Connecting an ordinary signal diode in series with R3,

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

threshold.

The current through R1 and R2 should be at least 1µA to

ensure that the 25nA (max over extended temperature

range) PFI input current does not shift the trip point. R3

should be larger than 10k

Ω so it does not load down the

–

P

—

F

—

O

–

pin. Capacitor C1 adds additional noise rejection.

Monitoring an Additional Power Supply

These µP supervisors can monitor either positive or

negative supplies using a resistor voltage divider to

PFI.

–

P

—

F

—

O

–

can be used to generate an interrupt to the

µP (Figure 5). Connecting

–

P

—

F

—

O

–

to

–

M

—

R

–

on the MAX704

and MAX806 causes reset to assert when the

monitored supply goes out of tolerance. Reset remains

asserted as long as

–

P

—

F

—

O

–

holds

–

M

—

R

–

low, and for 200ms

after

–

P

—

F

—

O

–

goes high.

Interfacing to µPs

with Bidirectional Reset Pins

µPs with bidirectional reset pins, such as the Motorola

68HC11 series, can contend with the MAX690_/

MAX704_/MAX802_/MAX806_

–

R

—

E

—

S

—

E

—

T

–

output. If, for

example, the

–

R

—

E

—

S

—

E

—

T

–

output is driven high and the µP

wants to pull it low, indeterminate logic levels may

result. To correct this, connect a 4.7k

Ω resistor

between the

–

R

—

E

—

S

—

E

—

T

–

output and the µP reset I/O, as in

Figure 6. Buffer the

–

R

—

E

—

S

—

E

—

T

–

output to other system

components.

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 7 shows maximum transient duration vs. reset-

comparator overdrive, for which reset pulses are not

generated. The graph was produced using negative-

the reset threshold by the magnitude indicated (reset

comparator 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 allowable pulse width decreases. Typically,

threshold and lasts for 40µs or less will not cause a

reset pulse to be issued.

pin provides additional transient immunity.

3.0V/3.3V Microprocessor Supervisory Circuits

_______________________________________________________________________________________

9

MAX690T/S/R

MAX704T/S/R

MAX802T/S/R

MAX804T/S/R

MAX805T/S/R

MAX806T/S/R

TO STATIC

RAM

VBATT

GND

1N4148

RESET

(RESET)

( ) ARE FOR MAX804T/S/R, MAX805T/S/R ONLY

TO

µP

0.47F

3.0V OR 3.3V

MAX690T/S/R

MAX704T/S/R

MAX802T/S/R

MAX804T/S/R

MAX805T/S/R

MAX806T/S/R

TO STATIC

RAM

VBATT

GND

1N4148

RESET

(RESET)

( ) ARE FOR MAX804T/S/R, MAX805T/S/R ONLY

TO

µP

0.47F

3.0V OR

3.3V

+5V

b

a

Figure 3. Using a SuperCap as a Backup Power Source