LTC2952CF datasheet(20/30 Pages) LINER

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LTC2952

2952fa

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 tWDE (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 tWDE (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 tRST (200ms) before again becoming high

impedance for tWDE (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 (t1), the EN pin goes

high impedance and an internal 500ms (tKILL, ON BLANK)

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 2)indicatingthatproperpower-upsequenceiscompleted.

Failure to set KILL pin high at the end of the 500ms KILL

On Blanking time (t3) 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 (t4), a

shutdown sequence is immediately started. From the start

of the shutdown sequence, the system power will turn off

in 500ms (tKILL,OFF WAIT), unless an edge (a high-to-low or

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 (t5) 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, an-

other 500ms (Enable Lockout time, tEN, 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 500ms Enable Lockout time (t6), the

LTC2952 goes into its reset state with the EN pin remains

strongly pulling down.

APPLICATIONS INFORMATION

2952 F10

KILL

INTERNAL

ON/OFF SIGNAL

tKILL, ON BLANKING

t3 t4

DON'T CARE

<tKILL, OFF WAIT tEN, LOCKOUT

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