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TXB0106 Pull up

sushruta shetty
Prodigy 100 points
Other Parts Discussed in Thread: TXB0106, LSF0108, TXS0108E

Hi

We have used TXB0106 IC in our design.

Though we do not need bi-directional translation, we are using this IC for BoM cost optimization.

We have used TXB0106 IC in between HCPL-6651 optocoupler and Zynq SoC. 

TXB0106 IC translates output from optocoupler to Zynq SoC inputs. Hence, it always operates in one direction only.

The output of optocoupler is an open collector pin. Hence, this signal is pulled up to 5V. 

I wanted to know if there are any constraints for choosing pull up resistor? 

It is given in datasheet that "For proper operation, the device driving the data I/Os of the TXB0106 must have drive strength of at least ±2 mA". So, is it ok if I use 2K resistor?

Awaiting for your reply.

Thanks and Regards

Sushruta

  • 0
    TI__Guru 62240 points
    Hi Sushruta ,

    I will be moving your post into voltage translation forum for support .
  • 0 in reply to ShreyasRao
    TI__Guru 62240 points

    Hi Sushruta ,

    You could consider LSF0108 or TXS solution which is better suited for open collector applications . The requirement would be to sink the IOL current from the pullup resistor when the optocoupler output is low and still be able to maintain the Vol level for the application .

    For e.g , using 1K pullup resistor with 5Vpullup , would have 5mA of IOL current flowing into the optocoupler through the TXB/LSF/TXS device . As long as the optocoupler is rated to have Vol at or above 5ma IOL , it should work. But using 100 ohm resistor would mean IOL of 50ma which may not be within the spec of optocoupler .

  • 0
    TI__Genius 16510 points
    Hello Sushruta,

    As Shreyas mentioned, the TXB family is not recommended for open-drain applications because it has the requirement that any pull-up resistors be >= 50k ohms. Therefore, 2K ohms would not be ok.

    I would recommend looking at the TXS family instead, for open-drain translation. If you require 6 channels, the closest options in the TXS family is the TXS0108E and the TXS0104.

    What are you trying to translate? Is there a specific interface you're trying to work with?
  • 0 in reply to Jonathan Valdez
    Prodigy 100 points
    Hi

    Thank you Shreyas and Jonathan for your reply.

    First I would like to clarify that its just a GPIO Interface.

    My concern here is, if I use pull-up resistors be >= 50k ohms, then the current at the input of TXB0106 will be very low.

    It is given in datasheet that For proper operation, the device driving the data I/Os of the TXB0106 must have drive strength of at least ±2 mA.

    So, will I not be violating this rule?

    Thanks and Regards
    Sushruta
  • 0 in reply to sushruta shetty
    TI__Genius 16510 points

    Sushruta,

    TXB is designed for systems which are push-pull. Most GPIOs are push-pull. Push-pull refers to 'totem-pole' output type of driver, where a FET is able to pull the voltage high or to ground through a low-impedance path. In this configuration, a single FET can be turned on at once, meaning the bus is 'shorted' to either VCC or GND through a low impedance path. These outputs are generally able to drive several mA of current without any issues. The TXB is designed for this type of interface.

    Open drain interfaces only have a pull-down FET to ground, and use a pull-up resistor to pull the bus to a high voltage. The issue with this is that when a 0 is transmitted, then all the current from the pull-up resistor must be sunk in order for a 0 to be present on the line. The TXB is unable to do this because of the 4k series resistor, and this is why a push-pull interface is only allowed.

    TXS parts feature a different architecture which do not have this series resistance and can support open-drain interfaces.

    Since you say you are using a GPIO, please check to see if the port is a totem-pole/push-pull output with at least 2mA of driving current (this information will be in the datasheet) or if it's an open drain and requires an external resistor. If it is a push-pull setup and can handle 2mA of drive current, then you may use the TXB0106 with no pull-up resistor (or greater than 50k), otherwise, you will need to look at another solution such as the TXS0108E or TXS0104