VIN, ENVLDO=VIN,IOUT(VLDO)=100mA,LVIN=1.5V VIN=2.7Vto5.5V,LVOUT=1.2V,ENBUCK=0V, ENVLDO=VIN,IOUT(LREG)=10mA LVIN=0.9Vto5.5V,VIN=5.5V,LVOUT=0.4V, ENBUCK=VIN,ENVLDO=VIN,MODE=VIN, IOUT(VLDO)=100mA LVIN=1.5V,ENBUCK=VIN,ENVLDO=VIN, MODE=VIN,IOUT(VLDO)=50mA,VLFB=0.3V ENBUCK=VIN,ENVLDO=0V,MODE=VIN(Note6)

VLDOVINLineRegulation (ReferredtoLFB) LinearRegulatorVINLine Regulation(ReferredtoLFB) LVIN(LINEREG) LVINLineRegulation (ReferredtoLFB) VLDODO VLOADREG LVIN-LVOUTDropoutVoltage (Note9) BuckOutputLoadRegulation VLDOOutputLoadRegulation LinearRegulatorOutputLoad
IOUT(VLDO)=1mA-300mA,LVIN=1.5V,LVOUT=1.2V, ENBUCK=VIN,ENVLDO=VIN,MODE=VIN IOUT(LREG)=1mA-30mA,LVOUT=1.2V, ENBUCK=0V,ENVLDO=VIN
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WW
W
ABSOLUTE
AXI U RATI GS
pIN CONFIGURATION
TOP VIEW VIN ENBUcK BUcKFB LFB 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)ISGND,MUSTBESOLDEREDTOPCB
MIN 0.8 -200 2.7
TYP 0.95 -40
MAX 1.25 50 5.5
UNITS A nA nA V %/V mV/V mV/V mV/V
0.04 0.6 0.6 0.3 28 0.5 0.25 0.25
0.4
60
mV %
0.5 0.5
% %
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LTC3541-1 ELECTRICAL CHARACTERISTICS
SYMBOL VBUCKFB PARAMETER ReferenceRegulationVoltage (Note6) ReferenceRegulationVoltage (Note7) 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 VINShutdownQuiescentCurrent OscillatorFrequency RDS(ON)ofP-ChannelMOSFET RDS(ON)ofN-ChannelMOSFET SWLeakage InputPinHighThreshold InputPinLowThreshold InputPinCurrent ISW=100mA ISW=100mA Enable=0V,VSW=0Vor6V,VIN=6V MODE,ENBUCK,ENVLDO MODE,ENBUCK,ENVLDO

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise specified (Note 2)
CONDITIONS ENBUCK=VIN,ENVLDO=0V,TA=25C ENBUCK=VIN,ENVLDO=0V,0CTA85C ENBUCK=VIN,ENVLDO=0V,-40CTA85C VLFB ENBUCK=0V,ENVLDO=VIN,TA=25C ENBUCK=0V,ENVLDO=VIN,0CTA85C ENBUCK=0V,ENVLDO=VIN,-40CTA85C IS LVIN=1.5V,LVOUT=1.2V,ENBUCK=VIN, ENVLDO=VIN,MODE=0V,IOUT(VLDO)=10A, VBUCKFB=0.9V LVIN=1.5V,LVOUT=1.2V,ENBUCK=VIN, ENVLDO=VIN,MODE=0V,IOUT(VLDO)=10A, VBUCKFB=0.7V LVIN=1.5V,LVOUT=1.2V,ENBUCK=VIN, ENVLDO=VIN,MODE=VIN,IOUT(VLDO)=10A, VBUCKFB=0.7V VBUCKFB=0.9V,IOUT(BUCK)=0A,ENBUCK=VIN, ENVLDO=0V,MODE=0V VBUCKFB=0.7V,IOUT(BUCK)=0A,ENBUCK=VIN, ENVLDO=0V,MODE=0V VBUCKFB=0.7V,IOUT(BUCK)=0A,ENBUCK=VIN, ENVLDO=0V,MODE=VIN LVOUT=1.2V,ENBUCK=0V,ENVLDO=VIN, IOUT(LREG)=10A ENBUCK=0V,ENVLDO=0V

MIN 0.784 0.782 0.78 0.392 0.391 0.390
TYP 0.8 0.8 0.8 0.4 0.4 0.4 85 315 300 55 300 285 50 2.5 0.1
MAX 0.816 0.818 0.82 0.408 0.409 0.410
UNITS V V V V V V A A A A A A A A A
LVINShutdownQuiescentCurrent LVIN=3.6V,ENBUCK=0V,ENVLDO=0V fOSC RPFET RNFET ILSW VIH VIL IMODE, IENBUCK, IENVLDO 1.8
2.25 0.25 0.35 0.01
2.7
MHz
1 0.3
A V V A
0.9 0.01 1
Note 1:StressesbeyondthoselistedunderAbsoluteMaximumRatings maycausepermanentdamagetothedevice.ExposuretoanyAbsolute MaximumRatingconditionforextendedperiodsmayaffectdevice reliabilityandlifetime. Note 2:TheLTC3541-1isguaranteedtomeetperformancespecifications from0Cto85C.VLDO/linearregulatoroutputistestedandspecified underpulseloadconditionssuchthatTJTA,andare100%production testedat25C.Specificationsoverthe-40Cto85Coperating temperaturerangeareassuredbydesign,characterizationandcorrelation withstatisticalprocesscontrols. Note 3:MinimumoperatingLVINvoltagerequiredforVLDOregulator regulationis: LVINLVOUT+VDROPOUTandLVIN0.9V
Note 4:MinimumoperatingVINvoltagerequiredforVLDOregulatorand linearregulatorregulationis: VINLVOUT+1.4VandVIN2.7V Note 5:TJiscalculatedfromtheambienttemperature,TA,andpower dissipation,PD,accordingtothefollowingformula: TJ=TA+(PD*43C/W) Note 6:TheLTC3541-1istestedinaproprietarytestmodethatconnects VBUCKFBtotheoutputoftheerroramplifier.Forthereferenceregulation andlineregulationtests,theoutputoftheerroramplifierissettothe midpoint.Fortheloadregulationtest,theoutputoftheerroramplifieris driventominimumandmaximumofthesignalrange.
