LTC3553 Linear Technology Corporation, LTC3553 Datasheet - Page 27

• Manufacturer Part Number: LTC3553
• Description: Micropower USB Power Manager
• Manufacturer: Linear Technology Corporation

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OPERATION
ON or LDO_ON is low or goes low after the fi ve second
period the corresponding regulator(s) will be shut down.
In the above example both pins are high at the end of the
fi ve second period and therefore both regulators continue
to stay on at the end of the fi ve 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
V
PUP1 state. For this to work reliably, the BAT pin voltage
must be kept well-behaved when no battery is connected.
DUVLO
), the pushbutton circuity immediately enters the
BUS
> V
Figure 10. Power-Up via Asserting the BUCK_ON or LDO_ON Pins
UVLO
STATE
BUCK_ON
LDO_ON
ON (PB)
PBSTAT
and V
BUCK
V
LDO
BAT
BUS
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
BUS
POFF/HR
– V
BAT
>
PON
Ensure this by bypassing the BAT pin to GND with an RC
network consisting 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
3553 TD03
LTC3553
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3553f