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LPV801 Common Mode question

steve wenner
Expert 2355 points
Other Parts Discussed in Thread: LPV801, LPV802

Good morning.

The lay of the land.....

I want to monitor a DC 4.2V level cut down by a resistor divider to an MSP analog pin for sampling.  The MSP is powered by 3.3V.  I would like to buffer this divider (high impedance so as to draw <1uA) through a TI low power LPV801 opamp in a voltage follower fashion.  The opamp would be single supply, powered off of 3.3V as well. This will guarantee that if the 3.3V supply drops out, that the signal to the MSP analog pin on the NOW unpowered MSP will not damage the MSP.  

My question....

If the power on VCC (3.3V) of the LPV drops out BUT it has an out of Common Mode range signal on its input (i.e. 2.8V on input, VCC =0V, Ground = 0V) will I damage the part?  If I were to put a 1Mohm in series with the input would this be an acceptable solution to limit the Common Mode current and not blow the part?  

Thx

Steve

  • 0
    TI__Guru 53946 points

    Hello Steve,

    There are ESD diode clamps from the LPV802 inputs to the supplies. If the supplies collapse, the upper diode will clamp the input to the V+ line.

    The maximum current for the inputs is 10mA. The series resistance of the divider will limit the current to well below that. .

    Assuming a 1M total series resistance, 4.2V/1M = 4.2uA, so it is not a problem.

    It will not damage the amplifier, however, it will discharge your battery since the input is no longer a high impedance input, so be aware of that..

    Regards,