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LTC3541-1 ELECTRICAL CHARACTERISTICS
Note 7:Measurementmadeinclosedlooplinearregulatorconfiguration withLVOUT=1.2V,ILOAD=10A. Note 8:Measurementmadeinaproprietarytestmodewithslope compensationdisabled. Note 9:Measurementisassuredbydesign,characterizationandstatistical processcontrol. Note 10:ThisICincludesovertemperatureprotectionthatisintended toprotectthedeviceduringmomentaryoverloadconditions.Junction temperaturewillexceed125Cwhenovertemperatureprotectionisactive. Continuousoperationabovethespecifiedmaximumoperatingjunction temperaturemayimpairdevicereliability.
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage for Buck (Burst)
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 2 3 4 5 INPUT VOLTAGE (V) 6
35411 G01
VOUT = 1.8V IOUT = 500mA EFFICIENCY (%) IOUT = 100mA IOUT = 30mA
80 75 70 65 60 55 50 2 3 4 5 INPUT VOLTAGE (V) 6
35411 G02
EFFICIENCY (%)
Efficiency vs Load Current for Buck (Pulse Skip)
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 100 LOAD CURRENT (mA) 1000
35411 G04
VOUT = 1.8V
VIN = 2.7V VIN = 3.6V VIN = 4.2V EFFIcIENcY (%)
60 50 40 30 20 10 VOUT = 2.5V 0 0.1 1 10 100 LOAD cURRENT (mA)
EFFICIENCY (%)
UW
Efficiency vs Input Voltage for Buck (Pulse Skip)
100 95 90 85 VOUT = 1.8V IOUT = 500mA 100 90 80 70 60 50 40 30 20 10
Efficiency vs Load Current for Buck (Burst)
VIN = 2.7V VIN = 4.2V VIN = 3.6V
IOUT = 100mA IOUT = 30mA
VOUT = 1.8V 0 0.1 1 10 100 LOAD CURRENT (mA)
1000
35412 G03
Efficiency vs Load Current for Buck (Burst)
100 90 80 70 VIN = 2.7V VIN = 3.6V VIN = 4.2V 100 90 80 70 60 50 40 30 20 10 1000
35411 G05
Efficiency vs Load Current for Buck (Pulse Skip)
VOUT = 2.5V VIN = 2.7V VIN = 3.6V VIN = 4.2V
0 0.1
1
10 100 LOAD CURRENT (mA)
1000
35411 G06
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LTC3541-1 TYPICAL PERFOR A CE CHARACTERISTICS
VLDO Dropout Voltage vs Load Current
100 VOUT = 1.5V VIN = 3V BIAS CURRENT (A) VIN = 3.6V VIN = 4.2V 250 200
DROPOUT VOLTAGE (mV)
80
60
40
20 0
0
50
100 150 200 LOAD CURRENT (mA)
Oscillator Frequency vs Temperature
2.50 2.45 2.40 FREQUENCY (MHz) FREQUENCY (MHz) 2.35 2.30 2.25 2.20 2.15 2.10 2.05 2.00 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125 2.0 VIN = 3.6V 2.5
REFERENCE (V)
Buck Reference vs Temperature
0.820 0.816 0.812 REFERENCE (V) 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 VIN = 3.6V 0.700 0.600 0.500 RDS(ON) () 0.400 0.300 0.200 0.100
UW
250
35411 G10
Buck (Burst) Plus VLDO Bias Current vs VLDO Load Current
VIN = 3.6V ILOAD(BUCK) = 0 IBIAS = IVIN + ILVIN - ILOAD VOUT 2V/DIV LVOUT 2V/DIV VIN 2V/DIV
Output (Auto Start-Up Sequence, Buck in Pulse Skip) vs Time
150
100
50
IVOUT = 400mA ILVOUT = 30mA 1 10 100 LOAD CURRENT (mA) 1000
35411 G08
4ms/DIV
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300
0 0.1
35411 G07
Oscillator Frequency vs Supply Voltage
VIN = 3.6V 0.410 0.408 0.406 0.404 0.402 0.400 0.398 0.396 0.394 0.392 3 5 4 SUPPLY VOLTAGE (V) 6
35411 G11
VLDO/Linear Regulator Reference vs Temperature
VIN = 3.6V
2.4
2.3
2.2
2.1
0.390 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
53411 G12
RDS(ON) vs Temperature
LVOUT 10mV/DIV AC COUPLED VOUT 10mV/DIV AC COUPLED
Buck (Burst) and VLDO Output
