View LTC3541_5052280.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

LTC3541 High Efficiency Buck + VLDO Regulator
High Efficiency, 500mA Buck Plus 300mA VLDOTM Regulator Auto Start-Up Powers Buck Output Prior to VLDO/Linear 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)3541combinesasynchronousbuckDC/DC converterwithaverylowdropoutlinearregulator (VLDO)toprovideuptotwooutputvoltagesfromasingle inputvoltagewithminimalexternalcomponents.When configuredfordualoutputoperation,theLTC3541'sauto start-upfeaturewillbringtheBuckoutputintoregulation inacontrolledmannerpriortoenablingtheVLDOregulator outputwithouttheneedforexternalpincontrol.VLDO/ linearregulatoroutputpriortoBuckoutputsequencing mayalsobeobtainedviaexternalpincontrol.Theinput voltagerangeisideallysuitedforLi-Ionbattery-powered applications,aswellaspoweringsub-3.3Vlogicfrom5V or3.3Vrails. 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 and LTM 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 Typical Application
VIN 2.9V TO 5.5V
EFFICIENCY (%)
SW 2.2H 243k VOUT1 2.5V 200mA 22pF VIN
ENVLDO 150k
MODE LTC3541 ENBUCK GND BUCKFB LVIN LFB
LVOUT PGND
10F
115k
U
TYPICAL APPLICATIO
Buck (Burst) Efficiency and Power Loss vs Load Current
100 90 80 70 60 50 40 30 20
VOUT2 1.5V 300mA
412k
10 0 1
VIN = 3.3V VOUT = 2.5V 10 100 LOAD CURRENT (mA)
2.2F
3541 TA01a
U
1 EFFICIENCY 0.1 POWER LOSS (W) POWER LOSS 0.01 0.001 0.0001 1000
3541 TA01b
FEATURES
DESCRIPTIO
U
3541fa
LTC3541
(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 .
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
ORDERPARTNUMBER LTC3541EDD
DDPARTMARKING LCBS
Order OptionsTapeandReel:Add#TR LeadFree:Add#PBFLeadFreeTapeandReel:Add#TRPBF LeadFreePartMarking:http://www.linear.com/leadfree/ ConsultLTCMarketingforpartsspecifiedwithwideroperatingtemperatureranges.
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)

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 0.784 0.782 0.78 0.392 0.391 0.8 0.8 0.8 0.4 0.4 0.4
0.4
VLDOVINLineRegulation (ReferredtoLFB) LinearRegulatorVINLine Regulation(ReferredtoLFB) LVIN(LINEREG) LVINLineRegulation (ReferredtoLFB) VLDODO VLOADREG LVIN-LVOUTDropoutVoltage (Note9) BuckOutputLoadRegulation VLDOOutputLoadRegulation LinearRegulatorOutputLoad VBUCKFB ReferenceRegulationVoltage (Note6) ReferenceRegulationVoltage (Note7)
60
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 ENBUCK=VIN,ENVLDO=0V,TA=25C ENBUCK=VIN,ENVLDO=0V,0CTA85C ENBUCK=VIN,ENVLDO=0V,-40CTA85C
0.5 0.5 0.816 0.818 0.82 0.408 0.409 0.410
VLFB
ENBUCK=0V,ENVLDO=VIN,TA=25C ENBUCK=0V,ENVLDO=VIN,0CTA85C ENBUCK=0V,ENVLDO=VIN,-40CTA85C
0.390
3541fa
U
mV % % % V V V V V V
W
U
U
WW
W
ABSOLUTE
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
LTC3541 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 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 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
TYP 85 315 300 55 300 285 50 2.5 0.1
MAX
UNITS 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:TheLTC3541isguaranteedtomeetperformancespecifications 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:TheLTC3541istestedinaproprietarytestmodethatconnects VBUCKFBtotheoutputoftheerroramplifier.Forthereferenceregulation andlineregulationtests,theoutputoftheerroramplifierissettothe midpoint.Fortheloadregulationtest,theoutputoftheerroramplifieris driventominimumandmaximumofthesignalrange. 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.
