NCP1501
http://onsemi.com
5
DETAILEDOPERATINGDESCRIPTION
The Buck regulator is a synchronous rectifier PWM
regulatorwithintegratedMOSFETs.Thisregulatorhasan
LDOfunctionforlowpowermodestoconservepowerand
lower ripple voltage associated with PFM mode. The
NCP1501 does not containan internaloscillator for the
switchingmode.TheDualPWM/LDOmodeisanexclusive
PatentPendingcircuit.
ThePWMclockisgeneratedviaanexternalclocksignal
ontheSynchronizationpin.Theoperatingfrequencyrange
forthePWMis500kHzto1000kHz.Theoutputcurrentof
the PWMistypically100mAwitha guarantee ofover
300mAforthe2.7to5.2inputvoltagerange.
Ifasynchronizationpulseisnotpresent,theNCP1501
changesintotheLDOmode.TheLDOfunctionassuresthe
userofanextremelylowoutputripplevoltageandgreatly
reducedquiescentcurrentwhentheuserssystemisinasleep
mode.InternallytotheNCP1501,theSynchronizationpin
hasapulldownresistortoforcethepartintoLDOmode
whenaclocksignalisnotpresent.ToplacetheNCP1501in
LDOmode,theusermustsettheSynchronizationpinlow.
TheLDOmodeguaranteesanoutputinexcessof50mA.
PinsCB0andCB1controltheoutputvoltageselection.
Thefourvoltagesare1.05V,1.35V,1.57V,1.8V.CB0
containsapulldownresistorandCB1containsapullup
resistorinternaltotheNCP1501.Theresistorsforcethe
outputoftheconverterto1.35Vifthepinsarefloating
connectionstotheexternalcircuit.
TheShutdownPinenablestheoperationofthedevice.
TheShutdownPinhasaninternalpulldownresistortoforce
theNCP1501intotheoffmodeifthispinisfloatingdueto
theexternalcircuit.DuringStartup,theNCP1501hasasoft
startfunctiontolimitfastdV/dtandeliminateovershooton
theoutput.
Figure3.BlockDiagramandCircuitSchematicoftheNCP1501
L1
V
bat
Sync
SHD
C
in
10m
DC/DC
CONTROL
LDO
CONTROL
Q2
EA
CB0
CB1
10mH
LX
FB
V
out
C
out
10m
I
lim
Q1
Q3
Theexternalcomponentsrequiredareaninputandan
output100mFceramiccapacitoranda10mHinductor.
PWMMode
Duringnormaloperation,asynchronizationpulseactsas
theclockfortheDC/DCcontroller.Therisingedgeofthe
clockpullsthegateofQ1lowallowingthe inductorto
charge.WhenthecurrentthroughQ1reacheseitherthe
currentlimitorfeedbackvoltagereachesitslimit,Q1will
turnoffandQ2willturnon.Q2replacesthefreewheeling
diodetypicallyassociatedwithBuckConverters.Q2will
turnoffwheneitherarisingedgesyncpulseispresentorall
thestoredenergyisdepletedfromtheinductor.Q3remains
offduringthismode.
TheoutputvoltageaccuracyinthePWMmodeiswell
within 3% of the nominal set value. An over voltage
protectioncircuitispresentinthePWMmodetolimitthe
positivevoltagespikeduetofastloadtransientconditions.
ThePWMalsohastheabilitytogoto100%dutycyclefor
transient conditions and low input to output voltage
differentials.
InPWMmode,eachswitchingcyclehasaguaranteed
on--timeof100ns.TheNCP1501hastwoprotectioncircuits
thatcaneliminatethecycle.Whentripped,theovervoltage
protectionorthethermalshutdownoverridesthegatedrive
ofthehighsideMOSFET.
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