SYNCH SWITCH
MAIN SWITCH
0 -50 -25
VIN = 2.5V VIN = 3.6V VIN = 5.5V 100 125
50 25 75 0 TEMPERATURE (C)
2s/DIV VIN = 3.6V LVOUT = 1.5V VOUT = 1.8V ILOAD = 50mA Burst Mode OPERATION
35411 G15
53411 G13
35411 G14
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LTC3541-1 TYPICAL PERFOR A CE CHARACTERISTICS
Buck (Pulse Skip) Load Step from 1mA to 500mA
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV 40s/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 1mA TO 500mA
35411 G16
VLDO Load Step from 100mA to 300mA
LVOUT 20mV/DIV AC COUPLED
ILOAD 250mA/DIV
400s/DIV VIN = 3.6V LVOUT = 1.5V ILOAD = 100mA TO 300mA
VLDO to Linear Regulator Transition, Load = 1mA
LVOUT 10mV/DIV AC COUPLED
ILOAD 50mA/DIV
VIN = 3.6V LVOUT = 1.5V ILOAD = 1mA
UW
Buck (Burst) Load Step from 1mA to 500mA
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV 40s/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 1mA TO 500mA
35411 G17
VLDO Load Step from 1mA to 300mA
LVOUT 20mV/DIV AC COUPLED
ILOAD 250mA/DIV 400s/DIV VIN = 3.6V LVOUT = 1.5V ILOAD = 1mA TO 300mA
35411 G18
Linear Regulator to VLDO Transition, Load = 1mA
LVOUT 10mV/DIV AC COUPLED VOUT 10mV/DIV AC COUPLED
Linear Regulator to VLDO Transition, Load = 30mA
LVOUT 10mV/DIV AC COUPLED
ILOAD 50mA/DIV
35411 G19
VIN = 3.6V LVOUT = 1.5V ILOAD = 1mA
40s/DIV
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VIN = 3.6V 2s/DIV LVOUT = 1.5V VOUT = 1.875V ILOAD = 50mA Burst Mode OPERATION
35412 G21
VLDO to Linear Regulator Transition, Load = 30mA
LVOUT 10mV/DIV AC COUPLED ILOAD 50mA/DIV
40s/DIV
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VIN = 3.6V LVOUT = 1.5V ILOAD = 30mA
40s/DIV
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LTC3541-1 PI FU CTIO S
VIN (Pin 1):MainSupplyPin.Thispinmustbeclosely decoupledtoGNDwitha10Forgreatercapacitor. ENBUCK (Pin 2):BuckEnablePin.Thispinenablesthe buckregulatorwhendriventoalogichigh. BUCKFB (Pin 3):BuckRegulatorFeedbackPin.Thispin receivesthebuckregulator'sfeedbackvoltagefroman externalresistivedivider. LFB (Pin 4):VLDO/LinearRegulatorFeedbackPin.This pinreceiveseithertheVLDOorlinearregulator'sfeedback voltagefromanexternalresistivedivider. LVOUT (Pin 5):VLDO/LinearRegulatorOutputPin.This pinprovidestheregulatedoutputvoltagefromtheVLDO orlinearregulator. LVIN (Pin 6): VLDO/Linear Regulator Input Supply Pin. ThispinprovidestheinputsupplyvoltagefortheVLDO powerFET. GND (Pin 7):AnalogGroundPin.
Table 1. LTC3541-1 Control Pin Truth Table
PIN NAME ENBUCK 0 0 ENVLDO 0 1 MODE X X LTC3541-1PoweredDown BuckPoweredDown,VLDORegulator PoweredDown,LinearRegulator Enabled BuckEnabled,VLDORegulatorPowered Down,LinearRegulatorPoweredDown, BurstModeOperation BuckEnabled,VLDORegulatorPowered Down,LinearRegulatorPoweredDown, Pulse-SkipModeOperation BuckEnabled,VLDORegulatorEnabled, LinearRegulatorPoweredDown,Burst ModeOperation BuckEnabled,VLDORegulatorEnabled, LinearRegulatorPoweredDown,PulseSkipModeOperation OPERATIONAL DESCRIPTION
1
0
0
1
0
1
1
1
0
1
1
1
U
U
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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, andenablestheVLDOwhentheENBUCKpinisdrivento alogichigh. 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 oftheLTC3541-1.
