Datasheet

LTC2952
21
2952fb
Figure 11. Aborted Power-On Sequence
2952 F11
KILL
EN
INTERNAL
ON/OFF SIGNAL
t
KILL, ON BLANKING
t
7
DON'T CARE
t
EN, LOCKOUT
2952 F12
KILL
EN
INTERNAL
ON/OFF SIGNAL
t
KILL, ON BLANKING
t
8
t
9
DON'T CARE
Figure 12. KILL Initiated Shutdown
Extended Power During Turn-Off
In the shutdown process, the availability of power can
be extended by providing edges to the WDE pin during
the KILL Off Wait time. The timing diagram in Figure 13
is similar to the power-on/power-off sequence timing
diagram (Figure 10) except for the edges on the WDE pin
during the shutdown process. At time t
10
, the internal ON/
OFF signal transitions low. When this happens, the DC/DC
converter providing power to the system will be shut off
in 400ms unless the WDE pin is toggled.
When the WDE pin transitions at t
11
, the LTC2952 resets
the 400ms KILL Off Wait timer. Before this second 400ms
wait time expires, the WDE pin transitions again (this time
from high to low) at t
12
, causing the 400ms timer to reset
again. Finally, the third 400ms timer which starts at t
12
expires without any further extension at t
13
causing the
EN pin to go low, shutting down the DC/DC converter.
APPLICATIONS INFORMATION
Figure 13. Power-On/Power-Off Sequence with Extended Shutdown/Housekeeping Wait Time
2952 F13
KILL
EN
INTERNAL
ON/OFF SIGNAL
t
KILL, ON BLANKING
< t
KILL, OFF WAIT
WDE
t
10
t
11
t
12
t
13
DON'T CARE DON'T CARE
EXTENDED HOUSE KEEPING TIME
t
KILL, OFF WAIT
t
EN, LOCKOUT
< t
KILL, OFF WAIT
Aborted Power-On Sequence
The power-on sequence is aborted when the μP fails to
set the KILL pin high before the 400ms KILL On Blanking
time expires, as shown in the timing diagram in Figure 11.
When the KILL On Blanking timer expires (t
7
), the KILL
pin is still low indicating that the μP/system has failed to
power on successfully. When the system failed to set the
KILL pin high within the specified 400ms time window,
the LTC2952 pulls the EN pin low (thus turning off the
DC/DC converter) and as a side effect resets the internal
ON/OFF signal.
KILL Power Turn-Off During Normal Operation
Once the system has powered on and is operating nor-
mally, the system can turn off power by setting KILL low,
as shown in the timing diagram in Figure 12. At t
9
, KILL is
set low and this immediately causes the LTC2952 to pull
EN low, turning off the DC/DC converter.