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Keywords: over/undervoltage detection, negative voltage monitors, window comparators, window detector
APPLICATION NOTE 4545
Flexible Overvoltage/Undervoltage Detector
Monitors Negative and Positive Voltages
By: Franco Contadini
Bich Pham
Dec 10, 2010
Abstract: Combining a window comparator (MAX6764) with a positive-voltage monitor
(MAX6887/MAX6888) enables the circuit to monitor a negative voltage as well, as is often required in
telecom systems.
A similar version of this article appeared in the January 1, 2008 issue of PD magazine.
Multivoltage supply supervisors (such as the MAX6887) provide several voltage-detector inputs for
positive voltages, each with factory-set thresholds for undervoltage and overvoltage. The acitve-low
RESET output asserts when any input drops below its undervoltage threshold, or when you assert the
manual reset (MR). The acitve-low OV output asserts when any input exceeds its overvoltage threshold.
These capabilities are useful, but telecom applications often require that you monitor a negative supply
voltage for the RF circuitry as well.
To monitor negative voltage you can make use of the MAX6887 adjustable-input option (Figure 1), in
which a level-shifting circuit connects one side of the resistive divider to a positive level and the other
side to the negative voltage. This approach, however, produces inverted output logic. If, for example, you
monitor -6V with thresholds at -6.5V and -5.5V, the circuit asserts acitve-low UV when V
IN
= -6.5V and
acitve-low OV when V
IN
= -5.5V.
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