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TCA6408A IO Protection

Cochise Fant
Prodigy 50 points
Other Parts Discussed in Thread: TCA6408A

Greetings,

I would like to use the TCA6408A in my application; however, I have a question about the IO protection on the device.  My application has certain processing running all of the time and other processes running only some of the time.  To save power, we pull power from the processes that don't need to be running.  Therefore, it may be possible for the input IOs to the TCA6408A to be transitioning while the TCA6408A itself is powered down (both VCCP and VCCI would be 0V).

The IOs would not be transitioning above 5.5V or anything, they would be no more than 1.8V, but I wondered how the IO interface would hold up to the higher voltages on the IO pins while the rest of the TCA6408A device is powered down.

Can anyone speak to this?

  • 0
    TI__Genius 16510 points
    Hello Cochise,

    Thanks for the question.

    When you say that the IOs would be transitioning, I assume that you mean they're being used as inputs and there is some signal being drive to them?

    In that case, It is unlikely you will damage the device with a 1.8V signal applied to it while VCCP and VCCI are at 0V.

    However, there is no guaranteed state of the ports during power off, and typically it will mimic the state it was in prior to shut down (residual charge on the gates), so if you were using the ports as an input, it is likely that they will remain in a high impedance state just after power off. There is a chance that the pull up or pull down FETs could become partially active and you could introduce a leakage path from the pins to ground or to VCCP.

    For this reason, if you are using the pins as inputs only, I would recommend putting a series resistor for the pins to protect the device driving the signal from possibly having a leakage path to rail. Since the input buffers are very high impedance, a large resistor is acceptable such as 1k to 10k ohms.
  • 0 in reply to Jonathan Valdez
    Prodigy 50 points
    Jonathan,

    Thanks for your reply and suggestion.

    You have a proper understanding of my question. I am concerned about some device driving these input signals high and/or low while the TCA6408A is off.

    I have seen in other designs where VCCP is powered from an 'always-on' power rail while the VCCI is powered by a rail that can be enabled and disabled for power savings or sleep mode.

    NOTE: When I say 'I have seen in other designs...' I mean that I have seen a different, but similar I2C-to-GPIO expander used in this way.

    Would this be another way to avoid the issue of potential damage to the IO pins? Are there any "gotchas" to this approach? Anything wrong with applying power to VCCP while VCCI is at 0V?
  • 0 in reply to Cochise Fant
    TI__Genius 16510 points
    Cochise,

    It is possible to leave VCCP up as well. Similar behavior will occur though, since internally, the logic is powered via the VCCI. Again, it will be likely that the ports will continue with the same behavior, but there is a chance they will enter a semi-high resistance path to VCCP or ground (albeit a small one).

    I would recommend the use of a series resistor for the most security against damaging the TCA6408A. As mentioned, though, even without a series resistor, it is highly unlikely any damage will result. The series resistor will guarantee that the current through the port will never be above the absolute maximum allowed 50 mA.