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Original question:

Technical question about TPS70950

TPS709: TPS70950 doesn't work when EN pin is floating?

Sean Chou
Prodigy 235 points
Part Number: TPS709

Tool/software:

Hi TI team,

     I have buiild a circuit according to the TPS70950 datasheet when VIN=10V, we letf the EN pin floating ,but it doen't work when power on. When I connect the EN pin to VIN, it works.

Do you your what' wrong with that? The datasheet shows that the EN pin can be floating. Thanks a lot.

BR

Sean

  • 0
    TI__Genius 15125 points

    Hello Sean,

    Thank you for your question.

    Have you tried a new device? 

    What are you measuring at the EN pin when it is left floating, but the device is being powered by 10V?

    Are you measuring anything at the output?

    One thing I can think could be the reason is that this device has a 300nA (typical) pull up current to enable the device when EN is left floating. 

    Have you verified that the device doesn't start up when the EN pin is not being probed? If the EN pin is being probed, because of the 300nA current source, the probe could be loading the device too much to allow the EN to pull up.

    Are you using a PCB that may have conformal coating on the board? 

    There have been instances of devices having an internal EN pull up that are unable to turn on due to board-level leakage caused by conformal coating.

    Hope this helps!

    Best Regards,

    Hannah

  • 0 in reply to Hannah Grace
    Prodigy 235 points

    Hi Hannah,

         Thankls for the quick response.

         Yes, I'm sure that the device doesn't start up when the EN pin is not bening probed. And when I probed this pin, I get alomost zero voltag on this pin.

          As you have mentioned that amy be the reason is the conformal coating, May be I need to add a resistor divider to avoid this phenomenon. whatever, it may a common issue that other gus may meet that later.

    BR

    Sean

  • 0 in reply to Sean Chou
    TI__Genius 15125 points

    Hi Sean,

    Yes, try the conformal coating solution and let me know if you still see the same concern with the EN pin.

    Best,

    Hannah