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Original question:

TMUX646: TMUX646

TMUX646: Open Short Test

Soo Cheng Yeap
Prodigy 50 points
Part Number: TMUX646
Other Parts Discussed in Thread: TS5MP645

Tool/software:

Hi,

I am looking for a multiplexer that allows me to switch between normal MIPI operation and open short testing. Is TMUX646 the right option?

Regards,

Soo Cheng

  • 0
    TI__Genius 16060 points

    Hi Soo,

    This device is a good choice for MIPI applications and should be fine for open short testing as long as the device is operated under recommended operating conditions.

    Here is a thread where someone did MIPI operation and open short testing in the same design.

    The best solution  at the time was using the  TS5MP645 which is the older version of the TMUX646.

    The TMUX646 is a higher bandwidth device that support faster mipi operation.

    Please let me know if you have any other concerns with this device in this use case as I can look into this some more.

    Regards,

    kameron

  • 0 in reply to Kameron Hill
    Prodigy 50 points

    Hi Kameron,

    For TMUX646, what's the recommended operating condition that you mean for open short testing? Generally we source or sink 2mA of current to check the DUT upper and lower diodes.  

    For TS5MP645, it is not recommended for new design.

    Regards,

    Soo Cheng

  • 0 in reply to Soo Cheng Yeap
    TI__Genius 16060 points

    Hi Soo

    The recommend operating conditions is in section 6.3 of the datasheet and 2ma is fine to be put through the io pins of this device according to the Is or Id specification i ln that section of the datasheet

    There are no other concerns I see in using the TMUX646 for mipi and open short testing but please let me know if there are any other concerns.

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 50 points

    Hi Kameron,

    When doing open short testing for the lower side diodes, we sinks 2mA of current from the DUT and the voltage shall be a negative value and that's shall violate the Vsw spec. Based on this thread, it says that TMUX646 contains a protection diode to ground which I think that shall affect the open short reading.

    Is there any method to make it works?

    Regards,

    Soo Cheng

  • 0 in reply to Soo Cheng Yeap
    TI__Genius 16060 points

    Hi Soo,

    Thanks for that extra info.

    Yes if a negative voltage is placed on the io pins of this mux when VDD= 1.5 to 5.5V and GND=0V the esd diode from gnd to the io pins will shunt current and can effect your readings. Here is just a quick faq that explains internal esd structure in multiplexers

    However I think there is a way you can have negative signals on the io pins of this multiplexer.

    The method to this is called ground shifting where you would set the power supplies of this device using the recommended supply VDD rating as a differential rating.

    For clarity you could set the the supplies of this device as VDD=2.5V and GND= -2.5V 

    Additionally this differential should apply to the  Vsw spec in the data sheet were you would be able to place signal from  1.1 to -2.5V on the io pins

    Please let me know if this is something you would be open to doing and if you could let me know what supply rails you are going to power this device with when can double confirm this operation.

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 50 points

    Hi Kameron,

    Thanks for the detailed explanation. 

    This is a generic board with the device to be tested undefined yet so the current objective is to able to test open short at +/-2V range. From your reply, I found that the allowed upper Vsw decreases by 1.4V to 1.1V. Is this fixed or vary depending on the load condition? Is that stated in the datasheet ?  

    Regards,

    Soo Cheng

  • +1 in reply to Soo Cheng Yeap
    TI__Genius 16060 points

    Hi Soo,

    This decrease isn't due to the load condition of the device and isn't stated in the datasheet.

    The 1.1 to -2.5V signal range came from my understanding on how ground  shifting multiplexers work. and looking at what the input voltage range could be based on the FAQ.

    The Vsw spec is more limited than the recommended supply voltage spec in the datasheet

    However if we were to set the supply voltages based of the max recommended Vsw spec, then in theory you could use a 1.8V and -1.8v supply and pass 1.8 to -1.8V signals and after talking to our design team this should be fine

    This is just to have a better idea of what signals we could actually pass through the mux.

    The abs max section states that Vsw can up to 4V or in our ground shifting application 4V differential. So in theory you could try to use +/-2V supplies and pass 2V and -2V signals. But I am not certain about the longterm reliably of this device in that scenario.

    For mipi and open short testing this might be our best solution.

    Please let me know if this helps

    Regards,

    Kameron