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TXS0102: Equal VCCx and Floating VCCx

Brian Angiel
Expert 3620 points
Part Number: TXS0102
Other Parts Discussed in Thread: SN74AXC2T245, SN74AXC4T245, SN74AVC2T245

Hi team,

I have a couple questions regarding TXS0102. First, can this device be used in situations where both VCCA and VCCB are 3.3V? The datasheet states that VCCA must be less than or equal to VCCB. Having VCCA = VCCB = 3.3V doesn't violate that or the Recommended Maximum Voltage of 3.6V on VCCA. I'm inclined to believe this is OK but I wanted to check.

Second, if VCCA is left floating for an extended period of time while VCCB = 3.3V, will this damage the device? I know you can leave the data pins floating, but I don’t see anything in the datasheet or on e2e that is conclusive one way or another about leaving VCCA floating.I know the device will not function, but will it be damaged?

Thanks,

Brian

  • 0
    TI__Guru 57680 points

    Hi Bryan,

    1) Vcca = Vccb is a valid condition and no issues with this.

    2) when the Vcca is floating, is this at 0V, or at unknown value? with 0V, the Ioff circuitry will ensure that the IOs will be HiZ and the device will be disabled. The Icc values will also be limited to 1uA.

    unknown value will mean that the Ioff is not ensured and likely be a problem..

  • 0 in reply to ShreyasRao
    TI__Expert 3620 points

    Understood.   What would be the minimum pulldown I could use on VCCA to hold it in a known state (100K, 200k?)

     

    Alternatively I am simply trying to isolate a UART Tx/Rx Pair so the method of isolation does not need to support full bidirectionality on the individual nets (like it would on an I2C line) - Tx is always an output and Rx is always an input.  Do you have level translators that can function as simple buffers that might solve this problem as well?

    Thanks,

    Brian Angiel

  • 0 in reply to Brian Angiel
    TI__Guru 57680 points

    Hi Brian,

    100k seems like a good value to hold the lines at a default value.

    For UART devices, I would suggest to use SN74AXC2T245 device ( or AVC2T245) which can be used for effective isolation. The TXS0102 doesnt have true isolation since they are internally a FET device with additional circuitry around it.

    Please watch the UART video for more info. This gives info for 4 channel UART, but i have suggested the 2 channel device since you asked about TXS0102 which is also 2 channel device. If its indeed a 4 channel UART, you could use SN74AXC4T245.

    Please let me know if any other questions.

  • 0 in reply to ShreyasRao
    TI__Expert 3620 points

    Hi Shreyas,

    Regarding SN74AVC2T245, the datasheet says: "The VCC isolation feature ensures that if either VCC input is at GND, both ports are put in a high-impedance state."

    Is this VCC isolation independent of the /OE pin? So, if I have /OE LOW (device active), but one of the power rails is not active, then the outputs will be High-Z. Is that correct?

     

    Thanks,

    Brian Angiel

  • 0 in reply to Brian Angiel
    TI__Guru 57680 points

    Hi Brian,

    Yes, the Vcc isolation feature is independent of the OE pin condition. It can be low and if one of the Vcc is at 0V, then the IO is in HiZ.