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SN74CBT3253: Questions for EOS solution on Multiplexer, TVS and ADC

Bobby Kim
Prodigy 86 points
Part Number: SN74CBT3253
Other Parts Discussed in Thread: ADS1248, TMUX4052, TMUX6209

Tool/software:

We are using Multiplexer() and ADC() protected by TVS diode to monitor 6ea 3-Wire RTD(PT100). However when 24V event occur, the TVS and multiplexer get damaged as below picture. so i'm looking for an appropriate solutions to deal with this issue. i considered using resistor in the range of 300~600ohm, but due to PCB size concentrations, this is difficult to implement. i have been thinking of using Poly Switch as an alternative. What do you think about that approach? if possible, what should i consider? 

(a picture of multiplexer with damaged)

(damage path of the TVS and multiplexer) 

(a solution for Poly switch)

(Specifications for Poly switch

(TVS damage Waveform when 32V EOS is applied to the port directly connected to ADC) 

1. 32V EOS

2. pin 14 on ADS1248

3. 5V(Power) Zener

4. Current

  • 0
    TI__Mastermind 35618 points

    Hey Bobby, 

    Well on the mux side, it simply won't be able to handle the voltage. You could select a mux that support higher voltage like TMUX4052 (up to 24V) or TMUX6209 (up to 36V but would require a 36V supply. 

    You could also select a protection device that would support your use case better. I'm going to loop in our protection devices team to see if they have thoughts on this.

    Thanks,
    Rami

  • 0
    Guru 317150 points

    TVS diodes are designed to handle transient (i.e., short) voltage spikes. I suspect that applying EOS for several seconds burnt them out, they failed open, and then the mux and ADC were no longer protected.

    In theory, you could use devices that can handle a larger power dissipation, but they would need to be quite large, and  you say that there is not enough space.

    So the only way is to reduce the current. I doubt that these poly switches are fast enough, and a resistor in the same package should be able to dissipate the same amount of power. Why do you think that 0603 resistor would not work?

  • 0 in reply to Clemens Ladisch
    Prodigy 86 points

    Why do you think that 0603 resistor would not work?

    → Thanks, your reply. The 0603 resistor(or Small size resistor) is 1/20W in mm size as below picture and a 24V input require a resistor rated at 1.08W. isn't it? Using a 1W resistor in my board seems excessive. Additionally, the waveform measured at 300ohm is shown below. Is it possible to use a Poly switch in an application with RTD using ADS1248? I would appreciate your opinion on temperature(Resistance) changes with respect to the current.

    (Waveform at 300ohm)

    1. ADC differential input : RTD resistor(A)

    2. ADC differential input : RTD resistor(B)

    (Waveform at don't have resistor)

  • 0 in reply to Bobby Kim
    Guru 317150 points

    0603 (1608 metric) resistors are available in 1/3 W, and Vishay RCP can do 1.5 W.

    A three-wire measurement requires the three resistors match. If the resistors are not accurate enough for you, you would need a four-wire circuit.