LTC2955
11
2955fa
For more information www.linear.com/LTC2955
ON Pin Turn-Off Sequence (DFN Package Only)
Figure 5 shows the turn-off sequence of the LTC2955-1
initiated by the ON pin. The timing sequence is slightly
different from that by the pushbutton as there is no de
-
bounce time (t
DB(OFF)
) before the INT pin pulls low. The
ON pin falling edge can start the turn-off sequence only
if the following two conditions are met: (1) The preceding
turn-on sequence was initiated by the ON pin rising edge,
and (2) The SEL pin must be tied high (>0.8V). For TS8
package, the ON pin falling edge is ignored.
In Figure 5, a high to low transition at ON (t
1
) starts the
turn-off sequence. The interrupt output (INT) is asserted
immediately when the ON pin goes low. Unlike the PB
pin turn-off sequence, there is no 32ms debounce time
(t
DB(FF)
) for the ON pin going low. This allows the system
to initiate a shutdown as soon as the monitored supply is
removed and allows the µP the maximum amount of time
to perform power-down functions. The INT pin will stay
low for at least 32ms (t
INT(MIN)
). When the µP receives
this interrupt signal, it should start to perform its power-
down functions and assert the KILL input low (t
2
) once it
is done. The LTC2955-1 will then release the EN and INT
outputs, thus turning off the system power, as shown with
dotted lines in Figure 5.
If the µP fails to respond (KILL pin remains high, as shown
with solid line in Figure 5) and the ON pin remains low 32ms
after INT goes low, the external timer will start counting
for 16 cycles (t
TMR
, t
3
to t
4
). If the ON pin remains low
at the end of the 16 cycles (t
4
), 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, the LTC2955-1 starts the
one second lockout time (t
EN(LOCKOUT)
). During this lockout
time (t
4
to t
5
), the PB and ON inputs are ignored.
operaTion
Figure 5. ON Pin Turn-Off Timing (DFN Only, V
SEL
> 1.2V and the Preceding Turn-On Sequence Was
Initiated by the ON Pin Rising Edge
16 CYCLES
ON
t
1
t
2
t
5
t
4
t
3
TMR
t
INT(MIN)
t
TMR
t
EN(LOCKOUT)
INT
PGD
EN
(LTC2955-1)
2955 F05
KILL
IGNORED
IGNORED
