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TXB0108: Issue with noisy output

Daniel Nanto
Intellectual 290 points
Part Number: TXB0108
Other Parts Discussed in Thread: MSP432E401Y, TXU0304, TXU0104, TXU0204

Good evening,

We recently used a TXB0108 chip in a custom board design. It is being used to translate 3.3v signals to 5v signals in order to interface a MSP432E401Y with a VFD. We had a miscommunication and failed to include both the bypass capacitors and the pulldown resistor on OE. We are going to fix our negligent error in the upcoming board revision.

Since we already have the board, I went ahead and tested the outputs. I noticed that some B-side outputs were functioning nicely, but others were not. Some outputs were producing 3-4V with considerable noise (instead of 5V) or were unable to output more than 0.5v. Do you think this is likely a consequence of the design flaws? 

Also, would a 100kR pulldown resistor on OE be appropriate?

Thank you,

Dan

  • 0
    Guru 316920 points

    You need a pull-down on OE only if you want to disable all outputs during power up. Its value is not critical.

    What is the load on the outputs? The TXB works only with short traces connected to high-impedance inputs, without pull-ups or pull-downs.

  • 0 in reply to Clemens Ladisch
    Intellectual 290 points

    Thank you for your response!

    Each output draws 1/200 W to 1/80 W (0.5mA - 2.5mA @ 5V).

    The output needs to travel roughly 3 inches total (~1" through traces and 2" through cable). If that distance is outside of the TXB's recommended operating parameters, would you be able to suggest a replacement translator chip?

    We're just looking to translate twelve 3.3v signals to 5v over a distance of ~3 inches. Bidirectional is optimal, but not technically necessary.

  • 0 in reply to Daniel Nanto
    Guru 316920 points

    You must not try to draw more than 20 µA from a TXB output.

    For unidirectional signals, use unidirectional translators like the TXU0104/TXU0204/TXU0304.

  • +1 in reply to Clemens Ladisch
    TI__Mastermind 40526 points

    Hey Daniel,

    Clemens' recommendation is correct- the TXU family devices will be able to provide drive strength in the mA range compared to the almost negligible drive of the TXB that is in the uA range.

    Its important to note here that the auto-bidirectional translators will not provide any drive strength as they are designed to be weak to "sense" which side of the driving through the device. In other words, auto-bi directional translators should not be chosen for the sake of the simplicity, rather for the nature of the driving requirements of the signals being interfaced. 

    Regards,

    Jack