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SN74AVC1T45: An unsuitable design question

JH Wu
Prodigy 30 points
Part Number: SN74AVC1T45
Other Parts Discussed in Thread: SN74AXC1T45

We use a SN74AVC1T45DCKR for a signal level shift. But we find our product only provide v1.05v for VCCA,
it doesn't satisfy minimum operation voltage 1.2V that is descripted in datasheet though pin3 to pin4 function can work normally by our experiment.

We would like to know if the insufficient VCCA will cause floating situation and unexpected trigger pin4 to low?
Or the Pin 4 just always keeping high (our product has an external pull-high 3v3 for pin4, pull-high 1.05v for pin3)?

  • +1
    Guru 282680 points

    Below 1.2 V, correct operation is not guaranteed.

    Just use the SN74AXC1T45 instead.

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Sure, we will change it. According to datasheet, it seems the pin4 will be Hi-Z if Vcca side doesn't work, but I am not sure. The pin4 function is shutdown machine. So I need to do a risk assessment for shipped machines.

  • 0 in reply to JH Wu
    Guru 282680 points

    The I/Os are high impedance when any power supply is off, but that is guaranteed only for less than 0.5 V.

    In practice, a supply voltage of 0.15 V below the limit is likely to work (there is a safety factor to account for temperature effects and manufacturing variations). But there is no guarantee.

    You can make a better assessment if you measure the actual threshold where the device ceases to work.

  • +1
    TI__Mastermind 26921 points

    Hey JH,

    If it is insufficient VCCA (<0.15V) then the device will enter VCC isolation that puts the I/O in high impedance. This is an additional feature on both the AVC1T45 and the AXC1T45. 

    Regards,

    Jack 

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Thanks, I use an external power supply to adjust Vcca, and wire pin3 to ground. Test result is when Vcca down to 0.60-0.55v, the pin4 will change to high. It shows this IC has good tolerance, so we never get any failed feedback. (It is found by engineer review datasheet)