Monitoring a Negative Voltage

The power-fail comparator can be used to monitor a

negative supply voltage using Figure 15’s circuit. When

the negative supply is valid, PFO is low. When the neg-

ative supply voltage drops, PFO goes high. This cir-

cuit’s accuracy is affected by the PFI threshold toler-

Backup-Battery Replacement

above the reset threshold. No precautions are neces-

sary to avoid spurious reset pulses.

Negative-Going VCC Transients

While issuing resets to the µP during power-up, power-

down, and brownout conditions, these supervisors are

transients (glitches). It is usually undesirable to reset

Figure 16 shows maximum transient duration vs. reset

comparator overdrive, for which reset pulses are not

generated. The graph was produced using negative-

reset threshold by the magnitude indicated (reset com-

parator overdrive). The graph shows the maximum

typically have without causing a reset pulse to be

issued. As the amplitude of the transient increases (i.e.,

goes farther below the reset threshold), the maximum

sient that goes 100mV below the reset threshold and

lasts for 40µs or less will not cause a reset pulse to be

issued.

pin provides additional transient immunity.

Connecting a Timing Capacitor to SWT

SWT is internally connected to a ±100nA current

source. When a capacitor is connected from SWT to

ground (to select an alternative watchdog timeout peri-

od), the current source charges and discharges the

timing capacitor to create the oscillator that controls the

watchdog timeout period. To prevent timing errors or

oscillator start-up problems, minimize external current

leakage sources at this pin, and locate the capacitor as

close to SWT as possible. The sum of PC board leak-

age + SWT capacitor leakage must be small compared

to ±100nA.

Microprocessor Supervisory Circuit

16

______________________________________________________________________________________

100

0

10

1000

10,000

40

20

80

60

RESET COMPARATOR OVERDRIVE (mV)

100

0.1

µF CAPACITOR

Figure 16. Maximum Transient Duration without Causing a

Reset Pulse vs. Reset Comparator Overdrive

MAX791

GND

PFI

R2

R1

+5V

PFO

PFO

+5V

0V

0V

V-

5 - 1.25

=

R1

R2

V-

Figure 15. Monitoring a Negative Voltage