method provides additional noise margin without com-

promising 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 10kΩ so it does not load down the

PFO 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. PFO can be used to generate an interrupt to the

µP (Figure 5). Connecting PFO to MR on the MAX704

and MAX806 causes reset to assert when the monitored

supply goes out of tolerance. Reset remains asserted

as long as PFO holds MR low, and for 200ms after PFO

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_ RESET output. If, for example, the

RESET output is driven high and the µP wants to pull it

low, indeterminate logic levels may result. To correct this,

connect a 4.7kΩ resistor between the RESET output and

the µP reset I/O, as in Figure 6. Buffer the RESET 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 the

Figure 7 shows maximum transient duration vs. reset-

comparator overdrive, for which reset pulses are

not

generated. The graph was produced using negative-going

threshold by the magnitude indicated (reset comparator

overdrive). The graph shows the maximum pulse width a

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 decreas-

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

reset pulse to be issued.

provides additional transient immunity.

Figure 3. Using a SuperCap as a Backup Power Source

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

MAX690T/S/R, MAX704T/S/R,

MAX802T/S/R, MAX804−MAX806T/S/R

3.0V/3.3V Microprocessor

Supervisory Circuits

www.maximintegrated.com

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