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TS5A3359: Over voltage when the multiplexer is powered off

newer_bee
Prodigy 30 points
Part Number: TS5A3359
Other Parts Discussed in Thread: TIDA-00879, , TIDA-01012

Hi.

I am inspired by the AFE design in TIDA-01012 and TIDA-00879 where the TS5A3359 multiplexer is used for resistive voltage divider network selection. I am designing a similar scenario where I want to measure three different sensors (A,B,C) as shown in the sketch below. I am planning to use a design similar to the one on the left. The voltages at A,B and C can be as high as +-100V peak with respect to COM (the yellow point).

The reference Design TIDA-01012 has a maximum input of 50V but due to the voltage divider I am guessing that 100 V is also possible in this design. My qestion is: what happens when  TS5A3359 is powered off (Vcc=0) but the input voltages (A,B, C) are still available. Does it create any damage to the multiplexer? Also do you see any problem when the mux is connected on the high side as in the sketch on the right?

The reason I want to use a mux is that the sensors (A, B and C) are also in a resistive network between eachother and I dont want to introduce parallel paths from the measuring circuit .

  • 0
    Guru 243120 points

    Footnote (4) of the absolute maximum ratings says:

    The input and output voltage ratings may be exceeded if the input and output clamp-current ratings are observed.

    So with the 10 MΩ resistors in the left circuit, the current would exceed 50 mA at ±500 kV.

    The right-side circuit has no current limit.

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Thank you Clemens,

    Out of curiosity, why does the right side circuit have no limit?

  • 0 in reply to newer_bee
    Guru 243120 points

    There are parasitic and ESD protection diodes between the I/Os and VCC/GND. If the I/O voltage is higher than VCC or lower than GND, a current flows through these diodes and can burn them out if it is too high.

    In the right-side circuit, there is no resistor in the path between the high voltage and VCC/GND.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3820 points

    Hi newwe_bee,

    in both cases you'll have 100V over the Switch while its off. As Clemens mentioned this will damage the ESDs between I/O and Vcc/GND. 
    It might also damage the MOSFET structure itself.

    If you want to measure such high voltages you can look into our Appnote on Current Shunt and Voltage measurement: http://www.ti.com/lit/ug/tidua81/tidua81.pdf

    Best regards,

    Ambroise

  • 0 in reply to Ambroise Suter
    Prodigy 30 points

    Ambroise,

    thank you for the reference and sorry for the late reply.

    I am a little confused as you said both setups are not allowed. the circuit on the left is very similar to the application in TIDA-01012 which has an input voltage of 50V.  I only changed the resistor values slightly and the high voltage Will appear at the terminal board (A to COM or B to COM or C to COM). 

    As Clemens pointed out, the current will be very very low in this setup, so is this still a problem and will cause damage to the switch? 

  • 0 in reply to newer_bee
    Guru 243120 points

    ±100 V in the off state is not a problem if a current flows (through the ESD diodes) because most of the voltage will be dropped over the resistors.

    +100 V in the off state is a problem only if there are no ESD diodes to VCC, which would be the case for devices that have overvoltage-tolerant pins. The TS5A3359 does not have this feature.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3820 points

    Hi newer_bee,

    Indeed the first solution you would only have very little voltage over the MUX.

    For the second solution you'd have to test it as the part is rated at an absolute max of -0.5V to Vcc+0.5V.

    Regards,

    Ambroise