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Maximum value for R_load to use with XTR108

Juliana Campos Silva
Prodigy 180 points
Other Parts Discussed in Thread: XTR108

Hello, 

I would like to know what is the maximum value for the R_LOAD connected to the current loop of XTR108. I didn't find any mention of it in the datasheet.

Thanks in advance.

 

Juliana

  • 0
    TI__Genius 12325 points

    Hi Juliana,

    The maximum Rload is determined by the power supply voltage and MOSFET used. Because there is a voltage drop across every part of the current loop the power supply must be of adequate voltage to allow 20mA to flow through the load resistor and the MOSFETs Rds(on). Have you selected a power supply voltage in your system?

  • 0 in reply to John Caldwell
    Prodigy 180 points

    Hi, 

     

    I've already selected my power supply, that is equal to 24V. So, how can I calculate correctly this value ot the maximum Rload?

    Thanks

  • 0 in reply to Juliana Campos Silva
    TI__Guru 62975 points

    Hi Juliana,

    The minimum supply voltage, VSmin, will have to be greater than the max current, Imax, multiplied by the load resistance, Rload. 
    Therefore:

     VSmin = Imax * Rload.

    There is an internal 51Ohm resistance in series with the output current of the XTR108 along with the internal or external MOSFET which will have a non-zero Drain-to-Source resistance, RDSon.  Also, any cable in the circuitry will have a non-zero resistance, Rcable.  These three resistances will contribute to the max Rload and should not be ignored. 

    Therefore If you desire a max output current of 20mA with a minimum supply voltage of 24V, plugging into the equation above yields:
    24V = 0.020A * (Rload+Rcable+RDSon+51Ohms). 

    Solving for Rload yields:  Rload = (24V/0.02A) - (RDSon + Rcable + 51Ohms)  =  1.2kOhms - (RDSon + Rcable + 51Ohms)

    Although this is the max ideal value for Rload, a safe design will provide some margin away from this value to compensate for other resistances in the design as well as resistance changes due to temperature, aging, and other factors.

    Best Regards,
    Collin Wells
    Linear Applications