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LTC3541-1 W
2.2H VIN(MIN) LVOUT + 1.4V 10 1 VIN SW IOUT(BUCK) = 500mA 10F 22pF BUCKFB PGND LVIN 3 6 VLDO/LINEAR REG ENBUCK ENVLDO MODE CONTROL LOGIC REF REF VIN LVIN LVOUT(MAX) < VIN - 1.4V IOUT = 300mA (VLDO REG) IOUT = 30mA (LINEAR REG) LVOUT GND LFB GND 7 PGND 11
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FU CTIO AL BLOCK DIAGRA U U
500mA BUCK VIN SW REF FB GND
2 9 8
+ -
5
LFB CNTRL
2.2F 4
Figure 1. LTC3541-1 Functional Block Diagram
TheLTC3541-1containsahighefficiencysynchronous buckconverter,averylowdropoutregulator(VLDO)and a linear regulator. It can be used to provide up to two outputvoltagesfromasingleinputvoltagemakingthe LTC3541-1idealforapplicationswithlimitedboardspace. Thecombinationandconfigurationofthesemajorblocks withintheLTC3541-1isdeterminedbywayofthecontrol pinsENBUCKandENVLDOasdefinedinTable1. WiththeENBUCKpindriventoalogichighandENVLDO driven to a logic low, the LTC3541-1 enables the buck converter to efficiently reduce the voltage provided at theVINinputpintoanoutputvoltagewhichissetbyan external feedback resistor network. The buck regulator canbeconfiguredforPulse-SkiporBurstModeoperationbydrivingtheMODEpintoalogichighorlogiclow respectively.Thebuckregulatoriscapableofprovidinga maximumoutputcurrentof500mA,whichmustbetaken
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intoconsiderationwhenusingthebuckregulatortoprovide thepowerforboththeVLDOandforexternalloads. WiththeENBUCKpindriventoalogiclowandENVLDO driventoalogichigh,theLTC3541-1enablesthelinear regulator,providingalownoiseregulatedoutputvoltage attheLVOUTpinwhiledrawingminimalquiescentcurrent fromtheVINinputpin.Thisfeatureallowsoutputvoltage LVOUTtobebroughtintoregulationwithoutthepresence oftheLVINvoltage. With the ENBUCK and ENVLDO pins both driven to a logic high, the LTC3541-1 enables the high efficiency buckconverterandVLDOregulator,providingdualoutput operationfromasingleinputvoltage.Whenconfigured inthismanner,theLTC3541-1'sautostart-upsequencing featurewillbringtheVLDO/linearregulatoroutputinto regulationinacontrolledmannerpriortoenablingthe
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OPERATIO
LTC3541-1
buckregulatorwithouttheneedforexternalpincontrol. AdetaileddiscussionofthetransitionsbetweentheVLDO and linear regulator can be found in the VLDO/Linear RegulatorLoopsection. Buck Regulator Control Loop TheLTC3541-1internalbuckregulatorusesaconstant 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,theLTC3541-1 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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WhentheMODEpinisdriventoalogichightheLTC3541-1 operatesinPulse-Skipmodeforlowoutputvoltageripple. In this mode, the LTC3541-1 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 IntheLTC3541-1,theVLDOandlinearregulatorloops consist of an amplifier and N-channel MOSFET output stages that, when connected with the proper external components,willservotheoutputtomaintainaregulatoroutputvoltage,LVOUT.Theinternalreferencevoltage providedtotheamplifieris0.4Vallowingforawiderange ofoutputvoltages.LoopconfigurationsenablingtheVLDO orthelinearregulatorarestablewithanoutputcapacitance aslowas2.2Fandashighas100F.BoththeVLDO andthelinearregulatorsarecapableofoperatingwithan inputvoltage,VIN,aslowas2.7V,butaresubjecttothe constraintthatVINmustbegreaterthanLVOUT+1.4V. TheVLDOisdesignedtoprovideupto300mAofoutput currentataverylowLVINtoLVOUTvoltage.Thisallows aclean,secondary,analogsupplyvoltagetobeprovided withaminimumdropinefficiency.TheVLDOisprovided with thermal protection that is designed to disable the VLDOfunctionwhentheoutput,passtransistor'sjunction temperaturereachesapproximately160C.Inadditionto thermalprotection,short-circuitdetectionisprovidedto disabletheVLDOfunctionwhenashort-circuitcondition is sensed. This circuit is designed such that an output currentofapproximately1Acanbeprovidedbeforethis circuitwilltrigger.AsdetailedintheElectricalCharacteristics,theVLDOregulatorwillbeoutofregulationwhen thiseventoccurs.Boththethermalandshort-circuitfaults when detected are treated as catastrophic fault condi35411fa
OPERATIO
LTC3541-1
tions.TheLTC3541-1willberesetuponthedetectionof eitherevent.TheN-channelMOSFETincorporatedinthe VLDOhasitsdrainconnectedtotheLVINpinasshown inFigure1.Toensurereliableoperation,theLVINvoltage mustbestablebeforetheVLDOisenabled.Forthecase wherethevoltageontheLVINpinissuppliedbythebuck regulator,theinternalpowersupplysequencinglogicassuresvoltagesareappliedintheappropriatemanner.For thecasewhereanexternalsupplyisusedtopowerthe LVINpin,thevoltageontheLVINpinmustbestablebefore theENVLDOpinisbroughtfromalowtoahigh.Further, theexternalLVINvoltagemustbereducedinconjunction withVINwheneverVINispulledloworremoved. Thelinearregulatorisdesignedtoprovidealoweroutput current(30mA)thanthatavailablefromtheVLDO.The linearregulator'soutputpasstransistorhasitsdraintied totheVINrail.Thisallowsthelinearregulatortobeturned onpriorto,andindependentof,thebuckregulatorwhich wouldordinarilydrivetheVLDOinatypicalapplication. Thelinearregulatorisprovidedwiththermalprotectionthat 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.TheLTC3541-1 willberesetuponthedetectionofeitherevent. TheN-channelMOSFETincorporatedinthelinearregulator hasitsdrainconnectedtotheVINpinasshowninFigure1. ThesizeofthisMOSFETanditsassociatedpowerbussing isdesignedtoaccommodate30mAofDCcurrent.Currents abovethiscanbesupportedforshortperiodsasstipulated intheAbsoluteMaximumRatingssection.
