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TXS0108E: About Vcca and Vccb

Tsukui Yusuke
Guru 21930 points
Part Number: TXS0108E

Hi,

Could you tell me about TXS0108E VccB?
The following contents are listed in the data sheet page 22 "10 Power Supply Recommendations".
"During operation, ensure that V CCA ≤ V CCB at all times."

However,
In the customer's application, VccB is connected only during debugging.
(It is OPEN during normal operation.)
Therefore, there is a case of VccB side is OPEN.

Is there a problem with using the VccB side in the OPEN state?
Please tell me the point of caution in this case.

Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Intellectual 2100 points
    Hey Tsukui,

    I would not operate the circuit like you are describing. Are you able to give the use case for this part in your project because I can't think of any reason why you'd implement it in this operational fashion?

    Best Regards,
    Nick
  • 0 in reply to Nicholas Mariani9
    Guru 21930 points

    Nick-san,

    Thank you for your response.
    I apologize for my lack of explanation.
    Let me explain a little further.
    I will attach a customer use case image.



    Customer would like to use a microcontroller on the Vcca side and a debugger on the Vccb side.

    In the data sheet, there is [during operation, ensuring that Vcca ≤ Vccb at all times.],
    During operation, it means that Vcca     ≤ Vccb.

    However, since the microcontroller and the debugger are not connected at all times,
    Customer would like to use the Vccb side (debugger side) with OPEN.


    How to use OPEN on one side is still bad, is not it?

    Is it necessary to keep Vcca ≤ Vccb during operation?



    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    Guru 317000 points

    Without the external device, the TXS is not operating.

    Section 10 of the datasheet also says:

    The sequencing of each power supply will not damage the device during the power up operation, so either power supply can be ramped up first. The output-enable (OE) input circuit is designed so that it is supplied by VCCA and when the (OE) input is low, all outputs are placed in the high-impedance state. To ensure the high-impedance state of the outputs during power up or power down, the OE input pin … must not be enabled until VCCA and VCCB are fully ramped and stable.

    So it is allowed to have one side open; you just have to ensure that OE is low before that side is unplugged.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 2100 points
    Hey Tsukui,

    Clemens beat me to the punch but the enable pin is what will help you out here. As long as you pull the OE pin low and let the device enter the hi-z state during regular operation (no debugger) and before you connect or disconnect your debugging instrument, you should be good to go.

    If this helped to answer your question please let us know that we could lend a hand in solving your problems by hitting the green verification button below! If you have additional questions please don't hesitate to reach out.

    Best Regards,
    Nick