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TMUX6208: Reverse current to VDD

ahmet selcuk silindir
Prodigy 160 points
Part Number: TMUX6208
Other Parts Discussed in Thread: TMUX7462F, TMUX7308F

Hi,

I have a design with 24 TMUX6208 multiplexers on the same 28V power rail. some of them sharing the output connections on the connector and the purpose is to route different input signals to the connector pins. The problem we have is while the board is powered down, if we apply voltage on S pin(connected to the connector) of the mux, the board works via the voltage on that pin. There is a reverse current flowing from this pin to VDD and to the 28V power rail which powers the board. What would be the reason and solution for this situation? Creating a power rail for all mux VDD pins and powering this from 28V through a diode would help? 

Please share your suggestions and possible root cause of the issue. 

Best regards

Ahmet 

  • +1
    Guru 243120 points

    The absolute maximum ratings forbid voltages above VDD + 0.5 V; there are clamping diodes from the I/O pins to VDD.

    You can create a separate power rail, but the muxes would still be powered up. And you must limit the clamp current to less than 30 mA.

    You might be able to replace the TMUX6208 with the TMUX7308F, which allows overvoltage on the S1…S8 pins. Alternatively, add TMUX7462F protectors in front of the muxes.

  • 0 in reply to Clemens Ladisch
    TI__Expert 8652 points

    Hi Ahmet,

    Clemens has some really good suggestions in response to your answer! I'd just like to add some more information here. 

    It sounds like you may need a device that has our powered-off protection feature. You can read more about this feature at our [FAQ] Do I need powered-off protection? (HI-Z when Vcc = 0V).  A TI switch with powered-off protection will maintain a high impedance state on the I/O and digital pins (no reverse current) when the switch's supply is not powered (VCC = 0 V).

    In this case, TMUX6208 does not have powered-off protection, so the internal ESD diodes will be forward biased when VI/O exceeds VCC.

    If you'd like a switch with this feature then you can use the online product search tool to select one. As Clemens mentioned, the TMUX7462F would make for a good protector at the front end of the multiplexers you have in your design (due to powered-off protection) while the TMUX7308F would be a solid replacement for the TMUX6208 since it has overvoltage protection. We have an FAQ on this feature as well. [FAQ] How do Overvoltage & Fault protected devices work and which devices have this feature?

    Thanks!

    Alex 

  • 0 in reply to Clemens Ladisch
    Prodigy 160 points

    Hi Clemens,

    This is a very swift and accurate response, thank you very much but I have doubt if I can use the one with powered off protection TMUX7308F because it seems to have a really high on resistance. I will use this mux to multiplex canbus. Do you think TMUX7308F will work on canbus considering it's Ron resistance?

    Kind regards

  • 0 in reply to ahmet selcuk silindir
    Guru 243120 points

    The on-resistance is larger than the typical termination resistor. So it's unlikely to work.