LTC3553_15 Datasheet, PDF (27/36 Pages) Linear Technology – Micropower USB Power Manager With Li-Ion Charger, LDO and Buck Regulator

LTC3553
OPERATION
BUCK_ON or LDO_ON is low or goes low after the five second
period the corresponding regulator(s) will be shut down.
In the above example both pins are high at the end of the
five second period and therefore both regulators continue
to stay on at the end of the five second period.
The BUCK_ON and LDO_ON inputs can be driven via
a μP/μC or one of the regulator outputs through a high
impedance (100kΩ typ) to keep the regulators 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.
Power-Up Via Asserting the BUCK_ON or LDO_ON Pins
Figure 10 shows the LTC3553 powering up by driving
BUCK_ON high. For this example the pushbutton circuitry
starts in the POFF or HR state with a battery connected
and all bucks disabled. Once BUCK_ON goes high, the
pushbutton circuitry enters the PON state and the buck
powers up. If LDO_ON is brought high at a later time, the
LDO will power up. The pushbutton circuitry remains in
the PON state.
Powering up via asserting the BUCK_ON or LDO_ON pins
is useful for applications containing an always-on μC that’s
not powered by the LTC3553 regulators. That μC can power
the application up and down for housekeeping and other
activities not needing the user’s control.
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
1
BUCK_ON
0
1
LDO_ON
0
1
BUCK
0
1
LDO
0
STATE
POFF/HR
PON
3553 TD03
Figure 10. Power-Up via Asserting the BUCK_ON or LDO_ON Pins
3553fc
27