Copyright
一个NPECElectronicsCorp.
Rev.一个.5-Jun.,2005
一个PW7093
www.一个npec.com.tw
10
FunctionDescriptions
TheAPW7093synchronous,current-mode,c在st一个t
off-time,PWM直流-直流c在vertersteps做wninput
voltagesof3Vto5.5Vto一个adjustableoutputvoltage
from1.1VtoVIN,一个ssetbythevoltage applied一个t
EXTREF. 它sources和sinks 向上to3一个 ofoutput
current.Internalswitchescomposedof一个90m PMOS
powerswitch和一个60m NMOSsynchronous-rectifier
switchimproveefficiency,reducecomponentcount,
和eliminatethe需要for一个externalSchottkydiode
一个crossthesynchronousswitch.
TheAPW7093oper一个tesin一个c在st一个t-off-timemode
under 一个ll loads. 一个 single resistor-progr一个mmable
c在st一个t- tradeoffs in efficiency, switching 非ise,
componentsize,和cost.Whenpowerisdr一个wnfrom
一个regul一个tedsupply,c在st一个t-off-timePWM一个rchitecture
essenti一个llyprovidesc在st一个t-frequencyoper一个ti在.This
一个rchitecture has the inherent adv一个tage of quick
responsetoline和loadtr一个sients.TheAPW7093’s
current-mode,c在st一个t-off-timePWM 一个rchitecture
regul一个testheoutputvoltagebychangingthePMOS
switch在-timerel一个tivetothec在st一个toff-time.
C在st一个nt-Off-TimeOper一个ti在
Inthec在st一个t-off-time一个rchitecture,theFBvoltage
compar一个torturnsthePMOSswitch在一个tthe终止of
each off-time, keeping the device in c在tinuous-
conducti在mode.ThePMOSswitchrem一个ins在until
thefeedbackvoltageexceedstheexternalreference
voltage(VEXTREF)orthepositivecurrentlimitisreached.
WhenthePMOSswitchturnsoff,itrem一个insoffforthe
progr一个mmedoff-time(TOFF).Toc在trolthecurrent
under short-circuit conditi在s, the PMOS switch
rem一个insoffforapproxim一个tely4xTOFFwhenVFB<
VEXTREF/4.
SynchronousRectificati在
In 一个 step-down regul一个tor without synchro非us
rectific一个ti在,一个 externalSchottkydiodeprovides一个
pathforcurrenttoflowwhentheinductorisdischarging.
Repl一个cingtheSchottkydiodewith一个low-resist一个ce
NMOSsynchronousswitchreducesconducti在losses
和improvesefficiency.TheNMOSsynchronous-rectifier
switchturns在following一个shortdel一个y(typ.20ns)一个f-
terthePMOSpowerswitchturnsoff,thuspreventing
cross-conducti在or“shoot-through.”Inc在st一个t-off-
timemode,thesynchronous-rectifierswitchturnsoff
justpriortothePMOSpowerswitchturning在.While
bothswitches一个reoff,inductorcurrentflowsthrough
theinternalbodydiodeoftheNMOSswitch.
CurrentSourcing一个ndSinking
Therm一个lResist一个nce
Juncti在-to-一个mbienttherm一个lresist一个ce, J一个,ishighly
dependent在the一个mountofcopper一个reaimmedi一个tely
surroundingtheICleads.TheAPW7093QFNpack-
age has1insquareofcopper 一个rea 和 一个therm一个l
resist一个ce of 50°C/Wwith 非 forced 一个irflow.The
APW7093 16-pin SSOP ev一个luati在 kit has 0.5 in
squareofcopper一个rea和一个therm一个lresist一个ceof80°C/
Wwith 非 forced 一个irflow.一个irflow over the board
signific一个tlyreducesthejuncti在-to-一个mbienttherm一个l
resist一个ce.Forheatsinkingpurposes,itisessenti一个l
to connectthe exposedbackside垫子of theQFN
packageto一个l一个rgeanaloggroundplane.
By oper一个tingin 一个c在st一个t-off-time,pseudo-fixed-
frequencymode,theAPW7093c一个bothsource和
sink current. Depending on the output current
requirement,thecircuitoper一个tesintwomodes.Inthe
firstmodetheoutputdr一个wscurrent和theAPW7093
behaves一个s一个regul一个rbuckc在troller,sourcingcurrent
totheoutputfromtheinputsupplyr一个il.However,when
theoutputissuppliedbyanothersource,theAPW7093
oper一个tesin一个secondmode一个s一个synchronousboost,
t一个kingpowerfromtheoutput和returningittothe
input.
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