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TXS0108E-Q1: In case of using pull down at output port

Lee, Sam
Expert 5575 points
Part Number: TXS0108E-Q1

Hi team,

 

Can you look into an output level issue in TXS0108E-Q1 application?

 

A customer are using pull down resistor at the output port to get a divided voltage. The datasheet specifies “RPUA and RPUB have a value of 4 kΩ when the output is driving high.”, so they expected above 2V at B port with 10kohm pulldown. However, when giving A port = high, the B port is observed about 940mV. Can you explain what is wrong? Is the 4kohm right for the the internal pullup?

 

Condition:

VCCA = 1.8V

VCCB = 3.3V

Ax port = High

Bx port is pulled down with 10kohm.

: No external pull up

: Not connected to other device was also tested

 

 

Thanks,

Sam Lee

  • 0
    Guru 284820 points

    Did you measure this with a multimeter or an oscilloscope? (The voltage might be oscillating.)

    Anyway; 4 kΩ is a nominal value; resistors on silicon have very bad tolerances.

  • 0 in reply to Clemens Ladisch
    TI__Expert 5575 points

    Hi Clemens,

    It's measured with oscilloscope. Can you explain why the voltage might be oscillatung?

    As an additional info,  it's observed normally to 3.0V when using 47kohm pulldown.

    Can you explain this symptom between 10k and 47k?

    Thanks,

    Sam Lee

  • 0 in reply to Lee, Sam
    TI__Mastermind 44775 points

    Hey Sam,

    It is not recommend using pull-downs with this device, trying to implement a voltage divider with the internal resistors will not work as a normal voltage divider since there is additional circuitry (as the output is also an input). I would just avoid this all together.

  • 0 in reply to Dylan Hubbard
    TI__Expert 5575 points

    Hi Dylan,

    Thank you for your reply. However, the customer followed below sentence in TXS0108E-Q1 datasheet. So, if it's not recommended, the datasheet should be modified.

    • An external pull-down resistor decreases the output VOH and VOL. Use Equation 1 to calculate the VOH as a result of an external pull-down resistor.
       VOH = VCCx × RPD / (RPD + 4 kΩ)


    Pleae refer to below configuration of current issue. They need a pull down resistor at the Enable pin of the IC. This issue was found during testing pull down resistor rather than using divider voltage. So, the 940mV @ A port=High is abnormal value. Can you advise how to solve this issue and the rootcause? Otherwise, are you recommending that TXS0104-Q1 can't be used for this application?

     

    Observed :

    Case 1:


     

    Case 2:


     

     

    Thanks,

    Sam Lee

  • 0 in reply to Lee, Sam
    TI__Mastermind 44775 points

    Hey Sam,

    Are you saying even without the pull-down they still measure 940mV? So the pull-down being there or not makes no difference?

  • 0 in reply to Dylan Hubbard
    TI__Expert 5575 points

    Hi Dylan,

    For case1, they checked the voltage in the condition of disconnecting to the IC.

        : with 10kohm pull down -> 940mV

        : with 47kohm pull down -> 3.0V

    For case2, they measured the voltage without pulldown in the condition of connecting to the IC. However, they still observed the similar voltage to 940mV. So, based on this result, they're seperately checking if the enable pin of IC has internal 10k pull down or not.

    Anyway, you should focus on case1 because they need to use the pull down to keep low state at startup.

    Thanks,

    Sam Lee

  • 0 in reply to Lee, Sam
    Guru 284820 points

    The TXS outputs an open-drain signal and should never be used when the output has to be default low.

    There are other translators that would work better. What are the other seven channels used for?