Datasheet

LTC2955
10
2955f
OPERATIONOPERATION
the PB and ON inputs are ignored. This is to allow time for
the voltage regulator to turn off and for its output to decay
to 0V. This ensures that the µP has completely powered
down before it is allowed to restart. After the one second
lockout time, the LTC2955-1 proceeds to the next stage
to wait for a turn-on command and the cycle will repeat
as above. If the PB pin remains low after the EN output
goes low, the LTC2955-1 will delay the start of this lockout
time until the PB pin goes high.
If the PB pin goes high when the external timer is still
counting, as shown in Figure 4, the turn-off sequence will
be aborted. The INT pin is forced high immediately and
the EN pin remains high.
high). This can be done by holding the pushbutton down
for a long period (> t
DB(OFF)
+ t
INT(MIN)
+ t
TMR
) as shown
with the solid lines in Figure 3. If the PB pin remains low at
the end of the 32ms minimum INT pulse width (t
INT(MIN)
),
the external timer will start counting (t
4
).
The capacitance at the TMR pin determines the duration
of this timer and it will count for 16 cycles (t
TMR
). If the
PB pin remains low at the end of the 16 cycles (t
5
), the
LTC2955-1 will force the EN pin low and the INT pin high.
See Applications Information for adjusting t
TMR
.
After the EN output goes low and the PB pin goes high,
the LTC2955-1 starts the one second lockout time
(t
EN(LOCKOUT)
). During this lockout time (t
6
to t
7
i n F i g u r e 3)
Figure 4. Pushbutton Turn-Off Aborted
t
DB(OFF)
PB IGNORED
t < t
TMR
PB
TMR
t
INT(MIN)
2955 F04
INT