LTC2954
11
2954fb
Aborted Power-On Sequence
The power-on sequence is aborted when the KILL remains
low at the end of the 512ms blanking time. Figure 2 is a
simplified version of an aborted power-on sequence. At
time t
ABORT
, since KILL is still low, EN pulls low (thus
turning off the DC/DC converter).
μP Turns Off Power During Normal Operation
Once the system has powered on and is operating nor-
mally, the μP can turn off power by setting KILL low, as
shown in (Figure 3). At time t
KILL
, KILL is set low by the
μP. This immediately pulls EN low, thus turning off the
DC/DC converter.
DC/DC Turn Off Blanking
When the DC/DC converter is turned off, it can take a sig-
nificant amount of time for its output to decay to ground. It
is desirable to wait until the output of the DC/DC converter
is near ground before allowing the user (via PB) to restart
the converter. This condition guarantees that the μP has
always powered down completely before it is restarted.
Figure 4 shows the μP turning power off. After a low on
KILL releases enable, the internal 256ms timer ignores the
PB pin. This is shown as t
EN/EN, LOCKOUT
in (Figure 4).
Figure 3. μP Turns Off Power (LTC2954-1) Figure 4. DC/DC Turn-Off Blanking (LTC2954-1)
low 512ms after it went high. Note that the LTC2954 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 LTC2954 monitors PB
and KILL for a turn-off command while consuming only
6μA of supply current.
A high to low transition on PB (t
5
) starts the power-off
sequence debounce timer. In order to assert the interrupt
output (INT), PB must stay low continuously (PB high
resets debounce timer) for 32ms (t
6
–t
5
). At the comple-
tion of the power-down debounce timer (t
6
), an internal
interrupt timer keeps the interrupt output low for at least
32ms, even if PB is released between t
6
and t
7
. If PB is
low at the end of this 32ms internal timer (t
7
), the external
adjustable power-down timer is started. The capacitor
placed at the PDT pin will determine the time period of
this timer. If the pushbutton is released prior to 16 cycles
of the PDT pin, the interrupt output will go high (t
8
). Note
that the enable output is not directly changed by this
interrupt pulse. The function of the interrupt signal is to
initiate a software shutdown. At t
9
, the μP has performed
its power-down functions and asserted the KILL input
low. This releases the enable output, which in turn shuts
down system power. Note that if the pushbutton is held
long enough to count 16 cycles at the PDT pin, the enable
pin would be released immediately after the 16th cycle.
The system is now in its reset state where the LTC2954 is
in low power mode (6μA) and PB is monitored for a high
to low transition.
APPLICATIONS INFORMATION
t
KILL
PB
EN
DC/DC
TURNS OFF
2954 F03
KILL
XXX DON’T CARE
μP SETS
KILL LOW
PB
PB IGNORED
EN
KILL
PB BLANKING
(INTERNAL
SIGNAL)
XXX DON’T CARE
256ms
POWER ON
2954 F04
DC/DC
TURNS OFF
μP SETS
KILL LOW
t
EN/EN, LOCKOUT
