MAX791EJE Maxim Integrated, MAX791EJE Datasheet - Page 16

• Manufacturer Part Number: MAX791EJE
• Description: Supervisory Circuits
• Manufacturer: Maxim Integrated
• Series: MAX791r

Specifications of MAX791EJE

• Number Of Voltages Monitored: 1
• Monitored Voltage: 4.65 V
• Undervoltage Threshold: 4.5 V
• Overvoltage Threshold: 4.75 V
• Output Type: Active Low, Open Drain, Push-Pull
• Manual Reset: Resettable
• Watchdog: Watchdog
• Battery Backup Switching: Backup
• Power-up Reset Delay (typ): 280 ms
• Supply Voltage - Max: 5.5 V
• Maximum Operating Temperature: + 85 C
• Mounting Style: Through Hole
• Package / Case: CDIP-16
• Chip Enable Signals: Yes
• Maximum Power Dissipation: 800 mW
• Minimum Operating Temperature: - 40 C
• Power Fail Detection: Yes
• Supply Current (typ): 150 uA
• Supply Voltage - Min: 0 V

close to SWT as possible. The sum of PC board leak-
age + SWT capacitor leakage must be small compared
to ±100nA.
A way to help the watchdog timer keep a closer watch
on software execution involves setting and resetting the
watchdog input at different points in the program,
rather than “pulsing” the watchdog input high-low-high
or low-high-low. This technique avoids a “stuck” loop
where the watchdog timer continues to be reset within
the loop, keeping the watchdog from timing out.
Figure 17 shows an example flow diagram where the
I/O driving the watchdog input is set high at the begin-
ning of the program, set low at the beginning of every
subroutine or loop, then set high again when the pro-
Microprocessor Supervisory Circuit
Figure 15. Monitoring a Negative Voltage
16
PFO
+5V
NOTE: V
______________________________________________________________________________________
5 - 1.25
0V
R1
+5V
Watchdog Software Considerations
=
TRIP
1.25 - V
IS NEGATIVE
R2
R2
R1
TRIP
V-
V
V-
TRIP
PFI
MAX791
GND
V
CC
PFO
0V
gram returns to the beginning. If the program should
“hang” in any subroutine, the I/O is continually set low
and the watchdog timer is allowed to time out, causing
a reset or interrupt to be issued.
The V
the battery switchover comparator and should not
exceed 0.03V/µs. A standard rule of thumb for filter
capacitance on most regulators is on the order of
100µF per amp of current. When the power supply is
shut off or the main battery is disconnected, the associ-
ated initial V
= 0.01V/µs. The V
V
maximum fall-time requirement.
Figure 16. Maximum Transient Duration Without Causing a
Reset Pulse vs. Reset Comparator Overdrive
CC
falls exponentially, which more than satisfies the
CC
fall time is limited by the propagation delay of
100
80
60
40
20
CC
0
10
fall rate is just the inverse or 1A / 100µF
RESET COMPARATOR OVERDRIVE (mV)
CC
(Reset Threshold Voltage - V
fall rate decreases with time as
Maximum V
100
V
T
0.1µF CAPACITOR
FROM V
CC
A
1000
= +25°C
= +5V
OUT
CC
CC
TO GND
)
10,000
Fall Time