LTC2954 Linear Technology, LTC2954 Datasheet - Page 11

• Manufacturer Part Number: LTC2954
• Description: Push Button On/Off Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
low 512ms after it went high. Note that the LTC2954 does
not sample  K  I  L  L and  P  B until after the 512ms internal timer
has expired. The reason  P  B is ignored is to ensure that
the system is not forced off while powering on. Once the
512ms timer expires (t
debounced with an internal 32ms timer. The system has
now properly powered on and the LTC2954 monitors  P  B
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
sequence debounce timer. In order to assert the interrupt
output (  I  N  T ),  P  B must stay low continuously (  P  B high
resets debounce timer) for 32ms (t
tion of the power down debounce timer (t
interrupt timer keeps the interrupt output low for at least
32ms, even if  P  B is released between t
low at the end of this 32ms internal timer (t
adjustable power down timer is started. The capacitor
placed at the PDT pin will determine the time period of
this timer. If the push button is released prior to 16 cycles
of the PDT pin, the interrupt output will go high (t
that the enable output is not directly changed by this
interrupt pulse. The function of the interrupt signal is to
initiate a software shutdown. At t
its power down functions and asserted the  K  I  L  L input low.
This releases the enable output, which in turn shuts down
system power. Note that if the push button is held long
enough to count 16 cycles at the PDT pin, the enable pin
would be released immediately after the 16th cycle. The
KILL
PB
EN
Figure 3. µP Turns Off Power (LTC2954-1)
XXX DON’T CARE
U
4
), the release of the  P  B pin is then
U
9
5
, the µP has performed
) starts the power off
W
6
–t
5
). At the comple-
6
KILL LOW
and t
7
µP SETS
6
), the external
TURNS OFF
t
), an internal
KILL
DC/DC
7
U
2954 F03
. If  P  B is
8
). Note
system is now in its reset state where the LTC2954 is in
low power mode (6µA) and  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 at the end of the 512ms blanking time. Figure 2 is a
simplified 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
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-
nificant 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, the internal 256ms timer ignores the
 P  B pin. This is shown as t
PB BLANKING
(INTERNAL
SIGNAL)
ABORT
KILL
PB
EN
Figure 4. DC/DC Turn Off Blanking (LTC2954-1)
, since  K  I  L  L is still low, EN pulls low (thus
XXX DON’T CARE
POWER ON
EN/  E  N , LOCKOUT
 K  I  L  L
,  K  I  L  L is set low by the
TURNS OFF
DC/DC
LTC2954
in (Figure 4).
t
EN/EN, Lockout
PB IGNORED
256ms
µP SETS
KILL LOW
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