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Question:About recommended resistance value of TXB0108.

Toshiya Hiramatsu
Intellectual 630 points
Other Parts Discussed in Thread: TXB0108

Hi,

I am using TXB0108.
Some I/O ports are unused.
The processing of an unused terminal is pull-up or pull-down.
Here, I have the question.
What is recommended value of resistor?

For instance,I think as follows.
PULL-UP.
MIN value:VCCx÷50mA.(note:x is A or B.)
Because it doesn't exceed absolute maximum rating IO(50mA).
MAX value:4kohm.
Because the unused terminal input voltage is sure to become VCCI/2 or more.


Is this correct?
Correct me if I am wrong.


Thanks for reading!
Toshiya.

  • 0
    TI__Genius 9085 points

    Hello,

    The TXS0108E is the recommended device if your application requires pull-up resistors. This device also has internal pull-up resistors, which can simplify your layout.

    The use of pull-up or pull-down resistors are not recommended with the TXB devices. TXB devices have an internal buffer. This buffer will contend with the pull-up/pull-down if the resistance of the pull-up/down is < 50kOhm, and effect the VOL and VOH values. If the pull-up/down is > 50kOhm, the buffer will overpower the external resistor and it may not have the desired effect. 

    If you do not plan to communicate on the I/Os, and only want to prevent them from floating, I would recommend a pull-up of > 50kOhm. Because the TXB is a buffered device, it will never leave the output floating when powered and enabled. It will always pull the output either high or low with the internal 4kOhm buffer. 

    Below is an application note which discusses the TXB translation devices in more detail.

    http://focus.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=scea043&docCategoryId=1&familyId=707

  • 0 in reply to Hattie
    Intellectual 630 points

    Hattie-san,

    Thank you for your explanation.
    I'll recommend the >50kohm resistor.
    Because I want to prevent them from floating.

    Regards,
    Toshiya.