LTC2955
8
2955fa
For more information www.linear.com/LTC2955
Figure 1. Pushbutton Turn-On Timing
operaTion
Overview
The LTC2955 is a pushbutton on/off controller that man-
ages system power based on a pushbutton input (PB
pin)
and a voltage monitor input (ON pin). In a typical application,
the enable output (EN/EN) turns on system power when
the pushbutton is pressed or the supply monitor detects
the presence of a primary or secondary supply such as a
wall adaptor or car battery.
After the system is powered up, a pushbutton event as
-
serts the interrupt output (INT) wh
ich can be used in menu
driven applications to request for a system power-down.
A power kill input (KILL) allows a microprocessor or sys
-
tem to release the enable output immediately, effectively
powering down the system. System power is also forced
off if the pushbutton remains asserted for more than the
forced turn-off activation time configurable via the TMR pin.
The LTC2955 is also designed with a blanking time after
each system turn on and off event. During this blanking
time, the LTC2955 ignores the KILL, ON and PB pins,
thus ensuring that the system stays on/off for a minimum
amount of time. This provides sufficient time for the voltage
regulator to turn on/off and allows it to charge/discharge
its output to the final voltage. It also allows the µP suf
-
ficient time to perform power on/off tasks.
T
he PG
D output indicates the status of the ON pin to allow
the system to differentiate between pushbutton turn-on
or supply plug-in events.
Turn-On Sequence and KILL Blanking Time
The LTC2955-1 asserts the EN output high if the pushbut
-
ton is pressed or the ON pin goes high. This is typically
used to turn on a DC/DC converter or linear regulator.
Figure 1 shows the turn-on sequence of the LTC2955-1
initiated by the PB pin. When the button is pressed at time
t
1
, a high to low transition on the PB pin initiates the turn-
on sequence. The PB pin must stay low continuously for
a period of 32ms (t
DB(ON)
) before the LTC2955-1 asserts
the EN output high. Once EN goes high (t
2
), an internal
512ms blanking time (t
KILL(OnBlank)
) is started. During
this blanking time, the LTC2955-1 ignores the KILL, ON
and PB pins and keeps EN high for at least 512ms. This
provides sufficient time for the voltage regulator to turn
on and charge its output to the final voltage and allows
the µP enough time to perform power on tasks. The µP
must pull the KILL pin high during this 512ms blanking
time, or else EN will go low at the end of this time (t
3
).
t
DB(ON)
t
KILL(ON BLANK)
PB AND KILL IGNORED
PB
2955 F01
EN
(LTC2955-1)
t
1
t
2
t
3
