Datasheet

LTC3553
26
3553fb
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
ex a mp le. If ei t he r B UCK _ ON or LD O_ ON is low or go es lo w
after the fi ve second period the corresponding regulator(s)
will be shut down. In the above example LDO_ON is low
at the end of the fi ve second period and therefore the LDO
is disabled at the end of the fi ve second period.
OPERATION
The BUCK_ON and LDO_ON inputs can be driven via a
P/C 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 fi ve second PUP period is over. If either
BAT
V
BUS
ON (PB)
PBSTAT
BUCK
LDO
SEQ
BUCK_ON
LDO_ON
STATE POFF/HR PONPUP2/PUP1
3553 TD02
5s
5s
100ms
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
Figure 9. Power-Up via Applying External Power
BAT
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
V
BUS
ON (PB)
PBSTAT
BUCK
LDO
SEQ
BUCK_ON
LDO_ON
STATE POFF/HR PONPUP2/PUP1
3553 TD01
5s
400ms
Figure 8. Power-Up via Pushbutton Press