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TPS2661 INA592: Back Powering Issue in 3-Wire Transmitter Design

doudu vamshi
Prodigy 90 points

Part Number: INA592
Other Parts Discussed in Thread: OPA2196, TMUX6219, OPA310

We have recently designed a V-to-I converter for our 3-wire transmitter using the INA592, OPA2196, and TMUX6219.

We are observing a back-powering issue when the loop wire is connected to ground (Source Mode) while the device ground supply is disconnected. Under this condition, the device appears to be powering through an unintended path.

 

The current through the R142 and R59 is almost same as like 4-20mA loop current but the current showing in the ammeter which is connected between Loop wire and Power supply GND is equal to total current consuming by the device.V to I Converter.pdf 

 

I have attached the schematic.

  • 0
    TI__Intellectual 1980 points

    Hi Doudu,

    In order for the device to work as expected, the negative supply needs to stay connected to a potential or else current will run through internal paths as you have demonstrated. We have a series of videos covering electrical overstress in our op amps, and in the video titled "Electrical overstress - introduction," power supply sequencing is covered.

    Usually a TVS diode on the negative supply connected to ground would help this issue, but depending on the way in which the ground is disconnected, this might not be possible.

    Is the condition where the ground supply becomes disconnected required for some safety requirements? Is the ground only being disconnected from the V- pin of the INA592?

    Best Regards,

    Taylor Allan

  • 0 in reply to Taylor Allan
    Prodigy 90 points

    Hi Allan,

    We are removing the Ground Wire for entire device that means total circuit will not have Ground reference.

    Our devices are SIL2 certified. If the Ground is disconnected or open the device should power off or loop current should be less than 1mA.

    one more observation when we disconnect the power supply(+33V) to the entire mA loop generation, Monitoring, Separation still device getting turn on. When we remove U7,U17,U19,U20 then problem is disappearing.

    Best Regards,

    Vamshi.

  • 0 in reply to doudu vamshi
    TI__Intellectual 1980 points

    Hi Doudu,

    Sorry for the delay here, I was out-of-office yesterday. I see, I will have to think about how we could fix this issue, as it would likely require different buffer amps. We do have the OPA310, which does include fail-safe input ESDs that prevent back powering, but this is a low voltage part that wouldn't work with a supply voltage of 33V. 

    I will reach out to see if we have any high voltage parts with the same feature and get back to you.

    Best Regards,

    Taylor Allan

  • 0 in reply to Taylor Allan
    Prodigy 90 points

    Hi Taylor,

    Can i use the TPS26610 in series with Loop with supply voltage less than 30V but loop voltage with 18V-36V as in Datasheet mentioned input can be -50V to +50V.

  • 0 in reply to doudu vamshi
    TI__Mastermind 31090 points

    Hi Doudu,

    I'm going to transfer this thread to the team that supports the TPS26610 so they can verify the device's behavior when you remove the local ground. 

    Best Regards,

    Robert Clifton