Low-Cost, µP Supervisory Circuits
The power-fail comparator can be used for various pur-
poses because its output and noninverting input are
not internally connected. The inverting input is internal-
ly connected to a 1.25V reference.
TRIGGERED BY MR
To build an early-warning circuit for power failure, con-
nect the PFI pin to a voltage divider (see
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.
Ensuring a Valid RESET
Output Down to V
falls below 1V, the MAX705-MAX708 RESET
output no longer sinks current—it becomes an open cir-
cuit. 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 con-
necting a resistor (with a value approximately 10 times
the sum of the two resistors in the potential divider net-
work) 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 V
line. Connect PFO to MR to initiate a RESET
pulse when PFI drops below 1.25V. Figure 6 shows the
MAX705-MAX708 configured to assert RESET when the
+5V supply falls below the reset threshold, or when the
+12V supply falls below approximately 11V.
( ) ARE FOR MAX813L ONLY.
Figure 3. MAX705/MAX706/MAX813L Watchdog TIming
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 resis-
tors 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 V
and the resistors.
MR EXTERNALLY DRIVEN LOW
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.