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LMV331 Comparator - output voltage with no power to ic

Chuck99
Prodigy 20 points
Other Parts Discussed in Thread: LMV331

I'm using an LMV331 comparator in a battery powered circuit.

The comparator is powered by +5V while the open collector output

is connected through a resistor to +3.3V.

Because the comparator function is needed only intermittently, the

+5V to the comparator is switched off when the comparator is not

needed, but the +3.3V is constant.

Will the open collector output stay off (no current flow) when the

power to the comparator is switched off?  I'm concerned about

parasitic diodes that might switch on.

 

  • 0
    Prodigy 130 points

    Chuck,

    Your concern is indeed justified! You will usually find that the data sheet for many integrated circuits call for voltage limits on input and output signal pins with respect to the power supply pins. This is usually due to the way the I/O pins are protected from electrostatic discharge (ESD) over-voltage. In many cases, each I/O uses a pair of reversed biased clamp diodes connected to the common power supply busses on chip. If you remove power to the supply pin and not the I/O pins, these clamp diodes will begin to forward bias and draw current from the I/O into the on chip supply rail.

    I have shown below, one simple way to isolate the open collector (or open drain) connection of a comparator for a dual supply system such as you have proposed. Use an NMOS device with a turn on threshold voltage much less than the 5V_Supply. Select the size of the pullup resistor and the NMOS transistor to ensure that the OUT voltage goes to the required low level when the comparator is triggered.

     \3716.LMV331_20091113.pdf

    NMOS Gate to 5V_SUPPLY
    NMOS Source to Comparator Output Pin
    NMOS Drain to PullUp resistor of other 3.3V Power Supply Domain.