3541fa
LTC3541 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
3541 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
3541 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
3541 G04
VOUT = 1.8V
VIN = 2.7V VIN = 3.6V VIN = 4.2V EFFIcIENcY (%)
60 50 40 30 20 10 VOUT = 2.5V 1 10 100 LOAD cURRENT (mA) 1000
3541 G05
EFFICIENCY (%)
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)
UW
250
3541 G07
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
3541 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
Efficiency vs Load Current for Buck (Pulse Skip)
VOUT = 2.5V VIN = 2.7V VIN = 3.6V VIN = 4.2V
0 0.1
0 0.1
1
10 100 LOAD CURRENT (mA)
1000
3541 G06
Buck (Burst) Plus VLDO Bias Current vs VLDO Load Current
VIN = 3.6V ILOAD(BUCK) = 0 IBIAS = IVIN + ILVIN - ILOAD
Output (Auto Start-Up Sequence, Buck in Pulse Skip) vs Time
VOUT 2V/DIV LVOUT 2V/DIV VIN 2V/DIV
150 100
50
IVOUT = 300mA ILVOUT = 200mA 1 10 100 LOAD CURRENT (mA) 1000
3541 G08
2ms/DIV
3541 G09
300
0 0.1
3541fa
LTC3541 TYPICAL PERFOR A CE CHARACTERISTICS
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 3 5 4 SUPPLY VOLTAGE (V) 6
3541 G11
VIN = 3.6V
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
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
3541 G16
UW
3541 G10
Oscillator Frequency vs Supply Voltage
2.5 VIN = 3.6V 0.410 0.408 0.406 0.404 0.402 0.400 0.398 0.396 0.394 0.392
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
5341 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
3541 G15
5341 G13
3541 G14
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
3541 G17
VLDO Load Step from 1mA to 500mA
LVOUT 20mV/DIV AC COUPLED
ILOAD 250mA/DIV 400s/DIV VIN = 3.6V LVOUT = 1.5V ILOAD = 1mA TO 300mA
3541 G18
3541fa
LTC3541 TYPICAL PERFOR A CE CHARACTERISTICS
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 Transient Step, Load = 1mA
LVOUT 10mV/DIV AC COUPLED
ILOAD 50mA/DIV
VIN = 3.6V LVOUT = 1.5V ILOAD = 1mA
UW
Linear Regulator to VLDO Transient Step, Load = 1mA
Linear Regulator to VLDO Transient Step, Load = 30mA
LVOUT 10mV/DIV AC COUPLED
LVOUT 10mV/DIV AC COUPLED ILOAD 50mA/DIV
ILOAD 50mA/DIV
3541 G19
VIN = 3.6V LVOUT = 1.5V ILOAD = 1mA
40s/DIV
3541 G20
VIN = 3.6V LVOUT = 1.5V ILOAD = 30mA
40s/DIV
3541 G21
VLDO to Linear Regulator Transient Step, Load = 30mA
LVOUT 10mV/DIV AC COUPLED ILOAD 50mA/DIV
40s/DIV
3541 G22
VIN = 3.6V LVOUT = 1.5V ILOAD = 30mA
40s/DIV
3541 G23
3541fa
LTC3541 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 Control Pin Truth Table
PIN NAME ENBUCK 0 0 ENVLDO 0 1 MODE X X LTC3541PoweredDown 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
U
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.
