LTC2950 LINER [Linear Technology], LTC2950 Datasheet - Page 11

• Manufacturer Part Number: LTC2950
• Description: Push Button On/Off Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
and ⎯ K ⎯ I ⎯ L ⎯ L (for a turnoff command) while consuming only
6µA of supply current.
A high to low transition on ⎯ P ⎯ B (t
off sequence. ⎯ P ⎯ B must stay low continuously ( ⎯ P ⎯ B high
resets debounce timer) for a period controlled by the
default 32ms and the external OFFT capacitor (t
the completion of the OFFT timing (t
is set, signifying that EN will be switched low after the
⎯ K ⎯ I ⎯ L ⎯ L turn off (t
timeout period. Once a system has fi nished performing
its power down operations, it can set ⎯ K ⎯ I ⎯ L ⎯ L low (t
thus immediately set EN low, terminating the ⎯ K ⎯ I ⎯ L ⎯ L turn off
timer. The release of the ⎯ P ⎯ B pin is then debounced with
an internal 32ms timer.
The system is now in its reset state: where the LTC2951
is in low power mode (6µA). ⎯ P ⎯ B is monitored for a high
to low transition.
Aborted Power On Sequence
The power on sequence is aborted when the ⎯ K ⎯ I ⎯ L ⎯ L remains
low after the end of the 512ms blanking time. Figure 2 is
a simplifi ed version of an aborted power on sequence.
At time t
turning off the DC/DC converter).
µP Turns Off Power During Normal Operation
Once the system has powered on and is operating nor-
mally, the µP can turn off power by setting ⎯ K ⎯ I ⎯ L ⎯ L low, as
shown in Figure 3. At time t
KILL
PB
EN
ABORT
Figure 3. µP Turns Off Power (LTC2951-1)
, since ⎯ K ⎯ I ⎯ L ⎯ L is still low, EN pulls low (thus
⎯ K ⎯ I ⎯ L ⎯ L
XXX DON’T CARE
,
U
OFF DELAY
U
+ t
⎯ K ⎯ I ⎯ L ⎯ L
PB & KILL IGNORED
⎯ K ⎯ I ⎯ L ⎯ L
, ⎯ K ⎯ I ⎯ L ⎯ L is set low by the
5
,
W
) initiates the power
6
OFF DELAY, ADDITIONAL
), an interrupt ( ⎯ I ⎯ N ⎯ T )
KILL LOW
µP SETS
t
TURNS OFF
KILL
U
DC/DC
6
–t
2951 F03
7
5
) and
). At
)
µP. This immediately pulls EN low, thus turning off the
DC/DC converter.
DC/DC Turn Off Blanking
When the DC/DC converter is turned off, it can take a sig-
nifi cant amount of time for its output to decay to ground. It
is desirable to wait until the output of the DC/DC converter
is near ground before allowing the user (via ⎯ P ⎯ B ) to restart
the converter. This condition guarantees that the µP has
always powered down completely before it is restarted.
Figure 4 shows the µP turning power off. After a low on
⎯ K ⎯ I ⎯ L ⎯ L releases enable, ⎯ P ⎯ B is ignored during the internal
256ms timer period. This is shown as t
Figure 4.
LTC2951-1, LTC2951-2 Versions
The LTC2951-1 (high true EN) and LTC2951-2 (low true
⎯ E ⎯ N ) differ only by the polarity of the EN/ ⎯ E ⎯ N pin. Both ver-
sions allow the user to extend the amount of time that the
⎯ P ⎯ B must be held low in order to begin a valid power off
sequence. An external capacitor placed on the OFFT pin
adds additional time to the turn off debounce time. If no
capacitor is placed on the OFFT pin, then the turn off de-
bounce time is given by an internally fi xed 32ms timer.
Both versions of the LTC2951 provide extendable ⎯ K ⎯ I ⎯ L ⎯ L
turn off timer, t
an optional external capacitor on the KILLT pin. The default
⎯ K ⎯ I ⎯ L ⎯ L turn off delay time is 128ms, t
PB BLANKING
KILL
PB
EN
Figure 4. DC/DC Turn Off Blanking (LTC2951-1)
XXX DON’T CARE
POWER ON
⎯ K ⎯ I ⎯ L ⎯ L , OFF DELAY, ADDITIONAL
PB & KILL IGNORED
TURNS OFF
⎯ K ⎯ I ⎯ L ⎯ L
DC/DC
LTC2951
,
EN/ ⎯ E ⎯ N , LOCKOUT
OFF DELAY
, by connecting
t
EN/EN, LOCKOUT
PB IGNORED
256ms
µP SETS
KILL LOW
11
.
2951f
2951 F04
in