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TXB0108: Interfacing with ISO3086T

Part Number: TXB0108
Other Parts Discussed in Thread: ISO3086T, TXS0108E, SN74LVC8T245,

Hi

We have interfaced our processor UART interface pins @ 3.3V with ISO3086T operating @ 5V using TXB0108E. We are facing an issue when the pins are driven from high to low.

We feel there is no enough drive strength  of the TXB0108E. (20uA) ..  We are thinking of using TXS0108E to solve the problem. 

One more point is , we previously used NVT2008PW IC for the same. As we are using other TI ICs our procurement department asked us to use TI part hence we used TXB0108E.

Can you please let us know if using TXS0108E will solve the issue ? We have also placed pull-ups on our board on D, R and DE/RE lines so we can populate them for Open-Drain output. The speed of operation is less than 1Mhz.

We are stuck and we have to give proper explanation if we have to go back to NVT2008PW part :)

Ramgopal Kota

  • Hi Ram,

    UART signal is push pull interface and TXB device are appropriate for them.
    TXS devices are ideal for open drain devices since they have internal pullup resistors internally. TXS doesn't have DC drive better than TXB either and the high to low is in fact dependent on the driver device which needs to take the line low.
    Is there any scopeshot to see what the issue is about? what is the sink current in your case? the TXB cannot drive/sink higher than 20uA due to the internal 4k limiting resistor.
    I would suggest looking into the SN74LVC8T245 which has enough drive current to take the line from high to low.
  • Hi Shreyas

    Thank you for your quick reply ... Due to holidays in India we will be back on thursday. We will send some captures the problems we are facing.

    Can you also tell us can we replace NVT2008 with TXB0108E ? I see that NVT2008 has only 5uA where as TXB has 20uA. Please suggest if it is ok to replace.

  • It depends on the drive requirements. The NVT device doesn't have any drive of its own as its a passive device. (cant comment much on NXP device)
    the TXB is active drive device,however the drive is limited by the 4k resistor internally. So, unless there is greater than 20uA load current required, there is no problem to use TXB0108 device.
  • Shreyas

    We are driving ISO3086T TI Chip. From the datasheet of ISO3086T the drive current on D , R ,DE,RE are 10uA.

    Can you please confirm this so that we can close this case ?

  • Ram,

    I think TXB is good to use in your case since the leakage is under the rated load current supported by the TXB.