A similar version of this article appeared in the February 8, 2008 issue of Portable Design
A simple but accurate circuit (Figure 1
) monitors the magnitude of a negative supply voltage. This capability is useful in multi-rail systems, particularly if the negative rail serves as the precision bias voltage for a GaAs device. The IC (MAX971
) includes an open-drain comparator and a precision 1% voltage reference. Its trigger threshold is set by the value of external resistor R4. For convenience, all other resistors have the value 1MΩ.
Figure 1. The output of this negative-voltage monitor goes low when the monitored negative voltage is above (more negative than) the threshold set by R4.
Resistors R1 and R2 divide the reference voltage, providing a trip point of Vref/2 for the comparator. R3 and R4 sample the negative voltage, and IC1 compares that sample to the trip-point voltage. R4 = 1MΩ + 2(1MΩ × VNEG
/1.182V), where VNEG
is the magnitude of the voltage being monitored (ignoring the minus sign). The circuit output goes low in response to a fault condition—i.e., when the magnitude of the monitored voltage drops below the set threshold. To ensure an overall accuracy better than 2%, all resistors should have 1% tolerance.
For monitoring more than one negative voltage, choose the MAX974
, which includes four open-drain comparators with a precision 1% reference. Feed the output of R1 and R2 to all the comparators, add an R3/R4 pair for each monitored voltage, and calculate each R4 to yield a trip point for the corresponding voltage. All the open-drain outputs can be connected together.