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SN74LVC1G00: Device is un-powered while inputs are still toggling. Is it safe to assume output in '0'?

Eyal Lasko
Expert 2300 points
Part Number: SN74LVC1G00

Hi, 

VCC to the device is not guaranteed however inputs keep on toggling.

Can it harm the device?

Can I safely assume that the output is '0' when unpowered?
(If not, in which device family can this be guaranteed)

Thanks

  • 0
    TI__Guru* 96591 points

    Hello,

    The inputs to the SN74LVC1G00 are always high impedance -- this is not dependent on the supply, so to answer your first question, no, it cannot harm the device to apply a logic signal to the inputs with the supply unpowered.

    There are some devices, including the SN74LVC1G00, that will force the output into a high-impedance state _if_ the supply is at 0V -- this is specified as "Ioff" in the electrical characteristics table. This is not the same thing as "unpowered" because disconnecting a pin or shutting off a switch-mode power supply will not always result in 0V at the supply pin.

    So, to answer your second question -- if you can ensure that VCC = 0V, then the output will be high-impedance, and you can use a pull-down resistor of relatively large value to keep the output at 0V.  I would recommend a 10kohm resistor.

    We do not offer any logic devices that hold the output low when the supply is at 0V.

  • 0
    Expert 2300 points

    Thanks for your prompt response.

    In our case, 'unpowered' is actually disconnecting the supply (shutting down FET switch)

    hence the 74LVC1g00 is left 'partially powered' with some residual voltage in its bypass capacitor rather than fully unpowered (VCC=0V ) as you have suggested.

    What would be the state of its output in such a case?

    Would you recommend adding a bleeding resistor on its VCC to ensure VCC=0V when power is disconnected (at the cost of faster battery drainage)?

    Any other recommendations?

    Thanks

  • 0 in reply to Eyal Lasko
    TI__Guru* 96591 points

    Can you give me more details regarding the status of the device?

    What are the input/output states before power is turned off? What is the voltage on the supply pin when it's in the unpowered state? Is there a load connected / what type? Can any current get through that shutdown FET, or are we saying that all current flow is off in the unpowered state?