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LM5166: Minimum size for feedback resistors

David Dean
Expert 2315 points
Part Number: LM5166

Is there a minimum size for the feedback resistors?  Datasheet section 7.3.4 recommends the range of 100KΩ to 1MΩ and warns against larger values, but does not mention smaller values.  I ask because I need to handle some reverse current from a diode and I would typically do this with a resistor from the anode to ground.  However, since this resistor will be in parallel with the feedback resistors, perhaps I could simply use smaller resistors there and allow them to handle the reverse current.  My reverse-current-handling resistor is 1KΩ, so the feedback resistors would be in the range of 250Ω - 750Ω.  Do I risk damage to the FB pin with such small resistors?

  • 0
    TI__Guru 72425 points

    Hello

    There is not necessarily a minimum value.

    It is just a matter of how much current drain you want in the divider.

    Of course at some point self heating may effect your accuracy.

    Is the diode on the FB pin for protection?

    You will need to be careful about the capacitance of the diode causing a pole in the FB loop.

    However, with low values of resistor that is mitigated somewhat.

    You might look at lower leakage current alternatives such as the C-B junction of an NPN.

    Thanks

  • 0 in reply to Frank De Stasi
    Expert 2315 points

    The diode is unrelated to the FB pin - the regulator charges a supercapacitor and I don't want that voltage coming back through the regulator when the input power source is removed.  I want to provide a path for that reverse current.  You are correct that there are options with less reverse current that would allow for a larger resistor.

    I will aim for using a third resistor to handle the reverse current, it seems like the "correct" solution.  But good to know that this would not cause a problem with the LM5166 if I wanted to do it with the feedback resistors.

  • 0 in reply to David Dean
    TI__Guru 72425 points

    Hello

    OK; glad we could help

    Thanks