LTC2953

12

2953f

OPERATION

The LTC2953 is a push button On/Off controller with dual

function input and output supply monitors. The part con-

tains all the circuitry needed to debounce a push button

input and provides a simple μP handshake protocol for

reliable toggling of system power. The LTC2953 operates

over a wide 2.7V to 27V input voltage range and draws

only 14μA of current.

The LTC2953 features dual function supply monitoring: a

power fail comparator generates an early warning and an

under voltage lock-out comparator initiates a controlled

system power down.

Push Button Controller

The push button input controls the enable and interrupt

outputs. The enable output toggles system power while

the interrupt output provides debounced push button

status. The interrupt output can be used in menu driven

applications to request a system power down. A power

kill input allows a microprocessor or other logic to release

the enable output, thus immediately powering down the

system.

To assert the enable output (turn on system power), press

the push button (

⎯P⎯B) input and hold for at least 32ms.

See Figure 1.

Once system power has been enabled, a user can request

a system power down by again pressing the push button

for at least 32ms and releasing it before the PDT timer

counts 16 cycles. The LTC2953 then asserts the interrupt

output and the μP subsequently sets the

⎯K⎯I⎯L⎯L input low

to turn off system power. Note that the UVLO input can

also assert the interrupt output. See Figure 2 and Figure 3

and Dual Function Supply Monitors section.

In the event that the μP does not respond to the interrupt

request, the user can force release of the enable output by

pressing and holding down the push button (or UVLO) until

the PDT timer times out. See Figure 4 and Figure 5.

Dual Function Supply Monitors

An uncommitted power fail comparator provides real

time supply threshold information. The power fail input

(PFI) is compared against an accurate internal 0.5V refer-

ence and the comparison result is passed directly to the

power fail output (

⎯P⎯F⎯O) pin. The operation of the power

fail comparator is de-coupled from all other functionality

and is always active. See Figure 7.

The under voltage lockout comparator provides the user

with another method to initiate a controlled system power

down. If the UVLO pin voltage falls below its falling thresh-

old (0.5V) for longer than 32ms, the interrupt output is

asserted for a minimum of 32ms. If the UVLO pin voltage

remains below its threshold (0.5V) for an additional time

given by the PDT external capacitor, then the enable pin is

automatically released (thus powering down the system).

See Figure 3 and Figure 5.

This comparator also serves as an under voltage lockout. If

system power is off (enable released) and UVLO < 0.5V, the

UVLO comparator prevents the push button from turning

on system power (asserting enable output).

Voltage Supervisor with 200ms μP Reset

The LTC2953 provides a single adjustable supply monitor

with a nominal 200ms reset delay. When the VM input

voltage drops below 0.5V, the

⎯R⎯S⎯T output is pulled low.

⎯R⎯S⎯T remains low for 200ms after the VM input has risen

above 0.5V. The input 0.5V threshold has a guaranteed

accuracy of ±1.5% over temperature and process. The

operation of the supply monitor is de-coupled from all

other functionality and is always active. See Figure 6.