LTC3553_15 Datasheet, PDF (26/36 Pages) Linear Technology – Micropower USB Power Manager With Li-Ion Charger, LDO and Buck Regulator

LTC3553
OPERATION
PBSTAT to go low. PBSTAT goes high impedance when
ON goes high, except the logic enforces a minimum pulse
width of 50ms on PBSTAT.
In the HR, POFF, PDN1, and PDN2 states, PBSTAT remains
high impedance regardless of the state of ON.
Power-Up Via Pushbutton Press
Figure 8 shows the LTC3553 powering up through ap-
plication of the external pushbutton. For this example
the pushbutton circuitry starts in the POFF or HR state
with a battery connected and both regulators disabled.
Pushbutton application (ON low) for 400ms transitions the
pushbutton circuitry into the PUP state and powers up the
buck followed by the LDO since the SEQ pin is low in this
example. If either 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 LDO_ON is low
at the end of the five second period and therefore the LDO
is disabled at the end of the five second period.
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
1
BUCK
0
1
LDO
0
1
SEQ
0
1
BUCK_ON
0
1
LDO_ON
0
STATE
POFF/HR
400ms
5s
PUP2/PUP1
PON
3553 TD01
Figure 8. Power-Up via Pushbutton Press
26
The BUCK_ON and LDO_ON inputs can be driven via a
μP/μC or by one of the regulator outputs through a high
impedance (100kΩ 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 pushbutton applications in the PUP1,
PUP2 or PON states.
Power-Up Via Applying External Power
Figure 9 shows the LTC3553 powering up through ap-
plication of external power (V_BUS). For this example the
pushbutton circuitry starts in the POFF or HR state with
a battery connected and both regulators disabled. 100ms
after V_BUS application the pushbutton circuitry transitions
into the PUP state and powers up the buck followed by
the LDO. The 100ms delay time allows the applied sup-
ply to settle. The regulators will stay powered as long as
their respective BUCK_ON and LDO_ON inputs are driven
high before the five second PUP period is over. If either
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
1
BUCK
0
1
LDO
0
1
SEQ
0
1
BUCK_ON
0
1
LDO_ON
0
STATE
POFF/HR
100ms
5s
5s
PUP2/PUP1
PON
3553 TD02
Figure 9. Power-Up via Applying External Power
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