LTC2952
20
2952fb
Figure 10. Power-On and Power-Off Sequence with
KILL Deasserting EN During KILL Off Wait Time
WDE pin. The watchdog timer can expire due to any of
the following conditions:
1. No valid edge on the WDE pin in a t
WDE
(1.6s) time
period after the RST pin transitions from pulling low
to high impedance.
2. No valid edge on the WDE pin in a t
WDE
(1.6s) time
period since the last valid edge on the WDE pin while
the RST pin is high impedance.
As shown in the Timing Diagrams section, when the
watchdog timer is allowed to expire while voltage at the
VM pin is higher than 0.5V, the RST pin strongly pulls down
to ground for t
RST
(200ms) before again becoming high
impedance for t
WDE
(1.6s). This will continue unless there
is an edge at the WDE pin, the voltage at VM goes below
0.5V, or the watchdog function is disabled (by leaving the
WDE in a Hi-Z state).
In certain PowerPath configurations where both of the
ideal diode drivers are disabled, the watchdog function of
the WDE pin is also disabled. Examples of such configura-
tions are configuration C (Figure 5) and configuration D
(Figure 6) when both of the ideal diode can be turned off
due to a valid pushbutton off or a digital off command.
Power-On/Power-Off Sequence
Figure 10 shows a normal power-on and power-off timing
diagram. Note that in this timing diagram only the clean
internal ON/OFF signal is shown. A transition at this in-
ternal ON/OFF signal can be caused by a valid debounced
pushbutton ON/OFF or a digital ON/OFF through the mode
input pins (M1/M2).
In this timing sequence, the KILL pin has been set low
since power is first applied to the LTC2952. As soon as the
internal ON/OFF signal transitions high (t
1
), the EN pin goes
high impedance and an internal 400ms (t
KILL
,
ON BLANK
)
timer starts. During this 400ms KILL On Blanking period,
the input to KILL pin is ignored and the EN pin remains in
its high impedance state. This KILL On Blanking period
is designed to give the system sufficient time to power
up properly.
Once the μP/system powers on, it sets the KILL pin high
(t
2
) indicating that proper power-up sequence is completed.
Failure to set KILL pin high at the end of the 400ms KILL
On Blanking time (t
3
) will result in immediate system
shutdown (see Aborted Power-On Sequence segment).
After the KILL On Blanking time expires, the system is
now in normal operation with power turned on.
When the internal ON/OFF signal transitions low (t
4
), a
shutdown sequence is immediately started. From the start
of the shutdown sequence, the system power will turn off
in 400ms (t
KILL, OFF
WAIT
), unless an edge (a high-to-low or
low-to-high transition) at the WDE pin is detected within
the 400ms period to extend the wait period for another
400ms.
This KILL Off Wait time (400ms/cycle) is designed to allow
the system to finish performing its housekeeping tasks
before shutdown. Once the μP finishes performing its
power-down operations, it can either let the 400ms KILL
Off Wait time expire on its own or set the KILL pin low
(t
5
) immediately terminating the KILL Off Wait time. When
the KILL Off Wait time expires, the LTC2952 sets EN low,
turning off the DC/DC converter connected to the EN pin.
When the DC/DC converter is turned off (EN goes low), it
can take a significant amount of time for its output level
to decay to ground. In order to guarantee that the μP
has always powered down properly before it is restarted,
another 400ms (Enable Lockout time, t
EN,
LOCKOUT
) timer
is started to allow for the DC/DC converter output power
level to power down completely to ground. During this
Enable Lockout time, the EN pin remains in its low state.
At the end of the 400ms Enable Lockout time (t
6
), the
LTC2952 goes into its reset state with the EN pin remains
strongly pulling down.
APPLICATIONS INFORMATION
2952 F10
KILL
EN
INTERNAL
ON/OFF SIGNAL
t
KILL, ON BLANKING
t
1
t
2
t
3
t
4
t
5
t
6
DON'T CARE
<t
KILL, OFF WAIT
t
EN, LOCKOUT
