LTC2956
15
2956fa
For more information www.linear.com/LTC2956
OPERATION
Shutting Off with a Long SLEEP Pulse
In an active system, the LTC2956 can also be placed into
the low power SHUTDOWN mode by holding SLEEP high
for a duration greater than t
LONG
(Figure 8). The OFFALERT
pin is pulled low when t
LONG
times out and remains low
during the Shutting Off state. Unlike a long push button
press event, however, the t
ONMAX
timer remains enabled
while SLEEP is pulled high. If t
ONMAX
is adjusted to be
less than t
LONG
, a long SLEEP pulse can never trigger a
shutdown.
If the SLEEP pulse width is shorter than t
LONG
, the LTC2956
pulls the EN output high but does not enter the Shutting
Off state. Once SLEEP goes low (or if t
ONMAX
times out
first), the EN output goes low and the device enters the
Sleep state.
Pulling SLEEP High During the Sleep State
During the Sleep state, the system can prompt the LTC2956
to enter the Awake state by pulling SLEEP pin high even if
the t
PERIOD
time is not up. This is illustrated in Figure 9.
The LTC2956 automatically restarts the t
PERIOD
timer when
it enters the Awake state in response to the logic high on
the SLEEP input.
Upon entering the Sleep state, the LTC2956 ignores all
inputs for t
SLEEP_MIN
. This allows time for the system’s
power supply to fully discharge before it is allowed to turn
on again. If the system pulls SLEEP high during t
SLEEP_MIN
,
the LTC
2956 will wait until t
SLEEP_MIN
expires before re-
entering the Awake state.
Prolonging Awake Time
The LTC2956 restarts the t
PERIOD
timer on every rising edge
of EN. This allows the Awake time, t
AWAKE
to vary from cycle
to cycle while the turn-on period stay constant. However, if
t
AWAKE
extends toward or beyond t
PERIOD
, the next Awake
state may be delayed due to the minimum Sleep duration,
t
SLEEP_MIN
(128ms typically). This t
SLEEP_MIN
ensures that
the system supply has sufficient time to discharge. Refer
to Figure 10, the second Awake time is longer, resulting
in a longer t
PERIOD
due to t
SLEEP_MIN
.
TONMAX
t
ONMAX
MUST BE LONGER THAN
t
LONG
SHUTDOWN MODE
TLONG
EN
OFFALERT
STATE
PERIOD
TIMER
SLEEP
OFF
STOP
0V
SLEEP AWAKE SHUTTING OFF
t
AWAKE_MIN
STOP
RESTART
IGNORED
t
OFF
t
LONG
Figure 8. Holding SLEEP High to Enter SHUTDOWN Mode
