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CD74ACT14: Negative Bias on Input

William Schiller
Prodigy 30 points
Part Number: CD74ACT14

Tool/software:

Hi TI team,

For these Schmitt inverter family of parts I'd like to know what the stress implications are with having an input bias of -12V.  There's a fairly large series input resistance of ~50K at the input, which limits the current to well below the clamp rating in datasheet.  Datasheet states voltage exceedances are ok if current ratings are observed.  I assume the internal diodes will have no issues with this under voltage over extended operation?

  • 0
    Guru 317370 points

    12 V / 50 kΩ = 240 µA, so this will not damage the device, not even over a long time.

    When you go outside the recommended operating conditions, the electrical characteristics or even correct operation are no longer guaranteed.

  • +1
    TI__Guru* 97046 points

    Hey William,

    Just to clarify why this doesn't cause any problems for the device -- it never actually "sees" the -12V input. As soon as the voltage exceeds -0.5V, the negative clamp diode turns on and starts conducting (diodes are very fast to respond). That diode will hold the input voltage to approximately -0.7V as long as the current is limited (which it is by the 50k resistor).