LTC2953

13

2953f

PUSH BUTTON CONTROL

Power On Sequence

To enable system power, the push button input (

⎯P⎯B) must

enable output (EN/

⎯E⎯N) is asserted, the LTC2953 starts

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

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

delay is set by placing an external capacitor on the PDT

pin. Use the following equation to calculate the capacitance

capacitor:

See Figure 11.

PB

KILL

EN

(LTC2953-1)

2953 F09

PB, UVLO AND KILL

IGNORED

DO NOT CARE

SYSTEM SETS

KILL HIGH

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

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)