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TXB0104: TXS0104 Rated Load Capacitance

Brahim Daouas
Prodigy 30 points
Part Number: TXB0104
Other Parts Discussed in Thread: SN74LVC8T245, SN74LVC2T45, SN74AHCT1G126, SN74LVC1G126, TXB0304

Hi,

We are working on a solution to drive several UART TTL 5V systems connected on a UART bus in a daisy-chain configuration (34 device maximum). The connected UART devices will communicate (as slaves) with a USB-to-UART converter (master) which has 3.3V voltage logic level. Each UART TTL slave device has an SN74LVC1G126 buffer on its TX and RX lines.
We used the TXB0104 in our design to level shift 3.3V to 5V (and vice-versa) for the USB-to-UART to drive the UART devices. We have noticed that the system works with 5 UART devices connected but as soon as we add a 6th device to the bus (or sometimes just a wire), it doesn't work anymore. This sounded like a load capacitance issue seen by the TXB0104 which is above the rated 70pF .
A calculation showed that the total load capacitance would be about 400pF.
We are searching for other solutions to level shift 3.3V-5V and drive the UART devices without issues.

  1. What is the rated load capacitance of the TXS0104 ?
  2. Do you think that this IC would work for our application ?
  3. Do you have any other suggestion that would work ?

Thank you,

  • 0
    TI__Guru 62050 points
    Brahim,

    Welcome to e2e.

    Yes, you are correct about the TXB0104 .
    The 400pF is almost 6times the rated 70pf, hence it wont be reliable in level translation.
    consider using the buffered device SN74LVC8T245 which can support high drive up to 32mA per channel and supports level translation from 3.3 to 5V(vice versa) at the same time depending on the DIR pin.
    we do not have the 4 channel version of the device, however, there is 2 channel SN74LVC2T45 device as well.
    TXS0104 is a passive device which would not support daisy chained configuration with high loading.
  • 0 in reply to ShreyasRao
    Prodigy 30 points

    Hi Shreyas,

    Thank you for your suggestion, we will look into it. For now, we are looking for a drop-in replacement chip for the TXB0104 with higher capacitance loading ratings.

    Best Regards,

  • 0 in reply to Brahim Daouas
    TI__Guru 62050 points
    Brahim,

    If there is a block diagram of the system, please send it.
    I guess that having the SN74AHCT1G126 buffer on the lines could enable the translation from 3.3V to 5V without the need for TXB, but it wont work for 5 to 3.3V.
    Having the sn74LVC1g126 buffer after the TXB output on the daisy chain could cut down the cap loading and hence the translation using the TXB could work as well.
  • 0 in reply to ShreyasRao
    Prodigy 30 points

    Here is attached a block diagram.

    Attached "Screenshot 1" shows the transition (high to low) oscillations on the Tx signal (both 5V and 3.3V) when 4 UART devices are attached to the bus.

    Attached "Screenshot 2" shows the 5V and 3.3V Tx signal when 6 UART devices are attached to the bus.

    Daisy Chain Block Diagram TXB0104.pdf

  • 0 in reply to Brahim Daouas
    TI__Guru 62050 points
    Hi Brahim,

    As a drop in replacement with higher cap loading capability, i would suggest TXB0304 device. It has 1K internal series resistance which is smaller than the 4K internal resistance of the TXB0104 and hence has higher drive strength and load cap ratings.
    Other than that, as i had mentioned about using buffer like the SN74LVC1g126 at the output of the TXB to increase the drive strength will be advised.