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PCA9539: Protection against output short to gnd fault?

Cassidy
Prodigy 610 points
Part Number: PCA9539

Hi,

If an output port is configured to output HIGH will this part experience damage if an external fault shorts this output to GND? Is there any internal protection against this? If there is protection is it only spec'd for a limited amount of time?

Thank you!

  • 0
    TI__Guru 72751 points

    Hey Cassidy,

    This device does not come with any fault protection. If you are driving the output high and a fault does occur, you will potentially damage the PFET of the device. The only thing I can think of to try to remedy this is to place a series resistor directly to the output of the device to limit the current the device can supply.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    TI__Prodigy 610 points

    Hi Bobby,

    Thanks for the quick response! There is a clamping current stated in the datasheet, though this seems to be a spec only applicable when the output voltage is below 0V or above Vcc. Is there any such clamping behavior during normal operation that could limit the current?

  • 0 in reply to Cassidy
    TI__Guru 72751 points

    Hey Cassidy,

    I just looked at the design files for the device. I saw that the output stage is exactly as I thought it was, a CMOS which drives high or low. The PFET connects directly to the Vcc net without any series resistance of any kind. This means there is not a clamping mechanism for the output stage during normal operation.

    "There is a clamping current stated in the datasheet, though this seems to be a spec only applicable when the output voltage is below 0V or above Vcc."

    I do see some other cells connected to the output but they look like they only trigger when there is a difference of GND-Vforward and (this one is a guess) if the pin sees a fast slew rate above Vcc. (seems like part of an ESD protection cell).

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    TI__Prodigy 610 points

    Hi Bobby,

    Thanks again for the quick responses and info. So, just to really double check this...Can you explain the Output Clamp current rating in table 6.1 in the datasheet (absolute maximum ratings). This will be the spec the customer will look at and will likely want a more detailed explanation of that spec, what it is, and why it doesn't apply to normal operation.

    Given that it is in the absolute max ratings table, I'm guessing it's not a characteristic behavior of the part but a limitation. Still though, the "clamp current" terminology makes we question.

    Thanks!

  • 0 in reply to Cassidy
    TI__Guru 72751 points
    Hey Cassidy,

    "Can you explain the Output Clamp current rating in table 6.1 in the datasheet (absolute maximum ratings)."
    This is related to the clamping diodes on the I/O pins. If you go below GND by Vforward then this diode will conduct to clamp the voltage (this protects the device.) However there is a limit to how much current this diode can handle before it breaks. The current value you see there states how much DC current this diode can take before it breaks.

    "why it doesn't apply to normal operation."
    Normally you do not drive the I/O pin below GND or above Vcc.

    Thanks,
    -Bobby