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SavingCurrentWhileUsingtheWatchdog
t
d
t
d
t
(tout)
V
DD
V
它
WDI
重置
t
t
t
电源-failcomparator(pfiandpfo)
PFI PFO
V
CC
地
TPS3306
V
(sense)
R1
1%
R2
1%
V
pfi,trip
= 1.25 v
×
R
1
+ r
2
R
2
tps3306-15
tps3306-18,tps3306-20
tps3306-25,tps3306-33
SLVS290A–APRIL2000–REVISEDMAY2004
detaileddescription(持续)
thewatchdoginputisinternallydrivenlowduringthefirst7/8ofthewatchdogtime-outperiod,thenmomentarily
pulseshigh,resettingthewatchdogcounter.forminimumwatchdoginputcurrent(minimumoverallpower
消耗量),leavewdilowforthemajorityofthewatchdogtime-outperiod,pulsingitlow-高-lowoncewithin
7/8ofthewatchdogtime-outperiodtoresetthewatchdogtimer.ifinsteadwdiisexternallydrivenhighforthe
majorityofthetime-outperiod,acurrentof5v/40k
Ω
9125µacanflowintowdi.
figure1.watchdogtiming
anadditionalcomparatorisprovidedtomonitorvoltagesotherthanthenominalsupplyvoltage.这
电源-失败-输入(pfi)willbecomparedwithaninternalvoltagereferenceof1.25v.iftheinputvoltagefalls
belowthepower-failthreshold(v
PFI
)oftyp.1.25v,thepower-failoutput(pfo)goeslow.ifitgoesabove1.25v
plusabout10mvhysteresis,theoutputreturnstohigh.byconnectingtwoexternalresistors,itispossibleto
superviseanyvoltagesabove1.25v.thesumofbothresistorsshouldbeabout1m
Ω
,tominimizepower
consumptionandalsotoassurethatthecurrentinthePFIpincanbeneglectedcomparedwiththecurrent
throughtheresistornetwork.thetoleranceoftheexternalresistorsshouldbenotmorethan1%toensure
minimalvariationofsensedvoltage.ifthepower-failcomparatorisunused,connectpfitogroundandleave
pfounconnected.
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