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XTR305: External protection diodes

Kuramochi Tadahiko
Guru 28715 points
Part Number: XTR305

Hi,

I have a question about Figure 46. on the datasheet.

It is described that "external protection diodes may be necessary to handle excessive currents" on the datasheet.

Does "external protection diodes" mean above diodes"1N4002" on figure 46?

If yes, how are these diodes handle excessive current?

Best Regards,

Kuramochi

  • 0
    Guru 140200 points

    Hi Kuramochi,

    the external diodes shunt overvoltages reaching the output arround the internal protection diodes of XTR305 directly to the supply rails. Rc limits the current through the internal protection diodes then, according to section 8.2.2.10 of datasheet of XTR305.

    Of course, the 1N4002 can only withstand short lasting overvoltages coming from ESD, Burst, Surge or similar, while C4 helps to slow down the overvoltage glitch. Longer lasting ovevoltages can damage the 1N4002, if there's no added current limiting (fuse, polyfuse, ...). And, there should be a TVS from each supply rail to 0V to prevent the supply voltages of XTR305 from rising up to dangerous values.

    Kai

  • 0 in reply to kai klaas69
    Guru 28715 points

    Kai,

    I had assumed that the load is low impedance.

    In the case, external diode doesn't work because output voltage doesn't hit supply rails, right?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Guru 55466 points

    Hi Kuramochi,

    The maximum current the XTR305 can drive is +/-38.5mA with M2=High, and +/-24mA with M2=Low. 

    In Current output mode, the Load Error flag indicates saturation into the supply rails due to high load resistance or open load.

    You are correct.  The external circuit on Figure 46 is to protect for a fault condition where the XTR305 output is driven above the voltage rails.

    Thank you and Regards,

    Luis

  • 0 in reply to Kuramochi Tadahiko
    Guru 140200 points

    Hi Kuramochi,

    a protection circuit is not just a goal in itself. You always must adapt it to the actual application. So, first you must find out what overvoltage can hit the output at all. Is it a current limited 24V supply of a process control? Or is it "only" ESD? Or is it ESD, Burst and Surge, because you connect a cable to the output? Or is it impossible for the output of XTR305 to get hit by an overvoltage? Usually the CE testing determines what sort of overvoltages the output of XTR305 has to withstand.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 55466 points

    HI Kai,

    Thank you very much for your valuable inputs, much appreciated.

    Thank you and Kind Regards,

    Luis

  • 0 in reply to Luis Chioye
    Guru 140200 points

    Hi Luis,

    thank you for your warm words!

    Kai