LTC2952 Linear Technology, LTC2952 Datasheet - Page 20

• Manufacturer Part Number: LTC2952
• Description: Pushbutton PowerPath Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC2952
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
2. No valid edge on the WDE pin in a t
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
impedance for t
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).
20
ON/OFF SIGNAL
period after the RST pin transitions from pulling low
to high impedance.
period since the last valid edge on the WDE pin while
the RST pin is high impedance.
INTERNAL
KILL
EN
Figure 10. Power-On and Power-Off Sequence with
KILL Deasserting EN During KILL Off Wait Time
RST
WDE
DON'T CARE
(200ms) before again becoming high
(1.6s). This will continue unless there
t
t
1
KILL, ON BLANKING
t
2
t
3
t
<t
4
KILL, OFF WAIT
WDE
WDE
t
5
t
(1.6s) time
(1.6s) time
EN, LOCKOUT
t
6
2952 F10
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
high impedance and an internal 500ms (t
timer starts. During this 500ms 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
Failure to set KILL pin high at the end of the 500ms KILL
On Blanking time (t
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
shutdown sequence is immediately started. From the start
of the shutdown sequence, the system power will turn off
in 500ms (t
low-to-high transition) at the WDE pin is detected within
the 500ms period to extend the wait period for another
500ms.
This KILL Off Wait time (500ms/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 500ms
KILL Off Wait time expire on its own or set the KILL pin
low (t
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, an-
other 500ms (Enable Lockout time, t
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 500ms Enable Lockout time (t
LTC2952 goes into its reset state with the EN pin remains
strongly pulling down.
2
) indicating that proper power-up sequence is completed.
5
) immediately terminating the KILL Off Wait time.
KILL, OFF WAIT
3
) will result in immediate system
), unless an edge (a high-to-low or
EN, LOCKOUT
1
), the EN pin goes
KILL
,
ON BLANK
) timer is
6
), the
4
2952fa
), a
)