LTC2956
10
2956fa
For more information www.linear.com/LTC2956
OPERATION
OVERVIEW
The LTC2956 is a micropower, highly configurable wake-up
timer with pushbutton control. It controls power to a con-
nected system to allow that system to perform a periodic
task. After completing the task, the LTC2956 turns the
system off to conserve power. Figure 1 shows a simpli-
fied state diagram of the LTC2956. The LTC2956 has two
modes of operation: RUN mode and SHUTDOWN mode.
In RUN mode, the LTC2956 cycles between the Awake
state and the Sleep state with a period of t
PERIOD
. During
the Awake state the EN pin is pulled high to turn the sys-
tem on, and the t
ONMAX
timer runs. The LTC2956 exits the
Awake state when a falling edge is detected on the SLEEP
pin or if the t
ONMAX
timer expires. The LTC2956 ignores
any SLEEP pin transitions for t
AWAKE_MIN
(typically 8ms)
after entering the Awake state to ensure that the minimum
on time is met.
In the Sleep state, the EN output stays low, and the con-
nected system turns off. The LTC2956 returns to the Awake
state if the t
PERIOD
time is reached, if the pushbutton
(connected to the PB pin) is pressed, or if a logic high
is detected on the SLEEP pin. The minimum duration for
the Sleep state is t
SLEEP_MIN
(typically 128ms), and any
input or t
PERIOD
transitions during this time are ignored in
order to allow the system supply to sufficiently discharge.
SHUTDOWN mode is a very low power mode with the timer
disabled, entered through a long press of the pushbutton
or a long high pulse at the SLEEP input. This is useful for
products that are shipped with their batteries installed. A
short press on the pushbutton brings the LTC2956 back
into RUN mode. The LTC2956 can be configured to start
in either RUN or SHUTDOWN mode at power-up via the
LONG pin (see the Applications Information section).
SHUTDOWN mode consists of the Shutting Off state and
the Off state. The Shutting Off state occurs first during
the transition from RUN mode to SHUTDOWN mode, and
allows time for the system to perform any shutdown tasks
before the EN pin goes low in the Off state. This state is
indicated by the OFFALERT pin pulling low and lasts for
t
OFF
(typically 1s). Once the t
OFF
timer completes, the
LTC2956 transitions to the Off state, the EN pin is pulled
low and all timers are disabled, and the quiescent current
drops to about 0.3μA.
When the LTC2956 returns to the RUN mode from the
SHUTDOWN mode, it first enters the From Off state to
allow the connected system to run initialization tasks. This
is indicated by the ONALERT pin pulling low. The EN pin
is pulled high to turn the connected system on.
The LTC2956 turns the connected system on and off
via the EN pin. The LTC2956-1's active high EN pin can
be connected to the system supply RUN pin while the
LTC2956-2's active low EN pin can directly control the
system supply through a P-channel MOSFET.
The wake-up timer period can be adjusted from 250ms to
39 days via resistors at the PERIOD and RANGE pins. The
timing of the long pushbutton press and SLEEP pulse can
be adjusted via resistors connected to the LONG pin. To
minimize the quiescent current, the RANGE, LONG, and
PERIOD pins are biased only when the LTC2956 performs
a configuration cycle.
Timer configuration is performed once every
66 seconds
while the part is operating in RUN mode, and it is also
performed following power-up and whenever the LTC2956
exits the SHUTDOWN mode. During configuration, the
LTC2956 biases and reads the voltages at the RANGE
and LONG pins that are defined by external configuration
resistors. See Configuration section for more details.
Operations with SLEEP Tied Low (Passive System)
The LTC2956 is designed to control the power of both
active and passive systems. With a passive system where
the SLEEP pin is not managed by a microcontroller, the
SLEEP pin can be tied low and a capacitor connected to
the ONMAX pin determines the awake time. The minimum
t
ONMAX
must be longer than the maximum expected time
the system takes to complete its routine tasks.
