MAX705-MAX813L

Low-Cost, µP Supervisory Circuit

www.artschip.com

7

Power-Fail Comparator

The power-fail comparator can be used for various purposes because its output and noninverting input are not internally

connected. The inverting input is internally connected to a 1.25V reference.

Figure 3. MAX705/MAX706/MAX813L Watchdog Timing.

Figure 4. MAX705/MAX706 RESET, MR, and WDO Timing

with WDI Three-Stated. The MAX707/MAX708/MAX813L

RESET output is the inverse of RESET shown.

To build an early-warning circuit for power failure, connect the PFI pin to a voltage divider (see Typical Operating Circuit). Choose

the voltage divider ratio so that the voltage at PFI falls below 1.25V just before the +5V regulator drops out. Use PFO to interrupt

the µP so it can prepare for an orderly power-down.

Applications Information

Ensuring a Valid RESET

Output Down to Vcc=0V When Vcc falls below 1V, the

MAX705-MAX708 RESET output no longer sinks current – it

becomes an open circuit. High-impedance CMOS logic inputs

can drift to undetermined voltages if left undriven. If a

pull-down resistor is added to the RESET pin as shown in

Figure 5, any stray charge or leakage currents will be drained

to ground, holding RESET low. Resistor value (R1) is not

critical. It should be about 100KΩ, large enough not to load

RESET and small enough to pull RESET to ground.

Monitoring Voltages Other Than the

Unregulated DC Input

Monitor voltages other than the unregulated DC by connecting

a voltage divider to PFI and adjusting the ratio appropriately. If

required, add hysteresis by connecting a resistor (with a value

approximately 10 times the sun of the two resistors in the

potential divider network) between PFI and PFO. A capacitor

between PFI and GND will reduce the power-fail circuit’s

sensitivity to high-frequency noise on the line being monitored.

RESET can be asserted on other voltages in addition to the

+5V Vcc line. Connect PFO to MR to initiate a RESET pulse

when PFI drops below 1.25V. Figure 6 shows the

MAX705-MAX708S configured to assert RESET when the

+5V supply falls below the reset threshold, or when the +12V

supply falls below approximately 11V.

Monitoring a Negative Voltage

The power-fail comparator can also monitor a negative supply

rail (Figure 7). When the negative rail is good (a negative

voltage of large magnitude), PFO is low, and when the

negative rail is degraded (a negative voltage of lesser

magnitude), PFO is high. By adding the resistors and

transistor as shown, a high PFO triggers reset. As long as

PFO remains high, the MAX705-MAX708/MAX813L will keep

reset asserted (RESET=low, RESET =high). Note that this

circuit’s accuracy depends on the PFI threshold tolerance, the

Vcc line, and the resistors.