Datasheet
MAX793/MAX794/MAX795
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
the ratio of R1 and R2 such that PFI sees V
PFT
when
V
IN
falls to its trip point (V
TRIP
). R3 adds the additional
hysteresis and should typically be more than 10 times
the value of R1 or R2. The hysteresis window extends
both above (V
H
) and below (V
L
) the original trip point
(V
TRIP
).
Connecting an ordinary signal diode in series with R3,
as shown in Figure 16b, causes the lower trip point (V
L
)
to coincide with the trip point without hysteresis (V
TRIP
),
so the entire hysteresis window occurs above V
TRIP
.
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 82kΩ 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 ONV
CC
OUT
S
D
G
PMOS FET
BATT ONV
CC
OUT
GND
(b)(a)
Figure 14. Driving an External Transistor with BATT ON
MAX793
MAX794
OUT
TO STATIC
RAM
BATT
V
CC
GND
1N4148
RESET
TO µP
0.47F
3.0V OR 3.3V
MAX793
MAX794
OUT
TO STATIC
RAM
BATT
V
CC
V
CC
GND
1N4148
RESET
TO µP
0.47F
3.0V OR
3.3V
+5V
(b)(a)
V
CC
Figure 15. Using a Super Cap as a Backup Source