参数资料
型号: LTC3677EUFF-3#PBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 电源管理
英文描述: POWER SUPPLY MANAGEMENT CKT, PQCC44
封装: 7 X 4 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, QFN-44
文件页数: 14/44页
文件大小: 4112K
代理商: LTC3677EUFF-3#PBF
LTC3677-3
36773f
operaTion
Ideal Diode from BAT to VOUT
TheLTC3677-3hasaninternalidealdiodeaswellasa
controllerforanoptionalexternalidealdiode.Boththe
internal and the external ideal diodes respond quickly
wheneverVOUTdropsbelowBAT.Iftheloadincreases
beyondtheinputcurrentlimit,additionalcurrentwillbe
pulledfromthebatteryviatheidealdiodes.Furthermore,if
powertoVBUS(USB)orVOUT(externalwallpowerorhigh
voltageregulator)isremoved,thenalloftheapplication
powerwillbeprovidedbythebatteryviatheidealdiodes.
TheidealdiodesarefastenoughtokeepVOUTfromdrop-
pingsignificantlybelowVBATwithjusttherecommended
outputcapacitor(seeFigure2).Theidealdiodeconsists
ofaprecisionamplifierthatenablesanon-chipP-channel
MOSFETwheneverthevoltageatVOUTisapproximately
15mV(VFWD)belowthevoltageatBAT.Theresistanceof
theinternalidealdiodeisapproximately200mΩ.Ifthisis
sufficientfortheapplication,thennoexternalcomponents
arenecessary.However,iflowerresistanceisneeded,an
externalP-channelMOSFETcanbeaddedfromBATto
VOUT.TheIDGATEpinoftheLTC3677-3drivesthegateof
theexternalP-channelMOSFETforautomaticidealdiode
control.ThesourceoftheMOSFETshouldbeconnectedto
VOUTandthedrainshouldbeconnectedtoBAT.Capableof
drivinga1nFload,theIDGATEpincancontrolanexternal
P-channelMOSFEThavingextremelylowon-resistance.
Using the WALL Pin to Detect the Presence of an
External Power Source
TheWALLinputpincanbeusedtoidentifythepresence
ofanexternalpowersource(particularlyonethatisnot
subjecttoafixedcurrentlimitliketheUSBVBUSinput).
Typically,suchapowersupplywouldbea5Vwalladapter
outputorthelowvoltageoutputofahighvoltagebuck
regulator.Whenthewalladapteroutput(orbuckregulator
output)isconnecteddirectlytotheWALLpin,andthevolt-
ageexceedstheWALLpinthreshold,theUSBpowerpath
(fromVBUStoVOUT)willbedisconnected.Furthermore,
theACPRpinwillbepulledlow.Inorderforthepresence
ofanexternalpowersupplytobeacknowledged,bothof
thefollowingconditionsmustbesatisfied:
1.The WALL pin voltage must exceed approximately
4.3V.
2.TheWALLpinvoltagemustbegreaterthan75mVabove
theBATpinvoltage.
The input power path (between VBUSandVOUT)isre-
enabledandtheACPRpinispulledhighwheneitherof
thefollowingconditionsismet:
1.TheWALLpinvoltagefallstowithin25mVoftheBAT
pinvoltage.
2.TheWALLpinvoltagefallsbelow3.2V.
Each of these thresholds is suitably filtered in time to
preventtransientglitchesontheWALLpinfromfalsely
triggeringanevent.
Suspend Mode
When ILIM0 is pulled high and ILIM1 is pulled low the
LTC3677-3 enters suspend mode to comply with the
USBspecification.Inthismode,thepowerpathbetween
VBUSandVOUTisputinahighimpedancestatetoreduce
theVBUSinputcurrentto50μA.Ifnootherpowersource
is available to drive WALL and VOUT, the system load
connectedtoVOUTissuppliedthroughtheidealdiodes
connectedtoBAT.
4.0V
3.8V
VOUT
3.6V
500mA
–500mA
0
IBAT
IVOUT
LOAD
1A
0A
10s/DIV
CHARGE
DISCHARGE
36773 F02
VBAT = 3.8V
VBUS = 5V
5x MODE
COUT = 10F
Figure 2. Ideal Diode Transient Response
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