Datasheet

LTC2953
13
2953f
PUSH BUTTON CONTROL
Power On Sequence
To enable system power, the push button input (
⎯
P
⎯
B) must
be held low continuously for 32ms (t
DB, ON
). Once the
enable output (EN/
⎯
E
⎯
N) is asserted, the LTC2953 starts
a 512ms internal timer (t
KILL, ON BLANK
). The
⎯
K
⎯
I
⎯
L
⎯
L input
must be driven high within this 512ms window. This blank-
ing time represents the maximum time allowed for the
system to power up and initialize the circuits driving the
⎯
K
⎯
I
⎯
L
⎯
L input. If
⎯
K
⎯
I
⎯
L
⎯
L remains low at the end of the blanking
period, the enable output is released (see “Aborted Power
On Sequence” section). Figure 9 shows a normal power
on sequence.
APPLICATIONS INFORMATION
Short Pulse Interrupt
To interrupt the μP, either
⎯
P
⎯
B or UVLO must be low for
at least 32ms (t
DB, OFF
). This signals the μP either that a
user has pressed the push button or that the supply is
running low. The μP would then perform power down
and housekeeping tasks and assert
⎯
K
⎯
I
⎯
L
⎯
L low when done.
This in turn releases the enable output, thus shutting off
system power. See Figure 10.
Note that either
⎯
P
⎯
B or UVLO can control the power down
sequence, but not both at the same time. For example, if
both
⎯
P
⎯
B and UVLO are high and the user presses the push
button,
⎯
P
⎯
B will be active and UVLO will be ignored until
⎯
P
⎯
B
is released or the power down sequence is complete.
Forced Power Off Sequence
The LTC2953 provides a failsafe feature that allows a user
to manually force a system power down. For cases when
the μP fails to respond to the interrupt signal, the user
can force a power down by pressing and holding either
the push button or the UVLO inputs low.
The length of time required to release the enable output
is given by a fi xed internal 64ms delay (t
PD, Min
) plus an
adjustable power down timer delay (t
PDT
). The adjustable
delay is set by placing an external capacitor on the PDT
pin. Use the following equation to calculate the capacitance
for the desired extra delay. C
PDT
is the PDT pin external
capacitor:
C
PDT
= 1.56E-4 [μF/ms] • (t
PDT
– 1ms)
See Figure 11.
t
DB, ON
t
KILL, ON BLANK
PB
KILL
EN
(LTC2953-1)
2953 F09
PB, UVLO AND KILL
IGNORED
DO NOT CARE
SYSTEM SETS
KILL HIGH
t
DB, OFF
t
INT, Min
PB OR UVLO
KILL
EN
(LTC2953-1)
INT
2953 F10
SHORT PULSE
DO NOT CARE
SYSTEM SETS
KILL LOW
SYSTEM
POWER OFF
Note that only the push button input can enable system
power. The LTC2953 provides two enable output polarities
to allow DC/DC converter control (LTC2953-1) and external
power PFET control (LTC2953-2).
PB OR UVLO
EN
(LTC2953-1)
PDT
2953 F11
16 CYCLES
LONG PULSE
t
PDT
t
PD, Min
Figure 10. Power Off Interrupt Timing
Figure 11. Forced Power Off Timing with Adjustable Delay
(See Figure 5 for More Details)
Figure 9. Power On Timing (UVLO > 0.55V)