- 25 -`

AUGUST08,2012|DATASHEET| Rev3.1

IR3894

12AHighlyIntegratedSupIRBuck

Single‐InputVoltage,SynchronousBuckRegulator

PD‐97745

MINIMUMONTIMECONSIDERATIONS

TheminimumONtimeistheshortestamountoftimefor

CtrlFETtobereliablyturnedon.Thisisverycritical

parameterforlowdutycycle,highfrequencyapplications.

Conventionalapproachlimitsthepulsewidthtoprevent

noise,jitterandpulseskipping.Thisresultstolowerclosed

loopbandwidth.



IRhasdevelopedaproprietaryschemetoimproveand

enhanceminimumpulsewidthwhichutilizesthebenefits

ofvoltagemodecontrolschemewithhigherswitching

frequency,widerconversionratioandhigherclosedloop

bandwidth,thelatterresultsinreductionofoutput

capacitors.AnydesignorapplicationusingIR3894must

ensureoperation

withapulsewidththatishigherthanthis

minimumon‐timeandpreferablyhigherthan60ns.

Thisisnecessaryforthecircuittooperatewithoutjitter

andpulse‐skipping,whichcancausehighinductorcurrent

rippleandhighoutputvoltageripple.

(3)

out

  





InanyapplicationthatusesIR3894,thefollowingcondition

mustbesatisfied:

(min)

(4)

(5)

(6)

on on

out

in s

out

in s

 



 





  



Theminimumoutputvoltageislimitedbythereference

voltageandhenceV

out(min)

=0.5V.Therefore,for

out(min)

=0.5V,

V/uS 33.8

ns 60

V 0.5

(min)







out



Therefore,atthemaximumrecommendedinputvoltageof

21Vandminimumoutputvoltage,theconvertershouldbe

designedataswitchingfrequencythatdoesnotexceed

396kHz.Conversely,foroperationatthemaximum

recommendedoperatingfrequency(1.65MHz)and

minimumoutputvoltage(0.5V).Theinputvoltage(PVin)

shouldnot

exceed5.05V,otherwisepulseskippingwill

happen.

MAXIMUMDUTYRATIO

Acertainoff‐timeisspecifiedforIR3894.Thisprovides

anupperlimitontheoperatingdutyratioatanygiven

switchingfrequency.Theoff‐timeremainsatarelatively

fixedratiotoswitchingperiodinlowandmidfrequency

range,whileinhighfrequencyrangethisratioincreases,

thusthe

lowerthemaximumdutyratioatwhichIR3894

canoperate.Figure21showsaplotofthemaximumduty

ratiovs.theswitchingfrequencywithbuiltininputvoltage

feedforward.





Figure21:Maximumdutycyclevs.switchingfrequency.



- 26 -`

AUGUST08,2012|DATASHEET| Rev3.1

IR3894

12AHighlyIntegratedSupIRBuck

Single‐InputVoltage,SynchronousBuckRegulator

PD‐97745

DESIGNEXAMPLE



Thefollowingexampleisatypicalapplicationfor

IR3894.TheapplicationcircuitisshowninFig.28.

=12 V ( 10% )

=1 2 V

= 12 A

Ripple Voltage= 1%*

6% * 50%

=600 kHz

V V load transient



 for )



EnablingtheIR3894

Asexplainedearlier,theprecisethresholdoftheEnable

lendsitselfwelltoimplementationofaUVLOforthe

BusVoltageasshowninFig.22.



Figure22:UsingEnablepinforUVLOimplementation

ForatypicalEnablethresholdofV

=1.2V

(min)

* 1.2 (7)

in EN

V V

  





min

(8)

in( ) EN

 





ForV



(min)

=9.2V,R

=49.9KandR

=7.5Kohmisagood

choice.

Programmingthefrequency

ForF

=600kHz,selectR

=39.2KΩ,usingTable1.

OutputVoltageProgramming

Outputvoltageisprogrammedbyreferencevoltageand

externalvoltagedivider.TheFbpinistheinvertinginputof

theerroramplifier,whichisinternallyreferencedto0.5V.

Thedividerratioissettoprovide0.5VattheFbpin

whenthe

outputisatitsdesiredvalue.Theoutputvoltageisdefinedby

usingthefollowingequation:

1(9)

oref





  







Whenanexternalresistordividerisconnectedtotheoutput

asshowninFig.23.

