Datasheet

LTC2951-1/LTC2951-2
11
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Figure 1. Simplified Power-On/-Off Sequence for LTC2951-1
Simplified Power-On/-Off Sequence
Figure 1 shows a simplified LTC2951-1 power-on and
power-off sequence. A high to low transition on PB (t
1
)
initiates the power-on sequence. This diagram does not
show any bounce on PB . In order to assert the enable
output, the PB pin must stay low continuously (PB high
resets timers) for 128ms (t
2
–t
1
). Once EN goes high (t
2
),
an internal 512ms blanking timer is started. This blank-
ing timer is designed to give sufficient time for the DC/
DC converter to reach its final voltage, and to allow the
µP enough time to perform power-on tasks.
The KILL pin must be pulled high within 512ms of the
EN pin going high. Failure to do so results in the EN
APPLICATIONS INFORMATION
PB
OFFT
EN
KILL
INT
PB & KILL IGNORED PB IGNORED
2951 F01
t
DB, ON
t
KILL, ON BLANK
t
1
t
2
t
3
t
4
t
5
t
6
t
7
t
DB, OFF
t
OFFT
<t
KILL,
OFF DELAY
pin going low 512ms after it went high. (EN = low, see
Figure 2). Note that the LTC2951 does not sample KILL
and PB until after the 512ms internal timer has expired.
The reason PB is ignored is to ensure that the system
is not forced off while powering on. Once the 512ms
timer expires (t
4
), the release of the PB pin is then
debounced with an internal 32ms timer. The system has
now properly powered on and the LTC2951 monitors PB
and KILL (for a turnoff command) while consuming only
6µA of supply current.
A high to low transition on PB (t
5
) initiates the power-off
sequence. PB must stay low continuously (PB high resets
debounce timer) for a period controlled by the default 32ms