LTC3553-2_15 Datasheet, PDF (26/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
Power-Up Via Applying External Power from Power Off
Figure 7 shows the LTC3553-2 powering up through ap-
plication of external power (V_BUS). For this example the
pushbutton circuitry starts in the POFF state with a battery
connected. 100ms after V_BUS application the pushbutton
circuitry transitions into the PUP2 state and powers up the
buck. The 100ms delay time allows the applied supply to
settle. The buck regulator will stay powered as long as the
BUCK_ON input is driven high before the five second PUP2
period is over. If the BUCK_ON is low or goes low after the
five second period the buck regulator will be shut down. In
the above example the BUCK_ON pin is high at the end of the
five second period and therefore the buck regulator con-
tinues to stay on at the end of the five second period. In
this example, PGOOD is initially high as the always-on
LDO is enabled. At the end of 100ms, the input power at
V_BUS is validated and the buck regulator is enabled at this
point PGOOD goes low and will stay low until the buck
regulator is within 8% of its regulation voltage for 1.8ms.
The BUCK_ON input can be driven via a μP/μC or one of
the regulator outputs through a high impedance (100kΩ
typ) to keep the buck regulator 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., V_BUS > V_UVLO and V_BUS – V_BAT > V_DUVLO),
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 con-
sisting of a 100μF ceramic capacitor in series with 0.3Ω.
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
1
BUCK
0
1
LDO
0
1
PGOOD
0
1
BUCK_ON
0
STATE
POFF
100ms
1.8ms
5s
PUP2
PON
35532 F07
Figure 7. Power-Up via Applying External Power
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