3541fa
LTC3541 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
3541 F01
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 Functional Block Diagram
The LTC3541 contains a high efficiency synchronous buckconverter,averylowdropoutregulator(VLDO)and a linear regulator. It can be used to provide up to two outputvoltagesfromasingleinputvoltagemakingthe LTC3541idealforapplicationswithlimitedboardspace. Thecombinationandconfigurationofthesemajorblocks withintheLTC3541isdeterminedbywayofthecontrol pinsENBUCKandENVLDOasdefinedinTable1. WiththeENBUCKpindriventoalogichighandENVLDO driventoalogiclow,theLTC3541enablesthebuckconvertertoefficientlyreducethevoltageprovidedattheVIN inputpintoanoutputvoltagewhichissetbyanexternal feedbackresistornetwork.ThebuckregulatorcanbeconfiguredforPulse-SkiporBurstModeoperationbydriving theMODEpintoalogichighorlogiclowrespectively.The buckregulatoriscapableofprovidingamaximumoutput currentof500mA,whichmustbetakenintoconsideration
U
whenusingthebuckregulatortoprovidethepowerfor boththeVLDOandforexternalloads. WiththeENBUCKpindriventoalogiclowandENVLDO driven to a logic high, the LTC3541 enables the linear regulator,providingalownoiseregulatedoutputvoltage attheLVOUTpinwhiledrawingminimalquiescentcurrent fromtheVINinputpin.Thisfeatureallowsoutputvoltage LVOUTtobebroughtintoregulationwithoutthepresence oftheLVINvoltage. With the ENBUCK and ENVLDO pins both driven to a logichigh,theLTC3541enablesthehighefficiencybuck converterandVLDOregulator,providingdualoutputoperationfromasingleinputvoltage.Whenconfiguredinthis manner,theLTC3541'sautostart-upsequencingfeature willbringthebuckoutputintoregulationinacontrolled mannerpriortoenablingtheVLDOregulatorwithoutthe
3541fa
OPERATIO
LTC3541
needforexternalpincontrol.Adetaileddiscussionofthe transitionsbetweentheVLDOandlinearregulatorcanbe foundintheVLDO/LinearRegulatorLoopsection. Buck Regulator Control Loop TheLTC3541internalbuckregulatorusesaconstantfrequency,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 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.
U
WhentheMODEpinisdriventoalogichightheLTC3541 operatesinPulse-Skipmodeforlowoutputvoltageripple. Inthismode,theLTC3541continuestoswitchataconstant frequencydowntoverylowcurrents,whereitwillbegin skippingpulsesusedtocontrolthemain(top)switchto maintaintheproperaverageinductorcurrent. 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,theVLDOandlinearregulatorloopsconsist ofanamplifierandN-channelMOSFEToutputstagesthat, when connected with the proper external components, willservotheoutputtomaintainaregulatoroutputvoltage,LVOUT.Theinternalreferencevoltageprovidedtothe amplifieris0.4Vallowingforawiderangeofoutputvoltages.LoopconfigurationsenablingtheVLDOorthelinear regulatorarestablewithanoutputcapacitanceaslowas 2.2Fandashighas100F.BoththeVLDOandthelinear regulatorsarecapableofoperatingwithaninputvoltage, VIN,aslowas2.7V,butaresubjecttotheconstraintthat VINmustbegreaterthanLVOUT+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-circuitconditionis sensed.Thiscircuitisdesignedsuchthatanoutputcurrent ofapproximately1Acanbeprovidedbeforethiscircuit willtrigger.AsdetailedintheElectricalCharacteristics,the VLDOregulatorwillbeoutofregulationwhenthisevent occurs. Both the thermal and short-circuit faults when detectedaretreatedascatastrophicfaultconditions.The
3541fa
OPERATIO
LTC3541
LTC3541willberesetuponthedetectionofeitherevent. TheN-channelMOSFETincorporatedintheVLDOhasits drainconnectedtotheLVINpinasshowninFigure1.To ensurereliableoperation,theLVINvoltagemustbestable beforetheVLDOisenabled.ForthecasewherethevoltageontheLVINpinissuppliedbythebuckregulator,the internalpowersupplysequencinglogicassuresvoltages areappliedintheappropriatemanner.Forthecasewhere anexternalsupplyisusedtopowertheLVINpin,thevoltageontheLVINpinmustbestablebeforetheENVLDOpin isbroughtfromalowtoahigh.Further,theexternalLVIN voltagemustbereducedinconjunctionwithVINwhenever VINispulledloworremoved. Thelinearregulatorisdesignedtoprovidealoweroutput current(30mA)thanthatavailablefromtheVLDO.The linearregulator'soutputpasstransistorhasitsdraintied totheVINrail.Thisallowsthelinearregulatortobeturned onpriorto,andindependentof,thebuckregulatorwhich ordinarilydrivestheVLDO.Thelinearregulatorisprovided with thermal protection that is designed to disable the linearregulatorfunctionwhentheoutputpasstransistor's junction temperature reaches approximately 160C. In additiontothermalprotection,short-circuitdetectionis providedtodisablethelinearregulatorfunctionwhena short-circuitconditionissensed.Thiscircuitisdesigned suchthatanoutputcurrentofapproximately120mAcan beprovidedbeforethiscircuitwilltrigger.Asdetailedin theElectricalCharacteristics,thelinearregulatorwillbe outofregulationwhenthiseventoccurs.Boththethermal andshort-circuitfaultsaretreatedascatastrophicfault conditions.TheLTC3541willberesetuponthedetection ofeitherevent. TheN-channelMOSFETincorporatedinthelinearregulator hasitsdrainconnectedtotheVINpinasshowninFigure1. ThesizeofthisMOSFETanditsassociatedpowerbussing isdesignedtoaccommodate30mAofDCcurrent.Currents abovethiscanbesupportedforshortperiodsasstipulated intheAbsoluteMaximumRatingssection.
