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74HC244

Dick Berg
Prodigy 100 points

All - I have a few questions about the subject IC.

I have been told that this IC can be damaged if there are logic inputs in the absence of Vcc. Is this correct?

If so, are there subtle failures, for example, changes in threshold, logic levels, propagation delays, etc?

What I'm trying to determine is, short of a catastrophic failure, can there be lesser failure modes that may not be directly obvious?

Thanks,

Dick Berg

  • 0
    TI__Mastermind 46480 points

    Hi Dick

     The HC244 has clamp diodes to Vcc and gnd so if there is a voltage on the input or output when the part is powered off it can backpower the device. It will still function somewhat even though the power is off. The part can be damaged if the current is not limited.  It could be catastrophic or just wounded or may not be damaged depending on how it is connected. 

    The best thing to do is to switch to a part that has Ioff and allows voltages on the inputs or outputs when Vcc is 0V. SN74LV244A

  • 0 in reply to Chris Cockrill
    Prodigy 100 points

    Chris - Thanks for the response.

    I was aware of the class of devices that are tolerant of signals in the absence of Vcc.

    I was more interested in the types of failures that others have experienced as the HC244 degrades. For example,  an increase in propagation delay, slower rise times, etc.

    Thanks,

    Dick Berg

  • 0 in reply to Dick Berg
    TI__Mastermind 46480 points

    More than likely the part will not degrade. It will simply power up and function.

    (While it is unententionally powered) If the part drives an output load that needs more current than the input clamp diode can handle then it could degrade. the failure mode would more than likely be a leakage to vcc or gnd or it could be catastrophic. I dont really know.