0
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TransitioningfromlinearregulatormodetoVLDOmode, accomplishedbybringingENBUCKfromalogiclowtoa logichighwhileENVLDOisalogichigh,isdesignedtobe asseamlessandtransientfreeaspossible.Theprecise transient response of LVOUT due to this transition is a functionofCOUTandtheloadcurrent.Waveformsgiven in the Typical Performance Characeristics show typical transientresponsesusingtheminimumCOUTof2.2Fand loadcurrentsof1mAand30mArespectively.Generally,the amplitudeofanytransientspresentwilldecreaseasCOUT isincreased.Toensurereliableoperationandadherence to the load regulation limits presented in the Electrical Characteristicstable,theloadcurrentmustnotexceed thelinearregulatorIOUTlimitof30mAwithin20msafter ENBUCKhastransitionedtoalogichigh.The300mAIOUT limitofVLDOappliesthereafter.Further,forconfigurations thatdonotusetheLTC3541-1'sbuckregulatortoprovide theVLDOinputvoltage(LVIN),theusermustensurea stableLVINvoltageispresentnolessthan1mspriorto ENBUCKtransitioningtoalogichigh. In a similar manner, transitioning from VLDO mode to linearregulatormode,accomplishedbybringingENBUCK fromalogichightoalogiclowwhileENVLDOisalogic high,isdesignedtobeasseamlessandtransientfreeas possible.Again,theprecisetransientresponseofLVOUT duetothistransitionisafunctionofCOUTandtheload current. Waveforms given in the Typical Performance Characeristics show typical transient responses using theminimumCOUTof2.2Fandloadcurrentsof1mA and30mArespectively.Generally,theamplitudeofany transientspresentwilldecreaseasCOUTisincreased.To ensurereliableoperationandadherencetotheloadregulationlimitspresentedintheElectricalCharactersticstable, theloadcurrentmustnotexceedthelinearregulatorIOUT limitof30mA1mspriortoENBUCKtransitioningtoalogic lowandthereafer.Further,forconfigurationsthatdonot usetheLTC3541-1'sbuckregulatortoprovidetheVLDO inputvoltage(LVIN),theusermustcontinuetoensurea stableLVINvoltagenolessthan1msafterENBUCKhas transitionedtoalogiclow.
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OPERATIO
LTC3541-1
ThebasicLTC3541-1applicationcircuitisshownonthe firstpageofthisdatasheet.Externalcomponentselection isdrivenbytheloadrequirementandrequirestheselection ofL,followedbyCIN,COUT,andfeedbackresistorvalues forthebuckandtheselectionoftheoutputcapacitorand feedbackvaluesfortheVLDOandlinearregulator. BUCK REGULATOR Inductor Selection Formostapplications,theappropriateinductorvaluewill beintherangeof1.5Hto3.3Hwith2.2Hthemost commonly used. The exact inductor value is chosen largely based on the desired ripple current and burst rippleperformance.Generally,largevalueinductorsreduceripplecurrent,andconversely,smallvalueinductors producehigherripplecurrent.HigherVINorVOUTmay alsoincreasetheripplecurrentasshowninEquation1. 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 ratherthanwhattheLTC3541-1requirestooperate.Table2 showssometypicalsurfacemountinductorsthatwork wellinLTC3541-1applications.
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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 1.5 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 0.06 0.10 2.0 1.5 1.3 1.1 0.950 0.770 2.7 2.2 1.0 1.3 1.00 0.79 1.00 0.72 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 Sumida CDRH2D11/HP 2.5x3.2x2.0 3.2x3.2x1.2
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.Thissimpleworst-caseconditioniscommonlyusedfordesign.Notethatthecapacitormanufacturer's ripplecurrentratingsareoftenbasedon2000hoursof life.Thismakesitadvisabletofurtherderatethecapacitororchooseacapacitorratedatahighertemperature thanrequired.Alwaysconsultthemanufacturerwithany questionregardingpropercapacitorchoice. TheselectionofCOUTforthebuckregulatorisdrivenby thedesiredbucklooptransientresponse,requiredeffective seriesresistance(ESR)andburstrippleperformance. TheLTC3541-1minimizestherequirednumberofexternal components by providing internal loop compensation forthebuckregulatorloop.Loopstability,transientresponseandburstperformancecanbetailoredbychoice ofoutputcapacitance.Formanyapplications,desirable stability,transientresponseandrippleperformancecan
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LTC3541-1
be obtained by choosing an output capacitor value of 10Fto22F.Typically,oncetheESRrequirementfor COUThasbeenmet,theRMScurrentratinggenerallyfar exceedstheIRIPPLE(P-P)requirement.Theoutputripple VOUTisdeterminedby: 1 VOUT IL ESR + 8 fc
OUT
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.SincetheLTC3541-1'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 thelongwirescanpotentiallycauseavoltagespikeatVIN, largeenoughtodamagethepart.
