Datasheet

LTC2951-1/LTC2951-2
10
295112fb
Description
The LTC2951 is a low power (6µA), wide input voltage range
(2.7V to 26.4V), pushbutton On/Off controller that can in-
terface to a µP and a power supply. The part incorporates
all the flexible timing needed to debounce the pushbutton
input (PB). The LTC2951 also provides a simple interface
(INT output, KILL input) to allow a system to power on and
power off in a controlled manner. The wide input voltage
range allows a system designer to operate from single cell
to multi-cell battery stacks. Very low quiescent current
makes the LTC2951 ideal for continuously monitoring the
on/off pushbutton of a handheld device.
Turn-On
When power is first applied to the LTC2951, the part initial-
izes the output pins. Any DC/DC converters connected to
the EN/EN pin will, therefore, be off. To assert the enable
output, PB must be held low for a minimum of 128ms
(t
DB
,
ON
).
Once the enable output is asserted, any DC/DC converters
connected to this pin are turned on. The KILL input from
the µP is ignored during the succeeding 512ms blanking
time (t
KILL
,
ON BLANK
). This blanking time represents the
maximum time required to power up the DC/DC converter
and the µP. If KILL is not brought high during this 512ms
time window, the enable output is released. The assumption
is that 512ms is sufficient time for the system to power up.
Turn-Off
To initiate a power-off sequence, PB must be held low for a
minimum of 32ms (t
DB
,
OFF
). Additional turn-off debounce
time may be added via an optional capacitor connected
to the OFFT pin (t
OFFT
). The following equation describes
the additional time that PB must be held low to initiate a
power-off sequence. C
OFFT
is the OFFT external capacitor:
C
OFFT
= 1.56E-4 [µF/ms] • (t
OFFT
– 1ms)
Once PB has been validly pressed, INT is switched low.
This alerts the µP to perform its power-down and house-
keeping tasks.
KILL Turn-Off Delay
The LTC2951 provides a failsafe feature that allows the
user to turn off system power (via PB) under system fault
conditions. During a normal power-down sequence, the
LTC2951 first interrupts the µP by setting INT low. The
µP then performs power-down and housekeeping tasks
and drives KILL low when done. The LTC2951 releases the
enable output, thus turning off system power. The KILL
turn-off timer starts when INT is driven low. If the µP fails
to respond during this timeout period, the enable output
will automatically release. The default power-down timeout
period is 128ms (t
KILL
,
OFF DELAY
), which can be extended
by placing an optional capacitor on the KILLT pin (t
KILL
,
OFF DELAY, ADDITIONAL
). The following equation describes
the additional power-down timeout period. C
KILLT
is the
KILLT external capacitor:
C
KILLT
= 1.56e-4 [µF/ms] • (t
KILL
,
OFF DELAY, ADDITIONAL
– 1ms)
Note that KILL can be driven low (thereby releasing the
enable output) at any time after t
KILL
,
ON BLANK
period.
APPLICATIONS INFORMATION