LTC2954
U U
W U
APPLICATIO S I FOR ATIO
Description
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 assump-
tion is that 512ms is sufficient time for the system to
power up.
The LTC2954 is a push button On/Off controller that pro-
vides control of system power via a push button interface.
An enable output toggles system power while an interrupt
output provides debounced push button status. The inter-
rupt output can be used in menu driven applications to
request a system power down. A power kill input allows
a microprocessor or system to release the enable output,
effectively powering down the system. Independently ad-
justable On and Off timers allow dependable push button
control of the enable output and resistance to accidental
toggling of system power.
Turn Off
To initiate a power down sequence, assert the INT output
low by pressing the push button for a minimum of 32ms
(t
). The interrupt signal serves as a power down
DB,OFF
request to the µP. The µP would then perform power down
and housekeeping tasks and assert KILL low when done.
This in turn releases the enable output, thus shutting off
system power.
The length of timethepush buttoninput (PB)must be held
low in order to toggle the enable (EN/EN) output on and
off is independently adjustable with external capacitors
at the ONT/PDT pins, respectively. During normal opera-
tion, the interrupt output (INT) is asserted 32ms after PB
goes low. INT then tracks PB until either PB or EN/EN is
released. See Timing Diagrams on page 8.
Adjustable Power Down Timer
The LTC2954 provides a failsafe feature that allows the
user to turn off system power (via PB) under system fault
conditions. For cases when the µP fails to respond to the
interrupt signal, the user can force an immediate power
down by pressing and holding down the push button. The
length of time that PB must be held low is given by a fixed
The KILL input is used to immediately release the enable
output. During a normal power down sequence, INT re-
quests a system power down. The µP then performs its
housekeeping tasks and then sets KILL low. If the µP fails
to set KILL low, the user can force a system shutdown
by pressing and holding the push button until the PDT
timer expires.
internal 64ms delay (t
) plus an adjustable power
PD,Min
down timer delay (t , see timing diagram on page 8).
PDT
The adjustable delay is set by placing an optional external
capacitor on the PDT pin. Use the following equation to
calculate the capacitance for the desired delay. C
PDT external capacitor (µF):
is the
PDT
Turn On
-4
C
PDT
= 1.56 × 10 [µF/ms] • (t
– 1ms)
PDT
When power is first applied to the LTC2954, the part ini-
tializes the output pins. Any DC/DC converters connected
to the EN/EN pin will therefore be held off. To assert the
enable output, PB must be held low for a minimum of
Simplified Power On/Off Sequence
Figure1showsasimplifiedLTC2954-1poweronandpower
offsequence.AhightolowtransitiononPB(t )initiatesthe
1
32ms (t , ). The LTC2954 provides additional turn on
DB ON
power on sequence. In order to assert the enable output,
the PB pin must stay low continuously (PB high resets
timers) for a time controlled by the default 32ms and the
debounce time (t ) via an optional capacitor connected
ONT
to the ONT pin. The following equation describes the ad-
ditional time that PB must be held low before asserting the
external ONT capacitor (t –t ). Once EN goes high (t ),
2
1
2
enable output. C
is the ONT external capacitor (µF):
ONT
-4
an internal 512ms blanking timer is started. This blanking
timer is designed to give sufficient time for the DC/DC
converter to reach its final voltage, and to allow the µP
enough time to perform power on tasks.
C
ONT
= 1.56 × 10 [μF/ms] • (t
– 1ms)
ONT
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 a succeeding 512ms blanking
The KILL pin must be pulled high within 512ms of the EN
pin going high. Failure to do so results in the EN pin going
time (t
,
). This blanking time represents the
KILL ON BLANK
2954f
9