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Whenchoosingtheinputandoutputceramiccapacitors, choose the X5R or X7R dielectric formulations. These dielectricshavethebesttemperatureandvoltagecharacteristicsofalltheceramicsforagivenvalueandsize. Output Voltage Programming TheoutputvoltageissetbytyingBUCKFBtoaresistive divideraccordingtothefollowingformula:
R2 VOUT = 0.8 V 1+ R1
APPLICATIO S I FOR ATIO W U U
SincetheimpedanceattheBUCKFBpinisdependantupon theresistordividernetworkused,andphaseshiftdueto thisimpedancedirectlyimpactsthetransientresponseof thebuck,R1shouldbechosen<125k.Inaddition,stray capacitanceatthispinshouldbeminimized(<5pF)topreventexcessivephaseshift.Finally,specialattentionshould begiventoanystraycapacitancesthatcancoupleexternal signalsontotheBUCKFBpinproducingundesirableoutput ripple. For optimum performance connect the BUCKFB pintoR1andR2withashortPCBtraceandminimizeall otherstraycapacitancetotheBUCKFBpin.
Theexternalresistivedividerisconnectedtotheoutput, allowingremotevoltagesensingasshowninFigure6. Checking Transient Response Theregulatorloopresponsecanbecheckedbylooking attheloadtransientresponse.Switchingregulatorstake severalcyclestorespondtoastepinloadcurrent.When aloadstepoccurs,VOUTimmediatelyshiftsbyanamount equalto(ILOAD*ESR),whereESRistheeffectiveseries
0.8V VOUT 5V R2 BUcKFB LTc3541-1 GND
35411 F06
R1
Figure 6. Setting the LTC3541-1 Output Voltage
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LTC3541-1
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 Adjustable Output Voltage TheLTC3541-1LVOUToutputvoltageissetbytheratioof twoexternalresistorsasshowninFigure7.Thedevice servos LVOUT to maintain the LFB pin voltage at 0.4V (referencedtoground).Thus,thecurrentinR1isequal to0.4V/R1.Forgoodtransientresponse,stability,andaccuracy,thecurrentinR1shouldbeatleast2A,thusthe valueofR1shouldbenogreaterthan200k.Thecurrent inR2isthecurrentinR1plustheLFBpinbiascurrent. SincetheLFBpinbiascurrentistypically<10nA,itcanbe ignoredintheoutputvoltagecalculation.TheoutputvoltagecanbecalculatedusingtheformulainFigure8.Note thatinshutdowntheoutputisturnedoffandthedivider currentwillbezeroonceCOUTisdischarged. TheLTC3541-1VLDOandlinearregulatorloopsoperate at a relativelyhigh gain of -3.5V/mA and -3.4V/mA respectively,referredtotheLFBinput.Thus,aloadcurrentchangeof1mAto300mAproducesa1.05mVdrop attheLFBinputfortheVLDOandaloadcurrentchange of1mAto30mAproducesa0.1mVdropattheLFBinput
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LVOUT LTc3541-1 LFB R1 GND
35411 F07
APPLICATIO S I FOR ATIO W U U
R2
VOUT = 0.4V 1 + R2 R1 cOUT
()
Figure 7. Programming the LTC3541-1
forthelinearregulator.Tocalculatethechangereferred totheoutputsimplymultiplybythegainofthefeedback network(i.e.,1+R2/R1).Forexample,toprogramthe outputfor1.2VchooseR2/R1=2.Inthisexample,an outputcurrentchangeof1mAto300mAproduces1.05mV *(1+2)=3.15mVdropattheoutput. SincetheLFBpinisrelativelyhighimpedance(depending ontheresistordividerused),straycapacitanceatthispin shouldbeminimized(<10pF)topreventphaseshiftinthe erroramplifierloop.Additionally,specialattentionshould begiventoanystraycapacitancesthatcancoupleexternal signals onto the LFB pin producing undesirable output ripple.ForoptimumperformanceconnecttheLFBpinto R1andR2withashortPCBtraceandminimizeallother straycapacitancetotheLFBpin. Output Capacitance and Transient Response TheLTC3541-1isdesignedtobestablewithawiderange ofceramicoutputcapacitors.TheESRoftheoutputcapacitoraffectsstability,mostnotablywithsmallcapacitors.A minimumoutputcapacitorof2.2FwithanESRof0.05 orlessisrecommendedtoensurestability.TheLTC3541-1 VLDOisamicropowerdeviceandoutputtransientresponse will be a function of output capacitance. Larger values ofoutputcapacitancedecreasethepeakdeviationsand provideimprovedtransientresponseforlargerloadcurrent changes.Notethatbypasscapacitorsusedtodecouple individual components powered by the LTC3541-1 will increasetheeffectiveoutputcapacitorvalue.HighESR tantalum and electrolytic capacitors may be used, but alowESRceramiccapacitormustbeinparallelatthe output.ThereisnominimumESRormaximumcapacitor sizerequirement.
