Datasheet
Monitoring an Additional Power Supply
These µP supervisors can monitor either positive or
negative supplies using a resistor voltage divider to
PFI. PFO can be used to generate an interrupt to the µP
or to cause reset to assert (Figure 12).
Interfacing to µPs with
Bidirectional Reset Pins
Since the RESET output is open drain, the MAX793/
MAX794/MAX795 interface easily with µPs that have
bidirectional reset pins, such as the Motorola 68HC11.
Connecting the RESET output of the µP supervisor
directly to the RESET input of the microcontroller with a
single pullup resistor allows either device to assert
reset (Figure 17).
Negative-Going V
CC
Transients
These supervisors are relatively immune to short-dura-
tion negative-going V
CC
transients (glitches) while issu-
ing resets to the µP during power-up, power-down, and
brownout conditions. Therefore, resetting the µP when
V
CC
experiences only small glitches is usually not rec-
ommended.
MAX793/MAX794/MAX795
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
______________________________________________________________________________________ 17
MAX793
MAX794
V
CC
GND
0V
TO µP
V
L
= R1
V
PFT
PFI
PFO
R1
R2 R3
*OPTIONAL
C1*
V
IN
V
TRIP
V
IN
PFO
0V
V
H
V
L
R1
+ R2
R2
V
H
= (V
PFT
+ V
PFH
) (R1)
V
TRIP
= V
PFT
+
R1
1
+
R2
1
R3
1
–
R3
V
CC
V
PFT
= 1.237V
V
PFH
= 10mV
WHERE
V
CC
GND
TO µP
PFI
PFO
R1
R2 R3
*OPTIONAL
C1*
V
IN
R1
+ R2
)
R2
V
H
= R1 (V
PFT
+ V
PFH
)
V
TRIP
= V
PFT
(
+
R1
1
+
R2
1
R3
1
–
R3
V
D
V
PFT
= 1.237V
V
PFH
= 10mV
V
D
= DIODE FORWARD VOLTAGE DROP
V
L
= V
TRIP
WHERE
MAX793
MAX794
0V
PFO
0V
V
H
V
IN
V
TRIP
(b)(a)
(
)
( )
( )
+
R1
1
+
R2
1
R3
1
( )
Figure 16. Adding Hysteresis to the Power-Fail Comparator: (a) Symmetrical Hysteresis, (b) Hysteresis Only on Rising V
IN
MAX793
MAX794
MAX795
V
CC
GND
V
CC
N
RESET
GENERATOR
GND
V
CC
RESET
RESET
µP
Figure 17. Interfacing to µPs with Bidirectional Reset I/O