The circuit shown monitors both rails of a bipolar power supply with respect to user-defined threshold voltages (see the figure). It will produce a reset signal when either rail passes the associated reset threshold. IC2 provides both active-low and active-high reset signal outputs.
The positive rail is monitored by a comparator and 1.7-V reference voltage internal to IC2, plus the R1/R2 divider. The negative rail is monitored by the internal power-fail comparator, which compares the same 1.7-V reference voltage against a signal produced by the R3/R4 divider. The required positive voltage for this divider, higher than 1.7 V and relatively independent of VCC, is provided by the 2.5-V reference IC1. VCC can range between 3.0 V and 5.5 V.
These connections produce an action opposite to the usual one—PFO (pin 5) goes high instead of low when the negative rail fails. This transition is inverted by transistor Q1 to pull Manual Reset low, which in turn signals a power failure by triggering the reset outputs.
To set the threshold voltages, first select values for R2 and R4. Lower values waste power, and higher values produce error due to leakage current into the comparators. Then, calculate R1 and R3 as follows:
Thresholds can be set to any reasonable level greater than ±1.7 V. For precision threshold values, the voltage-divider resistors should have a tolerance of ±1% or better.