0
U
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'sbuckregulatortoprovide theVLDOinputvoltage(LVIN),theusermustensurea stableLVINvoltageispresentnolessthan1mspriorto ENBUCKtransitioningtoalogichigh. 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 Characeristics show typical transient responses using theminimumCOUTof2.2Fandloadcurrentsof1mA and30mArespectively.Generally,theamplitudeofany transients present will decrease as COUT is increased. Toensurereliableoperationandadherencetotheload regulationlimitspresentedintheElectricalCharacterstics table,theloadcurrentmustnotexceedthelinearregulator IOUTlimitof30mA1mspriortoENBUCKtransitioningtoa logiclowandthereafer.Further,forconfigurationsthatdo notusetheLTC3541'sbuckregulatortoprovidetheVLDO inputvoltage(LVIN),theusermustcontinuetoensurea stableLVINvoltagenolessthan1msafterENBUCKhas transitionedtoalogiclow.
3541fa
OPERATIO
LTC3541
ThebasicLTC3541applicationcircuitisshownonthefirst pageofthisdatasheet.Externalcomponentselectionis drivenbytheloadrequirementandrequirestheselection 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 ratherthanwhattheLTC3541requirestooperate.Table2 showssometypicalsurfacemountinductorsthatwork wellinLTC3541applications.
U
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. TheLTC3541minimizestherequirednumberofexternal components by providing internal loop compensation forthebuckregulatorloop.Loopstability,transientresponseandburstperformancecanbetailoredbychoice ofoutputcapacitance.Formanyapplications,desirable stability,transientresponseandrippleperformancecan
3541fa
LTC3541
beobtainedbychoosinganoutputcapacitorvalueof10F to22F.Typically,oncetheESRrequirementforCOUThas beenmet,theRMScurrentratinggenerallyfarexceeds theIRIPPLE(P-P)requirement.TheoutputrippleVOUTis determinedby: 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 switching regulator applications. Since the LTC3541's controlloopdoesnotdependontheoutputcapacitor's ESRforstableoperation,ceramiccapacitorscanbeused freelytoachieveverylowoutputrippleandsmallcircuit size. 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.
