TC3404

DS21413B-page 10

© 2002 Microchip Technology Inc.

3.3

Power Fail Detector

The Power Fail detector is a comparator in which the

inverting input is connected to the internal voltage

reference. The non-inverting input is the PFI pin of the

TC3404 and the PFO pin is the active low, open drain

output. This comparator is suitable as an early warning

fail or low battery indicator. In a typical application,

where a voltage regulator is being used to supply

power to a system, the Power Fail comparator would

monitor would measure the output voltage of the

regulator. Both PFO and RESET would drive interrupt

pins of a microcontroller.

ThePower Fail detector may beusedas a Wake-upor

Watchdog Timer. The Typical Application circuit shows

an RC network on PFI with the capacitor tied to a

tristated

µC I/O pin. If R4 is 1 MΩ and C2 is 10µF, the

time constant is roughly ten seconds. The

µC resets

the RC network by driving the I/O tied to PFI low and

then tristating it. The RC network will ramp to 1.23V in

PFO tied to a

µC input or interrupt, the µC will see a low

to high transition on PFO when the voltage on PFI

exceeds 1.23V. The PFO output is specified to be valid