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Proper use of XTR115 - contribution of Vreg and Vref currents in loop current

Art Goldstein
Prodigy 170 points
Other Parts Discussed in Thread: XTR115

We are planning on using the XTR115 as a 4-20 ma loop controller.

To get to my question, let me first quote the section entitled " Minimum-scale current" (page 6 of datasheet.)

" The quiescent current of the XTR115/116 (typically 200 uA) is the lower limit of its output current. Zero input current (Iin = 0) will produce an Io equal to the quiescent current. Output current will not begin to increase until Iin > Iq/100. Current drawn from Vref or Vreg will add to this minimum output current. This means that more than 3.7 mA is available to power external circuitry while still allowing the output current to go below 4 mA"

As I read this - with reference to the bold, italic  selection - it seems like the maximun current from Vreg would be 4 mA if Vref is unused, and the quiescent current is neglected. In other word, the current used by external devices is tucked into the hole between 0 mA and 4 mA in a 4-20 mA loop.

Is this a correct reading?

If this is a correct reading, then the question arises as to how the loop current is affected by the dynamic current variations of current sourced via Vreg.

Any insight and clarification would be very welcome.

 

 

 

  • 0
    Prodigy 170 points

    Since there has not been any response, perhaps my question is unclear, so I'll state it another way.

    Suppose there is a 1K resistor from Vreg to Iret such that there is a current of 5 mA through the resistor. Will this current flow through R2 to Vloop battery and in so doing lead to Io current rising to 5 mA?

     

     

  • 0 in reply to Art Goldstein
    Prodigy 170 points

    I am not sure why this question is posing such difficulty.

    Let me try it this way:

    Iq = 200uA
    Iin = 3 uA

    Vreg current alternates between 1 ma and 2 ma.

    Now, with these conditions, what does Io do?

    Is there anyone reading this who has used this part and can answer it? I am looking for the big picture answer here.

  • 0 in reply to Art Goldstein
    TI__Mastermind 27715 points

    Art,

    I'm sorry that I didn't see your question posted here initially - I wasn't subscribed to the /etc. forum. I will re-post my answer from the internal forum here:

    • Any current drawn by input circuitry from Vreg and Vref is returned to the Iret pin, and is part of the total loop current. The total loop current cannot be less than the sum of the XTR115 quiescent current and the current drawn from Vreg and Vref.
    • The XTR115 regulates the loop current despite changes in input circuit current. It is able to do this through a feedback loop which compares the return current to the input current at pin 2, injecting additional current from the external transistor circuit as needed. Keep in mind, the part cannot remove current from the loop, hence the condition that Iq + Ireg (max) + Iref (max) must be less than whatever your desired minimum-scale output current is.
    • For purposes of illustration, let's assume that Iin = 40uA, Ireg = 1mA, and Iref = 0mA. The output current will be 4mA, the majority of which will be provided by the external transistor Q1. If Ireg changes to 2mA, the output current will still be 4mA, the only change being that the loop will adjust to provide 1mA less from Q1.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    High Performance Linear

  • 0 in reply to Ian Williams
    Prodigy 170 points

    Hi Ian,

    I want to thank you for the time you spent with me and giving me a very concise and crisp answer. I now understand what is going on and can use the part with confidence. It needs to be stated, as well, that TI's support ecosystem is extraordinarily broad and deep and serves its customers well. Thank you.

  • 0 in reply to Art Goldstein
    TI__Mastermind 27715 points

    Art,

    I am happy to have been able to help you achieve a better understanding of the XTR115 (and to a degree, two-wire current transmitters in general). I also thank you sincerely for the kind compliments about TI's customer support structure. If you run into any problems while implementing your 4-20mA design, I am available to help further.

    Best regards,

    Ian Williams