LTC3677-3 datasheet(21/44 Pages) LINER

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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

IBAT

IVOUT

LOAD

10µs/DIV

CHARGE

DISCHARGE

36773 F02

VBAT = 3.8V

VBUS = 5V

5x MODE

COUT = 10µF

Figure 2. Ideal Diode Transient Response

LTC3677-3 Datasheet(PDF) 21 Page - Linear Technology

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