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TPS272C45: Logic input limitations when VS voltage source is switched off

Lesser Horned Owl
Prodigy 140 points
Part Number: TPS272C45
Other Parts Discussed in Thread: TPS274160

Hello,

I am using the TPS272C45 as follows:

VS = 24V

Logic Vdd = 3.3V, inputs driven by a micro

In the Absolute Maximum Ratings, some of the upper limits are specified as VS:

Our use case is such that VS might switch off at its source (power supply) independent of the 3.3V supply. So VS=0V in this scenario, and the inputs at 3.3V would be greater than that. Is this really going to be a problem?

I am also using the TPS274160, but am not seeing the same requirement:

Could you also confirm whether or not this is a problem?


Thank you.

  • 0
    TI__Genius 16390 points

    Hi,

    Could you try to use the internal VDD from the device or shut off VDD along with VS for this part to avoid this scenario?

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Prodigy 140 points

    Hi Yichi,

    Sounds like this is indeed a problem? Could you elaborate on what the failure mode would be in this case?

    I neglected to mention that I'm using the A version of this part, in which case the datasheet implies that it needs an external supply.

    Also, I did a deeper scan of the datasheet, and found this section:

    The UVLO behavior in 9.3.3.4 seems to contradict what is indicated in the absolute maximum ratings, might we be OK?

    Thanks

  • 0 in reply to Lesser Horned Owl
    TI__Genius 16390 points

    Hi,

    For device with external VDD, we expect the VDD to be cut off along with VS. If you leave VDD on but VS off, there might be unexpected behavior. Or we recommend the internal regulator. We will work on the datasheet update soon to clarify this.

    Please let me know if you have additional questions. Thanks!

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Prodigy 140 points

    Hi Yichi,

    I can figure out how to cut the VDD power, but the micro driving the EN and SEL lines cannot have its power cut. Is it OK for signal lines to remain active?

    Also, can you confirm that this does NOT apply to the TPS274160?

    Thanks.

  • 0 in reply to Lesser Horned Owl
    TI__Genius 16390 points

    Hi,

    If VS is floating, then the risk will be the part can be unintentionally turned on. If VS is tie to 0V, then it can damage our device depending how much current is passing (the sourcing capability of the microcontroller). There are internal diodes from those control pins to VS. 

    These should not apply to TPS274160.

    Please let me know if you have additional questions.

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Prodigy 140 points

    Hi Yichi,

    VS is turned off with a relay, so it's floating. What is/are the condition(s) where the TPS272 would turn on unintentionally?

    The micro is the TM4C129. Its GPIOs source a maximum of 4mA. I also have 10K series resistors on those inputs to limit current in the reverse direction. Would that be an issue if we decided to pull VS down to 0V?

    Thanks.

  • 0 in reply to Lesser Horned Owl
    TI__Genius 16390 points

    Hi,

    If the control pin voltage minus a diode drop is over 2.63V (above the UVLO threshold), and EN is high, then the part has chances to be turned on. Also it will be depending on how much load it's driving since microcontroller can't support too much current draw.

    4mA should not damage our device assuming you have 10k series resistors with all control pins. However, I would suggest you to test on our EVM to make sure everything works okay as this operation condition is not what the part is designed for.

    Please let me know if you have additional questions.

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Prodigy 140 points

    Hi Yichi,

    OK, so by "turned on," you mean that the device could be powered to some small degree which could cause damage because it's out of operating range?

    And if VS is floating instead of 0V, this could still happen?

    So, to try and summarize:

    1) I cannot rely on the internal VDD for the "A" version of the part because of this: 

    2) So, I need to supply external VDD (3.3V) and shut it off when VS shuts off

    3) If I can't disconnect the micro's inputs, I need to use the EVM to test whether the current supplied by the micro through a 10K resistor is going to cause damage. I feel like this one is going to be harder to assess.

    Thanks.

  • 0 in reply to Lesser Horned Owl
    TI__Genius 16390 points

    Hi,

    With VDD=0 and protection resistor in place at IO pins, the device should not damage in either 0V/floating VS case. With VS floating, the VS might be brought to a voltage and power up some very light load unintentionally if EN is high. With VS = 0V, the device should remain off.

    You summary should be correct and yes I would suggest you to order an EVM to test.

    Please let me know if you have additional questions.

    Regards,

    Yichi

  • 0 in reply to Yichi Zhang.
    Prodigy 140 points

    Thank you, I see a path forward now. You provide terrific support!