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TXB0108 and TXS0108 OE control

tsuyoshi tokumoto
Expert 2630 points
Other Parts Discussed in Thread: TXB0108, TXS0108E

Hi all, I'm disty in Japan.

My customer will design the evaluation board using TXB0108PWR
 or TXS0108EPWR.

He will not use the OE control function.

My question is as follows.

   My customer intend to tie the OE pin of TXB0108 or TXS0108E to VCCA
   through the resistance of several kilo ohms.
  
   Is there something that he should care during power-up
   or power-down?
    (e.g. All outputs will not be high-impedance during power-up or power-down.)

Best regards

  • 0
    TI__Intellectual 2530 points

    Hi Tsuyoshi-san

    OE can be tied directly to Vcca or through  a resistor. There is no problem.

    If possible, please ensure that all the inputs are at a defined state and not floating.

    Thanks

  • 0 in reply to Jennifer Joseph
    Expert 2630 points

    Hi Jennifer-san

      Thank you very much for your prompt reply.

      Please let me ask some questions additionally.
     

      (1) I will propose the following to my customer,
           if he ties the OE pin to VCCA through the resistance
           of several kilo ohms.

         ・About TXB0108
              I propose connecting external pulldown resistors
              whose values are higher than 50kΩ to the data I/Os.

         ・About TXS0108E
              I will not propose anything because TXS0108E has
              internal resistance with I/Os. 

            Is my understanding correct?


     (2) If my customer designs the data I/O as floating,
           what will occur during power-up or power-down?
        
          Do you think that the high current may flow?

     Best regards.

  • 0 in reply to tsuyoshi tokumoto
    TI__Intellectual 2530 points

    Hi

     

    The I/O defined state was for unused I/O. Please read further below:

    1) TXB0108E - 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. 

     TXS have internal pull-up resistors, unused I/Os can be left floating. You can leave it floating or connected to VCC. These options will give you the lowest current consumption.

    2). When I/Os are floating, undesired outputs occur possibly with increased current consumption. A good app note on floating CMOS inputs is here http://www.ti.com/lit/an/scba004c/scba004c.pdf

     

    Thanks

  • 0 in reply to Jennifer Joseph
    Expert 2630 points

    Hi Jennifer-san,

    Thank you very much for your prompt reply and detail explanation.

    I almost understood your proposal and explanation.

    However, I would like to ask you one more question.

    Your recommend is pullup(>50kΩ), if the I/Os of TXB0108 become floating state.

    Would you tell me what is the reason that pull-up is better than pull-down?


    Best regards.