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TMUX1208: Is there a risk of overvoltage on unselected inputs, on unselected channels that are pulled above 6V, but only through large (current-limiting) resistors?

Ricardo Gorinstein
Prodigy 30 points
Part Number: TMUX1208
Other Parts Discussed in Thread: MUX508

I have a 3.3V MCU with an ADC channel that I'd like to use to measure 8 different voltages, some of which can be as high as 28V.  I have designed a simple multiplexed voltage divider circuit (see below) that takes these 8 inputs, each through their own (top) resistors, to the 8 inputs of a mux.  The output of this mux is then connected through a single (bottom) resistor to ground.  The output of the mux is also hooked (directly) to the ADC input on the MCU.  The several resistors are chosen so that, when a particular channel is selected, the voltage at the output of the mux is always no greater than 3.2V, and the current is under 1 mA.  This puts the bottom resistor at 10 kOhm, and top resistors in a range from 3.74 kOhm to 76.8 kOhm.

example circuit

However, the voltage on the mux inputs on the OTHER channels (that is, those that are NOT selected, so that current is NOT flowing on those channels), will be pulled up, through the top resistors, to as high as 28V.

My question is, in this setting, can I get by with a TMUX1208 (which is small/inexpensive, but rated only to 6V) as my mux, or should I switch to something like a MUX508 (which is rated to 36V, but is much larger and more expensive)?

My hope is that the presence of the substantial resistors in the above-described scenario would protect the TMUX1208, so that even though the voltages of unselected channels are pulled well above 6V, the currents could never actually get large, so it would be ok.

How should I handle this situation?

Thanks for your guidance,
Ricardo

  • +1
    TI__Genius 10974 points

    Hi Ricardo,

    Thanks for your detailed question! So you will be able to use the TMUX1208, however, there would need to be a few adjustments made and things to be aware of.

    1) These multiplexers incorporate protection diodes on the inputs of the mux, which will forward bias and begin conducting once the input channel exceeds VDD+0.5 (even if the channel is not selected). So, if you have 28V on your input pin and a 5V VDD supply, you will conduct around 300uA (assuming you have your maximum resistor of 76.8k and a 23V differential). While the ESD protection diodes can handle this current (as the maximum current is 30mA), you will alter the voltage at which the device is operating at, possibly all the way to 28V and can damage the rest of the device. 

    2) With that said, you will need to adjust your protection strategy for your design if you want to proceed to use the TMUX1208. An easy solution would be to add a 5V zener/TVS diode to each of the inputs so the input voltage gets clamped to 5V once exceeded. As long as the zener/TVS can handle the current (which you are already accounting for by adding those series resistors!), there will be no overvoltage event seen at the inputs of the mux and will not cause any damage or VDD fluctuations.

    If you would like to read up some more on this subject, please refer to this app note: https://www.ti.com/lit/an/sbaa227/sbaa227.pdf

    Thanks!

    Bryan