U
Whenchoosingtheinputandoutputceramiccapacitors, choose the X5R or X7R dielectric formulations. These dielectricshavethebesttemperatureandvoltagecharacteristicsofalltheceramicsforagivenvalueandsize. Output Voltage Programming TheoutputvoltageissetbytyingBUCKFBtoaresistive divideraccordingtothefollowingformula: R2 VOUT = 0.8 V 1+ R1 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 GND
3541 F06
APPLICATIO S I FOR ATIO W U U
R1
Figure 6. Setting the LTC3541 Output Voltage
3541fa
LTC3541
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 TheLTC3541LVOUToutputvoltageissetbytheratiooftwo externalresistorsasshowninFigure7.Thedeviceservos LVOUTtomaintaintheLFBpinvoltageat0.4V(referenced toground).Thus,thecurrentinR1isequalto0.4V/R1. Forgoodtransientresponse,stability,andaccuracy,the currentinR1shouldbeatleast2A,thusthevalueof R1shouldbenogreaterthan200k.ThecurrentinR2is thecurrentinR1plustheLFBpinbiascurrent.Sincethe LFBpinbiascurrentistypically<10nA,itcanbeignored intheoutputvoltagecalculation.Theoutputvoltagecan becalculatedusingtheformulainFigure8.Notethatin shutdowntheoutputisturnedoffandthedividercurrent willbezeroonceCOUTisdischarged. The LTC3541 VLDO and linear regulator loops operate at a relativelyhigh gain of -3.5V/mA and -3.4V/mA respectively,referredtotheLFBinput.Thus,aloadcurrentchangeof1mAto300mAproducesa1.05mVdrop attheLFBinputfortheVLDOandaloadcurrentchange of1mAto30mAproducesa0.1mVdropattheLFBinput
U
LVOUT LTC3541 LFB R1 GND
3541 F07
APPLICATIO S I FOR ATIO W U U
R2
VOUT = 0.4V 1 + R2 R1 cOUT
()
Figure 7. Programming the LTC3541
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 TheLTC3541isdesignedtobestablewithawiderangeof ceramicoutputcapacitors.TheESRoftheoutputcapacitoraffectsstability,mostnotablywithsmallcapacitors. A minimum output capacitor of 2.2F with an ESR of 0.05orlessisrecommendedtoensurestability.The LTC3541VLDOisamicropowerdeviceandoutputtransientresponsewillbeafunctionofoutputcapacitance. Larger values of output capacitance decrease the peak deviationsandprovideimprovedtransientresponsefor largerloadcurrentchanges.Notethatbypasscapacitors usedtodecoupleindividualcomponentspoweredbythe LTC3541willincreasetheeffectiveoutputcapacitorvalue. HighESRtantalumandelectrolyticcapacitorsmaybeused, butalowESRceramiccapacitormustbeinparallelatthe output.ThereisnominimumESRormaximumcapacitor sizerequirement.
3541fa
LTC3541
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
3541 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
3541 F09
Figure 9. Change in Capacitor vs Temperature
U
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 ofthelossesintheLTC3541circuits: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
3541fa
APPLICATIO S I FOR ATIO W U U
LTC3541
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 TheLTC3541requiresthepackagebackplanemetal(GND pin)tobewellsolderedtothePCboard.Thisgivesthe DFNpackageexceptionalthermalproperties.Thepower handling capability of the device will be limited by the maximum rated junction temperature of 125C. The LTC3541hasinternalthermallimitingdesignedtoprotect the device during momentary overload conditions. For continuous normal conditions, the maximum junction temperatureratingof125Cmustnotbeexceeded.Itis importanttogivecarefulconsiderationtoallsourcesof thermalresistancefromjunctiontoambient.Additional heatsourcesmountednearbymustalsobeconsidered.
U
Forsurfacemountdevices,heatsinkingisaccomplished byusingtheheat-spreadingcapabilitiesofthePCboard anditscoppertraces.Copperboardstiffenersandplated throughholescanalsobeusedtospreadtheheatgeneratedbypowerdevices. ToavoidtheLTC3541exceedingthemaximumjunction temperature,somethermalanalysisisrequired.Thegoal ofthethermalanalysisistodeterminewhetherthepower dissipatedexceedsthemaximumjunctiontemperatureof thepart.Thetemperatureriseisgivenby: TR=PD*JA wherePDisthepowerdissipatedbytheregulatorandJA isthethermalresistancefromthejunctionofthedieto theambienttemperature. Thejunctiontemperature,TJ,isgivenby: TJ=TA+TR whereTAistheambienttemperature. Asanexample,considertheLTC3541withaninputvoltage VINof2.9V,anLVINvoltageof1.8V,anLVOUTvoltageof 1.5V,aloadcurrentof200mAforthebuck,aloadcurrentof300mAfortheVLDOandanambienttemperature of85C.Fromthetypicalperformancegraphofswitch resistance,theRDS(ON)oftheP-channelswitchat85Cis approximately0.25.TheRDS(ON)oftheN-channelswitch isapproximately0.4.Therefore,powerdissipatedbythe partisapproximately: PD=(ILOADBUCK)2*RSW+(ILOADVLDO)* (LVIN-LVOUT)=167mW Forthe3mmx3mmDFNpackage,theJAis43C/W. Thus,thejunctiontemperatureoftheregulatoris: TJ=85C+(0.167)(43)=92C whichiswellbelowthemaximumjunctiontemperature of125C. Notethatathighersupplyvoltages,thejunctiontemperatureislowerduetoreducedswitchresistanceRDS(ON).
