TXB0108: Maximum Input Voltage Level

Part Number: TXB0108
Other Parts Discussed in Thread: SN74LXCH8T245

Hello,

I connect the following voltages to the part: VCCA = 3.3V, VCCB = 5V

Is it OK to apply at TTL signal (5V) to the A side of the device (with VCCA = 3.3V)?

The datasheet says that VIH Maximum = VCCI (3.3V), on the other hand the Absolute Maximum rating says 6.5V 

Can it affect the operation / failure / damage?

Thanks a lot!

Max

  • Hey Max,

    Is your voltage input changing from 3.3 to 5V periodically? I'm not seeing why you would need a translator when it looks like you're going from 5V inputs to a 5V output.

    I wouldn't recommend applying a 5V signal to the A side at 3.3Vcc. The inputs aren't overvoltage tolerant in the TXB. Working with conditions between the abs max and the recommended operating conditions may not immediately damage the device but it will degrade performance over time and you'll likely see damage down the line. We also can't guarantee that the performance mentioned in the datasheet will match with the given conditions.

    Thanks,
    Rami

  • Hi Rami, 

    Thanks for your reply.

    It's not changing, part of the inputs are 3.3V and the other part are 5V.

    My conclusion is to somehow reduce the 5V inputs also to 3.3V, 

    Again thanks a lot!

  • Hey Max,

    That's definitely a solution that could work but i'd be a little concerned about timing characteristics if the different signal lines require anything precise, since you'd be adding some to the 5V lines to drop them to 3.3V but keeping the 3.3V the same.
    However, what's your application here? 
    You could always use a device with over-tolerant input that could still translate from 3.3V to 5V such as the SN74LXCH8T245. This will be directional controlled rather than auto-bidirectional. 

    Thanks,
    Rami

  • Hi Rami,

    The device is already there on the pcb, is there any other pin to pin device that is 5V tolerant?

    The 5V level input signal to this device also comes from a level translating buffer on other board (2.5V to 5V), which I can bypass and get 2.5V signal instead of the 5V (Timing differences of several nsec is not an issue).

    Thanks,

    Max

  • Max,

    I see. Unfortunately there isn't a p2p device that is also 5V tolerant. 
    Bypassing the buffer may be your best bet. Just note that you'll have a 2.5V CMOS input on the 3.3V supply side. While this does cross the Vih threshold, you'll likely see an increase in the supply current as well. 

    Thanks,
    Rami