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TMUX7209: TMUX7209RUMR

Tyler Laber
Prodigy 20 points
Part Number: TMUX7209
Other Parts Discussed in Thread: TMUX7309F, TMUX7412F

I would like to use the TMUX7209 as a cell measuring multiplexer in a 4S lithium battery BMS.  Because of the construction of the pack and the fact that we need to assemble with live cells we will need to "hot plug" the cells into the BMS board.  What concerns would I need to be aware of when doing this.  Would the order that the nets make contact with the mux be important both in terms of power/ground as well as the mux input channels?  Would latch up be a concern if nets were attached in a certain sequence?

 

  • 0
    TI__Mastermind 24440 points

    Hello Tyler,

    So please correct me if I am wrong but when you are saying hot plug you mean there is a chance that in your current design the 7209 will see a voltage on the IOs before VDD is powered(Vdd=0V and Vio is greater than 0V)

    If this is the case I recommended you use the TMUX7309F which has power off protection and can ensure the device is not damaged during the "hot plug". The power off protection of the tmux7309F eliminates power sequencing concerns  and voltages present on the io pins of the mux before it is powered up will not damage downstream components in your design. 

    In addition both the TMUX7209 and TMUX7309F are latchup immune meaning latchup should not happen with these devices in your design.

    Please let me know if you have any further concerns!

  • 0
    Prodigy 20 points

    Voltage on the digital pins would not be higher than Vdd but it is possible that the analog switch pins of the 7209 would connect before the power pins (this could probably be avoided with proper assembly if you would advise).  It looks to me like the TMUX7309F might not be a good solution though since we need to measure voltages close to the Vdd supply voltage(V_S1A = Vdd) and that the on resistance of the switches gets high when switch voltage nears Vdd and our follow on amplifier has a fairly low input impedance.

  • +1 in reply to Tyler Laber
    TI__Mastermind 24440 points

    Hello Tyler,

    You are correct and I wanted to clarify that the TMUX7309F has the power off protection feature to block input signals  when the device is unpowered.

    That being said I cannot guarantee the performance of the TMUX7209 when a  analog signal is present and VDD is unpowered as this operation is  not rated in the datasheet. However I can provide you with an FAQ that talks about what happens when devices that don't have power off protection like the TMUX7209 experience power off events like the one you are describing in your application. 

    Basically if the current through the device is limited with a resistor on the input pin that will experience voltage before the device is powered up, then the device could survive your fault scenario.(The  Ik spec on the tmux7209 datasheet is the important spec you would need to consider in your scenario)

    Another option is to use the TMUX7412F  where you tie the drain pins together and create a 4:1 1 channel mux. If you do this with 2x of the TMUX7412F devices , you can get the same functionality  in your design as a 4:1 2channel mux like the (TMUX7209).

    Please let me know what you think and I am happy to help.

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    TI__Mastermind 24440 points

    Hello Tyler ,

    Any updates on this!

  • +1 in reply to Kameron Hill
    Prodigy 20 points

    Thank you for the information.  I think due to our size constraints and since we need a low input impedance into the mux that the best solution will be to stick with the TMUX7209 and change our assembly order to prevent voltage at the inputs prior to applying power to the mux.