LTC3553-2_15 Datasheet, PDF (29/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
POFF state, the state machine immediately recognizes that
valid external power is available and transitions into the
PUP2 state. Entering the PUP2 state will cause the buck to
power up as described previously in the power-up sections.
Not depicted here, but in cases where the BUCK_ON pin
is driven by a supply that remains high when entering the
POFF state, then as per the state diagram in Figure 7, the
pushbutton circuitry will enter the PON state once V_OUT
is no longer in UVLO. Upon entering the PON state, the
buck regulator will power up.
Note: If V_OUT drops too low (below about 1.9V) the LTC3553-2
will see this as a POR condition and will enter the PDN1
state rather than the PDN2 state. One second later the
part will transition to the HR state. Under these conditions
an explicit power-up event (such as a pushbutton press)
may be required to bring the LTC3553-2 out of hard reset.
Hard Reset Timing
HARD RESET provides an ultralow power-down state for
shipping or long term storage as well as a way to power
down the application in case of a software lockup. In the
case of software lockup, the user can hold the pushbutton
(ON low) for 14 seconds and a hard reset event (HRST) will
occur, placing the pushbutton circuitry in the power-down
(PDN1) state. At this point the buck regulator will be shut
down. Following a one second power-down period the
pushbutton circuitry will enter the hard reset state (HR).
At this point the LDO regulator will be shut down.
Holding ON low through the one second power-down
period will not cause a power-up event at end of the one
second period. ON must be brought high following the
power-down event and then go low again for 400ms to
establish a valid power-up event, as shown in Figure 11.
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
14s
50ms
1
LDO
0
1
BUCK
0
1
BUCK_ON
0
1
PGOOD
0
STATE
PON
1s
PDN1
400ms
1.8ms
HR
PUP1
35532 F11
Figure 11. Hard Reset via Holding ON Low for 14 Seconds
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