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TMUX6211: Powering

Zoltan Szabo
Prodigy 50 points
Part Number: TMUX6211

The project is to monitor large Fuel cell pack voltage. 


My customer have 24 fuel cells in series, so the overall pack voltage is 9-26.5V.

Still now they used LTC6808, which is a great chip, but as get in shortage they need a less chip dependent design.

Im thinking to use a TMUX6211/DG412 as input MUX as that is available at several manufacturers. This muxes (12 pcs) would select 1 by 1 the cells, and connect them to an ADC. Vs are inputs, while Vd are outputs to ADC.

Here I have already a question about powering the mux. Can I power them directly from the cells 9-26.5V ? Will they work fine close to the rails (VDD=Vs) ? Based on datasheet they can work, but as I need to measure at 10mV level, I need really full rail to rail working range for the switched signals.

The cells can go slightly on negative too, so if the pack GND is used as GND for the muxes (connected to VSS), than it can happen as cell 1 goes negative, so the switch  input Vs pin have for ex. -1V less than its main GND (GND and VSS).To avoid this actually I added 2 DC-DCs. One to boost the pack voltage (9-26.5V) to fixed 30V to power the VDD of the muxes, and other one like -3V to power the VSS as GND of the muxes used as GND of the system.

Based on datasheet "Source or drain voltage (Sx, Dx) VSS–0.5 VDD+0.5" so mainly should work well for inputs on + side (Vs=VDD) and down to -0.5V on negative side(Vs=VSS-0.5V), but we dont want to lose any mV due clamping or any internal protection.

Could anyone tell me if im on the right way and really both DC-DC are needed ?

              OR                      

Thank You !

Zoltan

  • 0
    TI__Genius 10974 points

    Hi Zoltan,

    The TMUX6211 is a good choice here given the requirements you mentioned. This device allows rail to rail switching, so you could pass a signal up to whatever your VDD voltage is and anything below that voltage as well (as long as it is above VSS-0.5). In short, these devices perform perfectly fine when VS is at the rails. The only difference you will see is the resistance being slightly higher at the rail voltage level (see RON curves in datasheet).

    In regards to powering from the cells, you can power the TMUX6211 from the cells, however, you would need to make sure the VDD voltage will always be equal to or higher than the other VS voltages otherwise you risk biasing the clamping diodes and potentially damaging the device. Essentially, if you do power from one cell, can you guarantee it will never cause VDD to be less than the other cells being monitored? This is critical to ensure proper operating of the switch.

    As you mentioned, powering from the cells but using a DC-DC converter may be the safest option here as you can guarantee the condition I described above will be met and not need to worry about an overvoltage event happening on the inputs. In addition, this same condition needs to be met on the VSS side as well (which I believe you are aware of given your dual DC-DC converter configuration).

    Again, both configurations could work, it really is just a matter of how confident you are in the fluctuations of these cells and if you can guarantee the VDD/VSS voltage supplied by the cells can always be above the other cells on the VS inputs.

    Let me know if you have other questions on this.

    Thanks!

    Bryan

  • +1 in reply to Bryan Marshall
    Prodigy 50 points

    Hi Bryan,

    Thank You for Your detailed review !

    Theoretically got answer for all my questions J

    Now I will look for some parts and do a trial first. Interesting that your TMUX has a clamping at 0.5V, while others like DG have a clamping at 2V. So to be safe and compatible with all manufacturers I think better to use the DC-DC as originally planned.

     Thank You again for fast support and have a great day !

    Z.

  • 0 in reply to Zoltan Szabo
    TI__Genius 10974 points

    You are welcome Zoltan! Good luck with your project and let me know any additional questions!