3541fa
APPLICATIO S I FOR ATIO W U U
LTC3541
PC BOARD LAYOUT CHECKLIST Whenlayingouttheprintedcircuitboard,thefollowing checklistshouldbeusedtoensureproperoperationof theLTC3541.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 As a design example, assume the LTC3541 is used in asinglelithium-ionbatterypoweredcellularphoneapplication.TheVINwillbeoperatingfromamaximumof 4.2Vdowntoabout3V.Theloadcurrentrequirementis amaximumof0.5Aforthebuckoutputbutmostofthe timeitwillbeinstandbymode,requiringonly2mA.Efficiencyatbothlowandhighloadcurrentsisimportant. Theoutputvoltageforthebuckis1.8V.Therequirement fortheoutputvoltageoftheVLDOis1.5Vwhileprovid-
U
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.
3541fa
APPLICATIO S I FOR ATIO W U U
LTC3541 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
SW 2.2H VIN ENBUCK 22pF VOUT1 2.5V 200mA 10F 73k 154k
ENVLDO MODE GND 165k LFB 576k 2.2F
3541 TA02a
LTC3541 BUCKFB LVIN
LVOUT PGND
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
SW 2.2H VIN ENBUCK 22pF VOUT1 2.5V 200mA 10F 73k 154k
ENVLDO MODE GND 165k LFB 576k 2.2F
3541 TA03a
LTC3541 BUCKFB LVIN
LVOUT PGND
U
VOUT 2V/DIV LVOUT 2V/DIV
VOUT2 1.8V 300mA
VIN 2V/DIV
IVOUT = 200mA ILVOUT = 300mA
4ms/DIV
3541 TA02b
VOUT 2V/DIV LVOUT 2V/DIV
VOUT2 1.8V 300mA
VIN 2V/DIV
IVOUT = 200mA ILVOUT = 300mA
4ms/DIV
3541 TA03b
3541fa
LTC3541 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 SW 2.2H VIN ENBUCK 22pF VOUT1 2.5V 200mA 10F 73k 154k ENVLDO MODE GND 165k LFB 576k 2.2F
3541 TA04a
LTC3541 BUCKFB LVIN
LVOUT PGND
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 SW 2.2H VIN ENBUCK 22pF VOUT1 1.8V 200mA 143k ENVLDO MODE GND 150k LFB 412k 2.2F
3541 TA05a
LTC3541 BUCKFB
10F
LVIN 115k
LVOUT PGND
U
VOUT 2V/DIV LVOUT 2V/DIV
VOUT2 1.8V 300mA
VIN 2V/DIV
IVOUT = 200mA ILVOUT = 300mA
4ms/DIV
3541 TA04b
LVOUT 2V/DIV
VOUT2 1.5V 300mA
VIN 2V/DIV
IVOUT = 200mA ILVOUT = 30mA
4ms/DIV
3541 TA05b
3541fa
LTC3541 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
3541fa
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 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:2.9Vto5.5V,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 HighEfficiencyBuckplusVLDORegulator HighEfficiencyBuckplusVLDORegulator Dual300mA(IOUT),2.25MHz,Synchronous Step-DownDC/DCConverter
LTC3446 LTC3448 LTC3541-2 LTC3541-3 LTC3547
LTC3548/LTC3548-1 Dual800mA/400mA(IOUT),2.25MHz,Synchronous Step-DownDC/DCConverter LTC3548-2 LTC3700 Step-DownDC/DCControllerwithLDORegulator PowerPathisatrademarkofLinearTechnologyCorporation.
3541fa
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 2006

▲Up To Search▲

Price & Availability of LTC3541

	To Download LTC3541 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .