Full-Featured 碌P Supervisory Circuit with
卤1.5% Reset Accuracy
Backup-Battery Replacement
The backup battery may be disconnected while V
CC
is
above the reset threshold, provided BATT is bypassed
with a 0.1碌F capacitor to ground. No precautions are
necessary to avoid spurious reset pulses.
START
MAX807L/M/N
Negative-Going V
CC
Transients
While issuing resets to the 碌P during power-up, power-
down, and brownout conditions, these supervisors are
relatively immune to short-duration negative-going V
CC
transients (glitches). It is usually undesirable to reset
the 碌P when V
CC
experiences only small glitches.
The
Typical Operating Characteristics
show Maximum
Transient Duration vs. Reset Comparator Overdrive, for
which reset pulses are not generated. The graph was
produced using negative-going V
CC
pulses, starting at
5V and ending below the reset threshold by the magni-
tude indicated (reset comparator overdrive). The graph
shows the maximum pulse width that a negative-going
V
CC
transient may typically have without causing a
reset pulse to be issued. As the amplitude of the tran-
sient increases (i.e., goes farther below the reset
threshold), the maximum allowable pulse width
decreases.
Typically, a V
CC
transient that goes 40mV below the
reset threshold and lasts for 3碌s or less will not cause a
reset pulse to be issued.
VIN
R1
V
CC
PFI
C1*
R3
R2
PFO
GND
TO
碌P
+5V
PFO
0V
0V
R1 + R2
V
TRIP
= 2.265
R2
V
H
= 2.265 /
R2
||
R3
R1 + R2
||
R3
V
L
V
TRIP
V
H
V
IN
*OPTIONAL
SET
WDI
LOW
SUBROUTINE
OR PROGRAM LOOP,
SET WDI
HIGH
RETURN
END
Figure 16. Watchdog Flow Diagram
A 0.1碌F bypass capacitor mounted close to the V
CC
pin provides additional transient immunity.
Watchdog Software Considerations
To help the watchdog timer keep a closer watch on soft-
ware execution, you can use the method of setting and
resetting the watchdog input at different points in the
program, rather than 鈥減ulsing鈥?the watchdog input high-
low-high or low-high-low. This technique avoids a 鈥渟tuck鈥?/div>
loop where the watchdog timer continues to be reset
within the loop, keeping the watchdog from timing out.
Figure 16 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-
gram returns to the beginning. If the program should
鈥渉ang鈥?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.
+5V
MAX807
Maximum V
CC
Fall Time
The V
CC
fall time is limited by the propagation delay of
the battery switchover comparator and should not
exceed 0.03V/碌s. A standard rule for filter capacitance
on most regulators is on the order of 100碌F per amp of
current. When the power supply is shut off or the main
battery is disconnected, the associated initial V
CC
fall
rate is just the inverse or 1A / 100碌F = 0.01V/碌s. The
V
CC
fall rate decreases with time as V
CC
falls exponen-
tially, which more than satisfies the maximum fall-time
requirement.
15
V
L
- 2.265 + 5 - 2.265 = 2.265
R1
R3
R2
Figure 15. Adding Hysteresis to the Power-Fail Comparator
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