LTC2953

15

2953f

APPLICATIONS INFORMATION

2953 F14

LTC2953-1

–

+

–

+

+

0.5V

0.5V

R14

2150k

R13

23.2k

R12

196k

UVLO

COMPARATOR

POWER FAIL

COMPARATOR

PFO

PFI

UVLO

EN

INT

8.4V

DEBOUNCE

AND DELAY

DEBOUNCE

50mV

4mV

Which Input Initiated Power Down:

⎯P⎯B or UVLO?

The circuit in Figure 14 determines whether a power

down was initiated by a user pressing the push button

or by a battery drooping too low. If both

⎯I⎯N⎯T and ⎯P⎯F⎯O

outputs are low, then a low battery condition initiated a

power down.

PFI and UVLO Thresholds

The circuit depicted in Figure 14 uses one resistive divider

network for both power fail and low battery comparators.

The power fail comparator trips at a higher battery voltage

than the low battery comparator, thus providing a battery

warning before a power down sequence is initiated. Due

to the low offset architecture of the comparators, the

UVLO and

⎯P⎯F⎯I thresholds can be set to as close as ±5mV

apart. The trip thresholds of the circuit of Figure 14 are

6.04V and 5.40V for the power fail and low battery (UVLO)

comparators, respectively.

Push Button Lockout

The LTC2953 provides a push button lock out feature that

prevents a user from turning on a system with a dead

battery. The push button input is ignored when the UVLO

input voltage is less than the falling threshold (0.5V). See

Figure 15B.

UVLO

INT

2953 F15a

0.5V

0.55V

SUPPLY

GLITCH

UVLO

EN

INT

2953 F15b

0.5V

LOW SUPPLY

LOCKS OUT

ENABLE

LOW SUPPLY CONDITION

Figure 14. Dual Function Battery Comparators

Figure 15A. Supply Glitch Generates μP Interrupt

Figure 15B. Low Supply Initiates System

Power Down and Locks Out Enable