This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMUX7219: is it useful for protecting the DAC8760?

Part Number: TMUX7219
Other Parts Discussed in Thread: DAC8760,

Hi guys.

I am looking for a solution for protecting the voltage and current output of the DAC8760.

You can see the thread about DAC8760 trouble here.

The problem is that the customer does not want to apply the connector at our sensor's cable. Therefore it is possible to mistakenly connect the cable.

The cable has three terminal: positive power supply +VBATT (from 10V to 32V), GND and DAC8760 voltage&current output.

The board is protected from power supply reverse polarity by the series diode apply on +VBATT.

Now the problem is: how can I protect the DAC output from customer miswiring in all possibile combinations?

I think is a good idea to introduce an analog switch after the DAC output (TMUX7219). I would connect 0-10V out to S1, 4-20mA out to S2 and D to the board output. So I place the TVS to the board output to protect the TMUX7219 from the ESD events.

I can supply TMUX7219 with +5V internally generated from buck converter or directly from +VBATT supply.

My question is: what happens if the customer apply, for example, +VBATT to the board output (pin D of TMUX7219) and, for example, output cable to +VBATT pin board? And in the other cases?

What happens if the TMUX7219 is not powered and, for example, a voltage is applied to the output? Is it possible the voltage pass through the analog switch and is applied to the DAC8760 output.

BR,

Gabriele.

  • Hi Gabriele,

    A few things to note on your application for the TMUX7219:

    1) Need to make sure your inputs/outputs (S1, S2, D1) do not exceed your VDD/VSS voltage. You mentioned you may be powering the TMUX7219 with a 5V supply, so this needs to be taken into account that if your inputs go above supply, you most likely will cause permanent damage to this device. To avoid this, you could also put in current limiting resistors to mitigate this if a possibility. Powering from VBATT will be your best bet.

    2) If the device is not powered on, there is a possibility of back powering (basically, the input can be passed to the VDD rail, which in turn could power on the logic and close the switch). We do have muxes with a feature called powered off protection that will prevent this, however, not at this input voltage range unfortunately.

    3) My question is: what happens if the customer apply, for example, +VBATT to the board output (pin D of TMUX7219) and, for example, output cable to +VBATT pin board? And in the other cases?

    I am a bit confused on what you are asking here, so you have VBATT at the D pin of the 7219...and what exactly about this output cable? Not sure where this is connected or how it is incorporated. If you could provide a visual/schematic, could help clarify what you mean.

    Thanks,

    Bryan

  • Hi Bryan and thanks for your answer.

    I am attaching an image that describes the block diagram of my system.

    As you can see the cable pinout have three position (+VBATT, GND and OUT).

    The customer can do a miswiring because there isn't a connector on the cable. So he can place, for example, +VBATT instead of OUT pin; GND instead of OUT; +VBATT instead of OUT, etc.

    I would to protect the DAC8760 ouputs (Iout 4-20mA and Vout 0-10V).

    I thought the TMUX7219 could do the protection I need.

    One problem is: what happens when TMUX7219 is unpowered because +VBATT ad OUT pins are reversed?

    And what happens in the other cases?

    BR,

    Gabriele.

  • Hi Gabriele,

    Thanks for providing this block diagram. Definitely helps clear things up!

    See table below for possible scenarios:

    Combinations Result
    VCC D GND
    VBATT OUT GND Operates without issue
    VBATT GND OUT GND pin/circuit is now floating or at the potential the OUT cable is connected to. This could be a problem depending what is on that side of the OUT cable. Remainder of circuit is OK.
    OUT VBATT GND VCC is now floating or at the potential the OUT cable is at. VBATT is now attached to the input and most likely at a higher potential than VCC, thus turning on the ESD protection diode and possibly back powering device
    OUT GND VBATT VCC is now floating or at the potential the OUT cable is at. VBATT is now attached to the GND and the input most likely has a lower potential than VBATT, thus turning on the ESD protection diode and causing current to flow, possibly damaging the device if no current limiting resistor is in place.
    GND VBATT OUT VCC is now grounded. GND is floating or at potential of OUT cable. Input is at VBATT which will cause the ESD diode to turn on and possibly back power the device.
    GND OUT VBATT VCC is now grounded. GND is at VBATT and input is floating or at potential of OUT cable. S pin voltage will be lower than GND, thus turning on ESD protection diode and allowing current to flow. Could cause damage if current is too high. Need current limiting resistor.


    Essentially, your biggest concern will be the ESD diode conduction when the input pin from the S pin (0-10V) or the D pin (10-32V or 0V) is biased incorrectly. To mitigate any possible damage, I would suggest putting a current limiting resistor in series with your inputs such that the maximum input current to the ESD diodes will be the maximum current (80mA at max temp) divided by a factor of 3 (so limit to around 26.5 mA per input/output).

    In addition, to prevent overvoltage being seen at the input to the DAC in the case of a back powering situation, a zener/TVS diode would also be needed to clamp the voltage to a level that would not damage the DAC input.

    Hope this helps!

    Thanks!

    Bryan