• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 33 subscribers
  • Views 272 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TCA9535: Output characteristics with a pull-up above Vcc

Steve Widener
Genius 11010 points

Part Number: TCA9535

With Vcc at 3.3V and a 7.5kohm pull up to 5V on the I/O (configured as an output), what will the output voltage be? Will there be significant leakage current back to the 3.3V Vcc supply?

Is this an acceptable configuration?

  • 0
    TI__Genius 11010 points

    To add detail, the output would be driven high

  • 0 in reply to Steve Widener
    TI__Guru* 78861 points

    Hi Steve,

    I was going to say that's fine as long as the output isn't driven high.....

    When you have a higher Voltage tied to the output and drive high, the PFET that is turned on will switch its drain and source when the Vcc is lower than the outside bias voltage. This basically looks like a short from the external signal to Vcc. 

    This configuration is not acceptable.

    What exactly are they trying to do here? 

    -Bobby

  • 0 in reply to BOBBY
    TI__Genius 11010 points

    Bobby -

    Does this violate an ABS max rating? Since there is a current limiting resistor, why does this cause problems with the device?

    I'll check to see why there is pull-up to 5V, it may be legacy or perhaps the levels need to be that high for noise margin.

  • +1 in reply to Steve Widener
    TI__Guru* 78861 points

    This doesn't appear to violate the ABS max rating.... the damage would likely be at a system level (I'm not sure how it would affect our device). Anything connected to VCC that cannot tolerate ~5V could be damaged. This may cause issues related to our device operation though as I recall, when I did testing doing this, our device ended up locking up. I had to power cycle the device to resolve the lock up. 

    "Since there is a current limiting resistor, why does this cause problems with the device?"

    This port was meant to output current when driven high, if you suddenly sink current while driving high then you have current going the opposite direction that it was designed for (maybe to paths it's not supposed to go). I doubt silicon designers for this device were expecting current to go into the device when it was supposed to output current so there likely wasn't any design safety mechanisms to prevent this. Placing things such as a diode infront of the PFET would have lowered the VoH of the device so this kind of safety mechanism wouldn't have been practical and would have made our VoH inferior to competition. 

    -Bobby