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XTR117: Emitter Resistance Recommendation

Garret Holt
Intellectual 1050 points
Part Number: XTR117
Other Parts Discussed in Thread: XTR115

Hello,

We are working with XTR117. I saw on another forum post that RLIM is around 100 Ohm. We are looking to add an additional resistor, R3, as shown in the image to help dissipate some power when the loop voltage is high. Do you have any recommendations for the value of R3? Would you advise us to use R3 or is there an alternative method you would recommend? Thank you!

Regards,


Garret

  • 0
    TI__Mastermind 25503 points

    Hi Garret,

    This e2e post asks a similar question about adding the additional resistor to the XTR115, but the same will apply to the XTR117. Adding the extra resistor will decrease the existing current limit of 32mA. Are you looking to decrease the current limit? Adding too much resistance would decrease the current limit past 20mA, and it seems like it's not a reliable solution to add a fixed external resistance because Rlim is a dynamic resistance.     

    From the linked post: "After looking deeper into the issue however I do not believe this is a very robust solution because Rlim is a dynamic resistance implemented with active circuitry and thus it would be very difficult to hone in on a 25mA limit with an external resistor. You may be able to tune in an appropriate value for this resistor based on your supply voltage and Vbe of the selected transistor, but if you want a more reliable and robust solution for your customer I would suggest implementing an external active current limiting circuit per Collin's recommendation"

    The external active current limiting circuit recommended in that post is found on pages 21 and 22 of this design guide. Current Limiter and Circuitry to Protect Current-Sensing Circuitry.pdf

    A Q1 that can handle the power dissipation should be selected, but I will research if there are any other methods to do what you're asking. 

    Best,

    Katlynne Jones