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TXB0304: txb0304

yang laichun
Prodigy 40 points
Part Number: TXB0304
Other Parts Discussed in Thread: SN74AXC4T774, SN74LVC245A, SN74LVC244A, SN74HCS245-Q1

I have used the txn0304 as a buffer which vcc were both 3.3v.  There is a long  pcb trace about 38cm.When i have a quick changing signal from high to low i found a oscillating on the output.  The external pull up resister is 20k. Please help me to concelling the oscillating .Thanks!

  • 0
    TI__Mastermind 24885 points

    Hello Yang,

    For the TXB0304, when driving long traces that have high parasitic elements, there is a possibility of oscillations such as what you are noticing. 

    Are your signals all going in the same direction? 

    Is the directionality of the channels fixed? (in other words, a given channel does not need to switch directionality)

    For such an application, I would recommend the SN74AXC4T774. 

    This device will better be able to support driving into capacitive loads and each channel can be configured individually to the desired direction. Also it can support both supplies as 3.3V.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Prodigy 40 points

    Yes, the direction is unique. But how can i  decrease the oscillation?

  • 0 in reply to yang laichun
    TI__Guru* 96842 points

    Hi Yang,

    Michael was suggesting that changing to a fixed direction voltage translator would remove the oscillation problem.

    I expect that you are seeing some reflections on the 38cm microstrip transmission line which are causing multiple triggers of the one-shot edge accelerators in the TXB device. You could try adding a small series ressitor at the output of the TXB device. I would not exceed 100 ohms with that resistor (22-47 ohms typically will help some). There's an FAQ on driving transmission lines with logic here that explains a bit more:  https://e2e.ti.com/support/logic/f/151/t/763609 

    Since you are driving everything at 3.3V, I would recommend an even simpler solution -- just use a logic buffer like SN74LVC244A or SN74LVC245A. These will redrive the signal without any chance of producing a feedback loop like the auto-bidirectional TXB translator.  You can also add series output resistors to these devices to help dampen out ringing, which is likely to happen with such long traces.

    You might consider even going to a Schmitt-trigger device, which will help to reduce false triggers from ringing on the line.  The SN74HCS245-Q1 is one choice that would work.