Adding Hysteresis to the Power-Fail

Comparator (MAX793/MAX794)

The power-fail comparator has a typical input hystere-

sis of 10mV. This is sufficient for most applications

where a power-supply line is being monitored through

an external voltage divider (see the section

Monitoring

an Additional Power Supply).

If additional noise margin is desired, connect a resistor

between PFO and PFI as shown in Figure 16a. Select

hysteresis and should typically be more than 10 times

the value of R1 or R2. The hysteresis window extends

Connecting an ordinary signal diode in series with R3,

This method provides additional noise margin without

compromising the accuracy of the power-fail threshold

when the monitored voltage is falling. It is useful for

accurately detecting when a voltage falls past a thresh-

old. The current through R1 and R2 should be at least

1µA to ensure that the 25nA (max over temperature)

PFI input current does not shift the trip point. R3 should

be larger than 82k

Ω so it does not load down the PFO

pin. Capacitor C1 is optional, and adds noise rejection.

3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

16

______________________________________________________________________________________

MAX793

MAX794

MAX795

3.0V OR 3.3V

TO CMOS RAM

BODY DIODE

GND

MAX793

MAX794

MAX795

BATT ON

OUT

S

D

G

PMOS FET

BATT ON

OUT

GND

(b)

(a)

Figure 14. Driving an External Transistor with BATT ON

MAX793

MAX794

OUT

TO STATIC

RAM

BATT

GND

1N4148

RESET

TO

µP

0.47F

3.0V OR 3.3V

MAX793

MAX794

OUT

TO STATIC

RAM

BATT

GND

1N4148

RESET

TO

µP

0.47F

3.0V OR

3.3V

+5V

(b)

(a)

Figure 15. Using a Super Cap as a Backup Source