(10)

ref

oref



  









ForthecalculatedvaluesofR5andR6,seefeedback

compensationsection.



Figure23:TypicalapplicationoftheIR3894

forprogrammingtheoutputvoltage

BootstrapCapacitorSelection

TodrivetheControlFET,itisnecessarytosupplyagate

voltageatleast4VgreaterthanthevoltageattheSWpin,

whichisconnectedtothesourceoftheControlFET.

Thisisachievedbyusingabootstrapconfiguration,which

comprisestheinternalbootstrapdiode

andanexternal

bootstrapcapacitor(C1).Theoperationofthecircuitisas

follows:WhenthesyncFETisturnedon,thecapacitornode

connectedtoSWispulleddowntoground.Thecapacitor

chargestowardsV

throughtheinternalbootstrapdiode

(Fig.24),whichhasaforwardvoltagedropV

.ThevoltageV



acrossthebootstrapcapacitorC1isapproximatelygivenas:

(11)

cccD

VVV



 



- 27 -`

AUGUST08,2012|DATASHEET| Rev3.1

IR3894

12AHighlyIntegratedSupIRBuck

Single‐InputVoltage,SynchronousBuckRegulator

PD‐97745

WhenthecontrolFETturnsoninthenextcycle,the

capacitornodeconnectedtoSWrisestothebusvoltage

.However,ifthevalueofC1isappropriatelychosen,

thevoltageV

acrossC1remainsapproximately

unchangedandthevoltageattheBootpinbecomes:



(12)

Boot in cc D

VVVV   



Figure24:BootstrapcircuittogenerateVcvoltage

Abootstrapcapacitorofvalue0.1uFissuitableformost

applications.

InputCapacitorSelection

Theripplecurrentgeneratedduringtheontimeofthe

controlFETshouldbeprovidedbytheinputcapacitor.

TheRMSvalueofthisrippleisexpressedby:

(1 ) (13)

RMS o

IIDD    



(14)

 



Where:

DistheDutyCycle

RMS

istheRMSvalueoftheinputcapacitorcurrent.

Ioistheoutputcurrent.

ForI

=12AandD=0.1,theI

RMS

=3.6A.

Ceramiccapacitorsarerecommendedduetotheirpeak

currentcapabilities.TheyalsofeaturelowESRandESLat

higherfrequencywhichenablesbetterefficiency.

Forthisapplication,itisadvisabletohave4x10uF,25V

ceramiccapacitors,C3216X5R1E106MfromTDK.

Inadditiontothese,althoughnotmandatory,

a1x330uF,25VSMDcapacitor

EEV‐FK1E331PfromPanasonic

mayalsobeusedasabulkcapacitorandisrecommendedif

theinputpowersupplyisnotlocatedclosetotheconverter.

InductorSelection

Theinductorisselectedbasedonoutputpower,operating

frequencyandefficiencyrequirements.Alowinductorvalue

causeslargeripplecurrent,

resultinginthesmallersize,faster

responsetoaloadtransientbutpoorefficiencyandhigh

outputnoise.Generally,theselectionoftheinductorvalue

canbereducedtothedesiredmaximumripplecurrentinthe

inductor(Δi).Theoptimumpointisusuallyfoundbetween

20%and50%ripple

oftheoutputcurrent.

Forthebuckconverter,theinductorvalueforthedesired

operatingripplecurrentcanbedeterminedusingthe

followingrelation:



;

(15)

in o

in s

VVL tD

LVV

ViF



 











Where:

=Maximuminputvoltage

=OutputVoltage

Δi=InductorPeak‐to‐PeakRippleCurrent

=SwitchingFrequency

Δt=OntimeforControlFET

D=DutyCycle



IfΔi≈30%*I

,thentheoutputinductoriscalculatedtobe

0.5μH.SelectL=0.51μH,59PR9876N,fromVITECwhich

providesacompact,lowprofileinductorsuitableforthis

application.



Part #	ERJ-3EKF1000V
Description	RES THKFLM 0603 100 OHM 1% 1/10W 100PPM/ C SMD - Cut TR (S
Category	CHIP RESISTOR
Availability	Out of Stock
Qty	0

ERJ-3EKF1000V

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