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LTC3541-1
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 8 and 9. Whenusedwitha2Vregulator,a1FY5Vcapacitorcan loseasmuchas75%ofitsinitialcapacitanceoverthe operatingtemperaturerange.TheX5RandX7Rdielectrics resultinmorestablecharacteristicsandareusuallymore suitableforuseastheoutputcapacitor.TheX7Rtypehas betterstabilityacrosstemperature,whiletheX5Risless
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
35411 F08
Figure 8. 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
35411 F09
Figure 9. Change in Capacitor vs Temperature
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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,threemainsourcestypicallyaccountforthemajority ofthelossesintheLTC3541-1circuits:VINquiescentcurrent,I2RlossesandlossacrossVLDOoutputdevice.When operatingwithboththebuckandVLDOactive(ENBUCK andENVLDOequaltologichigh),VINquiescentcurrent lossandlossacrosstheVLDOoutputdevicedominate theefficiencylossatlowloadcurrents,whereastheI2R lossandlossacrosstheVLDOoutputdevicedominate theefficiencylossatmediumtohighloadcurrents.At lowloadcurrentswiththepartoperatingwiththelinear regulator(ENBUCKequaltologiclow,ENVLDOequalto logichigh),efficiencyistypicallydominatedbytheloss acrossthelinearregulatoroutputdeviceandVINquiescent current.Inatypicalefficiencyplot,theefficiencycurveat verylowloadcurrentscanbemisleadingsincetheactual powerlostisoflittleconsequence. 1.TheVINquiescentcurrentlossinthebuckisduetotwo components:theDCbiascurrentasgivenintheElectrical Characteristicsandtheinternalmainswitchandsynchronousswitchgatechargecurrents.Thegatechargecurrent resultsfromswitchingthegatecapacitanceoftheinternal powerswitches.Eachtimethegateisswitchedfromhigh tolowtohighagain,apacketofcharge,dQ,movesfrom VINtoground.TheresultingdQ/dtisthecurrentoutof VINthatistypicallylargerthantheDCbiascurrentand
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APPLICATIO S I FOR ATIO W U U
LTC3541-1
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. OtherlosseswhenthebuckandVLDOareinoperation (ENBUCKandENVLDOequallogichigh),includingCIN andCOUTESRdissipativelossesandinductorcorelosses, generallyaccountforlessthan2%totaladditionalloss. THERMAL CONSIDERATIONS The LTC3541-1 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. TheLTC3541-1hasinternalthermallimitingdesignedto protectthedeviceduringmomentaryoverloadconditions. Forcontinuousnormalconditions,themaximumjunction temperatureratingof125Cmustnotbeexceeded.Itis importanttogivecarefulconsiderationtoallsourcesof thermalresistancefromjunctiontoambient.Additional heatsourcesmountednearbymustalsobeconsidered.
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Forsurfacemountdevices,heatsinkingisaccomplished byusingtheheat-spreadingcapabilitiesofthePCboard anditscoppertraces.Copperboardstiffenersandplated throughholescanalsobeusedtospreadtheheatgeneratedbypowerdevices. ToavoidtheLTC3541-1exceedingthemaximumjunction temperature,somethermalanalysisisrequired.Thegoal ofthethermalanalysisistodeterminewhetherthepower dissipatedexceedsthemaximumjunctiontemperatureof thepart.Thetemperatureriseisgivenby: TR=PD*JA wherePDisthepowerdissipatedbytheregulatorandJA isthethermalresistancefromthejunctionofthedieto theambienttemperature. Thejunctiontemperature,TJ,isgivenby: TJ=TA+TR whereTAistheambienttemperature. As an example, consider the LTC3541-1 with an input voltage VIN of 2.9V, an LVIN voltage of 1.8V, an LVOUT voltageof1.5V,aloadcurrentof200mAforthebuck, aloadcurrentof300mAfortheVLDOandanambient temperatureof85C.Fromthetypicalperformancegraph ofswitchresistance,theRDS(ON)oftheP-channelswitch at85Cisapproximately0.25.TheRDS(ON) oftheNchannelswitchisapproximately0.4.Therefore,power dissipatedbythepartisapproximately: PD=(ILOADBUCK)2*RSW+(ILOADVLDO)* (LVIN-LVOUT)=167mW Forthe3mmx3mmDFNpackage,theJAis43C/W. Thus,thejunctiontemperatureoftheregulatoris: TJ=85C+(0.167)(43)=92C whichiswellbelowthemaximumjunctiontemperature of125C. Notethatathighersupplyvoltages,thejunctiontemperatureislowerduetoreducedswitchresistanceRDS(ON).
