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TXB0104: input floating

Shu-Cheng LIN
Guru 11785 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXS0104E, SN74AXC4T245, SN74AVC4T774

Hi Sirs,

Sorry to bother you.

We would like to know

1. TXB0104 how to auto detect signal are input or output?

2. If TXB0104 B side floating, is possible to make A side (output) to low?

Because our application are find this phenomenon that effect our A side of system to be low, so M/B couldn't work.

Thanks!!

  • 0
    Guru 11785 points

    Hi Sirs,

    The issue location is SU31 component.

    Our BATLOW# in B-side is floating.

    When system power on, the A-side PMC_BATLOW#_1.8 signal is low level.

    Schematic:

    txb0104-edited.pdf

    Waveform:

  • 0
    TI__Guru 56050 points
    Hi Shu-Cheng,

    Please refer to the app note on the working of TXB devices:
    www.ti.com/.../scea043.pdf

    I see on the schematic page 2 that the TXB0104(SU140 and SU139) has been listed as having internal 10k pullup resistance on A /B ports. This is not true. Are the signals open drain which requires pullup resistors and hence the device must be TXS0104e instead of TXB0104?

    Is the B side is always floating or only during sometimes when there is power off from the driver side? If the B channel is always floating, then you can ground both the B and the corresponding A channel using pulldown resistors. A weak pulldown resistor on A side can also be used in case B side is floating intermittently.
    You could also consider using the SN74AXC4T245 device or SN74AVC4T774 devices assuming all signals are pushpull type.
  • 0 in reply to ShreyasRao
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply.

    The schematics description is old. Please ignore.

    Our A-side PMC_BATLOW#_1.8 signal have a pull-high resistor.

    You mean the TXB0104 can’t use for open drain signal. Right?

    We have study TXB application before but we still not sure how does the IC auto detect the input or output signal?

    Could you help to explain it?

    Thanks

  • 0 in reply to Shu-Cheng LIN
    TI__Guru 56050 points
    Shu-Cheng,

    TXB0104 is not recommended for open drain signals. the reason is that the device has internal 4K series resistors which form a voltage divider with the external pull-up resistors used for open drain signals which can cause higher Vol close to the point of failure.

    When there is a low to high transition on the input, the one-shot edge acceleration circuitry triggers when the signal reaches about Vcc/2. This pulls up the signal to the Vcc level and the driver keeps it at this state.
    When the input driver switches from high to low( it has to overdrive the present state and hence requires 2mA of drive strength as mentioned in the ds) the one shot switches state and pulls the output low.