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Original question:

TXB0108: Output signal at high side (VCCb) is garbled

TXB0108: Oscillation of outputs when using a ferrite

Guenther Mayer
Prodigy 90 points
Part Number: TXB0108
Other Parts Discussed in Thread: SN74LVC8T245

Hi,

we use the TXB0108 buffer for some 3,3V->5V bi-dir GPIOs. Currently the MSP432 drives the line to the TXB0108. The TXB0108 translates it to 5V with no problems at an open connector.

There is a series ferrite bead in the line at the connector. However, if I connect the output to an input pin of the board (74LVC541), the output of the TXB0108 starts oscillation. Our test pattern is 1Hz toggling low-high.

I know that >70pF of capacitance can get the TXS/TXB to oscillation, but it don't think that we're that close to it. If the ferrite bead is bridged by a soldering wire, everything is fine.

Can the phase shift of the ferrite bead be the reason? Any other thoughts?

Thanks,

Gunther

  • 0
    Guru 284570 points
    >70 pF can get the TXB's edge accelerator to time out before the output voltage has reached GND or VCC; the resulting overdampened edge would be the opposite of oscillation.

    Is the oscillation around the target voltage (0 V or 5 V), or somewhere the middle? Can you show the waveform?
  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Hi Clemens,

    attached is a pdf with are several measurements. I tried to show the working circuit with an open connector. Then the oscillating circuit, when the output of the TXB is connected to an input of a 74LVC.

    Some measurements are focused on the TXB input and output, while others are focused on the ferrite bead left and right side.

    Thanks,

    GüntherMeasurements_TXB0108_v01.pdf

  • 0 in reply to Guenther Mayer
    Prodigy 90 points

    Additional remark: we have also a board with TXS instead of TXB.

    Same issue there, that’s why I assume the ferrite to be the reason of the oscillation.

  • 0 in reply to Guenther Mayer
    Guru 284570 points
    Your measurements show that the ferrite bead at least contributes to the problem. I do not know the layout of your board, but I'd guess that the impedance changes of the connector (and the board on the other side) cause reflections, and thus ringing. If this ringing becomes too large (and here, it certainly is), it triggers the TXB's edge accelerators to go the wrong way, which will then generate oscillations.

    Where are the ferrite beads placed on the board? If they are near the connector, they would be more useful for signal in the B→A direction.

    Would it be possible to use a direction-controlled translator like the SN74LVC8T245?
  • 0 in reply to Guenther Mayer
    TI__Guru 58860 points

    Mayer,

    Where is the connector going to? this can add cap loading as well.