LP3941A Datasheet, PDF (27/32 Pages) National Semiconductor (TI) – Cellular Phone Power Management Unit

Flowchart Operation (Continued)
passed. If LDO1 reaches 93%, the state machine advances
to the RESET OFF DELAY state. If 60 ms have passed
before the 93% level is achieved, the state machine returns
to the STANDBY state and waits for another wakeup source.
The RESET OFF DELAY state counts off 40 ms. If the
battery voltage drops below the UVLO threshold of 2.5V, the
state machine goes to the ENABLE RESET state and power
down sequence. If LDO1 output drops below 85% of the
nominal voltage the state machine returns to the TURNON
LDOs state in an attempt to restart the LDO. If neither of
these conditions occurs the state machine advances to the
PS_HOLD DETECT state.
In the PS_HOLD DETECT, RESET is deasserted and the
state machine waits 35 ms for the PS_HOLD signal to go
high. If PS_HOLD goes high within 35 ms of RESET going
low the state machine advances to the IDLE state. If
PS_HOLD in still low after 35 ms the state machine goes to
ENABLE RESET state and the power down sequence. If
battery voltage pins drops below the UVLO threshold, the
state machine advances to the ENABLE RESET state and
the power down sequence. If LDO1 output drops below its
85% point the state machine returns to the TURNON LDOs
state in an attempt to try restart the LDOs.
The state machine remains in the IDLE state until PS_HOLD
goes low for 35 ms. If PS_HOLD is low for less than 35 ms
the state machine remains in the IDLE state. If PS_HOLD
stays low for more than 35 ms, the state machine advances
to the ENABLE RESET state and the power down sequence.
If LDO1 output falls below its 85% point the state machine
returns to the TURNON LDOs state in an attempt to restart
the LDOs. The UVLO is disabled in the IDLE state. The
back-up battery charger is on.
In the ENABLE RESET state RESET is asserted. After
60 ms all LDOs are turned off. UVLO as well as the back-up
battery charger are on. Once LDO1 falls to its 85% point the
state machine returns to the STANDBY state.
The RTC_LDO is powered by the back-up battery and is
always on (unless specifically disabled via the I²C interface).
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