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APPLICATIO S I FOR ATIO W U U
LTC3541-1
PC BOARD LAYOUT CHECKLIST Whenlayingouttheprintedcircuitboard,thefollowing checklistshouldbeusedtoensureproperoperationofthe LTC3541-1.Checkthefollowinginyourlayout: 1.Thepowertraces,consistingoftheGNDtrace,theSW traceandtheVINtraceshouldbekeptshort,directand wide. 2.DoestheLFBpinconnectdirectlytothefeedbackresistors?TheresistivedividerR1/R2mustbeconnected betweenthe(+)plateofCOUTandground. 3.Doesthe(+)plateofCINconnecttoVINascloselyas possible?ThiscapacitorprovidestheACcurrenttothe internalpowerMOSFETs. 4.Keeptheswitchingnode,SW,awayfromthesensitive LFBnode. 5.Keepthe(-)platesofCINandCOUTascloseaspossible. DESIGN EXAMPLE Asadesignexample,assumetheLTC3541-1isusedin asinglelithium-ionbatterypoweredcellularphoneapplication.TheVINwillbeoperatingfromamaximumof 4.2Vdowntoabout3V.Theloadcurrentrequirementis amaximumof0.5Aforthebuckoutputbutmostofthe timeitwillbeinstandbymode,requiringonly2mA.Efficiencyatbothlowandhighloadcurrentsisimportant. Theoutputvoltageforthebuckis1.8V.Therequirement fortheoutputvoltageoftheVLDOis1.5Vwhileprovid-
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ingupto0.3Aofcurrent.Withthisinformationwecan calculateLusingEquation2: L= V 1 VOUT 1- OUT VIN ( f )( IL ) (2) SubstitutingVOUT=1.8V,VIN=3.6V(typ),IL=0.2Aand f=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)/2attemperature.COUTforthebuckischosen as22FwithanESRoflessthan0.2.Inmostcases,a ceramiccapacitorwillsatisfythisrequirement. Forthefeedbackresistorsofthebuck,chooseR1=80k. R2canthenbecalculatedfromEquation4tobe: V R2 = OUT - 1 R1= 100k 0.8 (4) ForthefeedbackresistorsoftheVLDO,chooseR1=200k. R2canthenbecalculatedfromEquation5tobe: V R2 = OUT - 1 R1= 550k 0.4 COUTfortheVLDOischosenas2.2F.
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APPLICATIO S I FOR ATIO W U U
LTC3541-1 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 3.2V TO 4.2V
VIN VOUT1 2.5V 400mA 22pF 154k 2.2H
ENBUCK MODE SW LVIN GND 165k 576k BUCKFB LVOUT PGND 2.2F VOUT2 1.8V 30mA LFB LTC3541-1
10F
73k
Dual Output with Minimal External Components Using Auto Start-Up Sequence, Buck in Pulse Skip Mode for Low Noise Operation
VIN 3.2V TO 4.2V
VIN VOUT1 2.5V 400mA 22pF 154k 2.2H
ENBUCK MODE SW LVIN GND 165k 576k BUCKFB LVOUT PGND 2.2F VOUT2 1.8V 30mA LFB LTC3541-1
10F
73k
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VOUT 2V/DIV ENVLDO LVOUT 2V/DIV
VIN 2V/DIV
35411 TA02a
IVOUT = 400mA ILVOUT = 30mA
4ms/DIV
3541 TA02b
VOUT 2V/DIV ENVLDO LVOUT 2V/DIV
VIN 2V/DIV
35411 TA03a
IVOUT = 400mA ILVOUT = 30mA
4ms/DIV
3541 TA03b
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LTC3541-1 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 3.2V TO 4.2V VIN VOUT1 2.5V 200mA 22pF 154k 2.2H ENVLDO
ENBUCK MODE SW LVIN BUCKFB GND 165k 576k LVOUT PGND 2.2F VOUT2 1.8V 300mA LFB LTC3541-1
10F
73k
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 2.9V TO 4.2V VOUT 2V/DIV VIN VOUT1 1.8V 200mA 22pF 143k 2.2H ENVLDO LVOUT 2V/DIV 150k 412k BUCKFB 10F 115k LVOUT PGND 2.2F VOUT2 1.5V 300mA VIN 2V/DIV ENBUCK MODE SW GND LTC3541-1 LVIN LFB
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VOUT 2V/DIV LVOUT 2V/DIV
VIN 2V/DIV
35411 TA04a
IVOUT = 200mA ILVOUT = 300mA
4ms/DIV
35411 TA04b
35411 TA05a
IVOUT = 200mA ILVOUT = 30mA
4ms/DIV
35411 TA05b
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LTC3541-1 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.
LTC3541-1 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)VLDO=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 VIN:2.9Vto5.5V,VOUT(BUCK)=1.875V,VOUT(VLDO)=1.5V,3mmx3mm 10-PinDFNPackage VIN:3Vto5.5V,VOUT(BUCK)=1.8V,VOUT(VLDO)=1.575V,3mmx3mm 10-PinDFNPackage 95%Efficiency,VIN:2.5Vto5.5V,VOUT(MIN)=0.6V,IQ=40A,ISD<1A, 8-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 HighEfficiencyBuckplusVLDORegulator HighEfficiencyBuckplusVLDORegulator Dual300mA(IOUT),2.25MHz,Synchronous Step-DownDC/DCConverter
LTC3446 LTC3448 LTC3541 LTC3541-2 LTC3541-3 LTC3547
LTC3548/LTC3548-1 Dual800mA/400mA(IOUT),2.25MHz,Synchronous Step-DownDC/DCConverter LTC3548-2 LTC3700 Step-DownDC/DCControllerwithLDORegulator PowerPathisatrademarkofLinearTechnologyCorporation.
35411fa
0 Linear Technology Corporation
1630 Mccarthy Blvd., Milpitas, cA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0407 REV A * PRINTED IN USA
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2007

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