LTC3554EPD datasheet(28/36 Pages) LINER

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LTC3554

3554f

typical) to keep the bucks enabled as described above.

PBSTAT does not go low on initial pushbutton application

for power-up, but will go low with subsequent ON pushbut-

ton applications in the PUP1, PUP2 or PON states.

Power-Up Via Applying External Power

Figure 8 shows the LTC3554 powering up through ap-

plication of external power (VBUS). For this example the

pushbutton circuitry starts in the POFF or HR state with a

battery connected and both bucks disabled. 100ms after

VBUS application the pushbutton circuitry transitions into

the PUP state and powers up buck1 followed by buck2.

The 100ms delay time allows the applied supply to settle.

The bucks will stay powered as long as their respective

PWR_ON inputs are driven high before the 5 second PUP

period is over. If either PWR_ON is low or goes low after

the 5 second period the corresponding buck(s) will be shut

down. In the above example both PWR_ONs are high at

OPERATION

the end of the 5 second period and therefore both bucks

continue to stay on at the end of the 5 second period.

PGOOD is asserted once all enabled bucks are within 8%

of their regulation voltage for 230ms.

The PWR_ON inputs can be driven via a μP/μC or one of

the buck outputs through a high impedance (100kΩ typ)

to keep the bucks enabled as described above.

Without a battery present, initial power application causes

a power-on reset which puts the pushbutton circuitry in

the PDN1 state and subsequently the HR state one second

later. At this time, if a valid supply voltage is detected

at the BUS pin (i.e., VBUS > VUVLO and VBUS – VBAT >

VDUVLO), the pushbutton circuity immediately enters the

PUP1 state. For this to work reliably, the BAT pin voltage

must be kept well-behaved when no battery is connected.

Ensure this by bypassing the BAT pin to GND with an RC

network consisting of a 100μF ceramic capacitor in series

with 0.3Ω.

BAT

VBUS

ON (PB)

PBSTAT

BUCK1

BUCK2

PGOOD

PWR_ON1

PWR_ON2

STATE

POFF/HR

PON

PUP2/PUP1

3554 TD02

230ms

100ms

Figure 8. Power-Up Via Applying External Power

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