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LM5155: Are there any maximum +/- limits on bias current into FB pin 8?

Bob Zwicker
Prodigy 210 points
Part Number: LM5155

I am designing my converter around the LM5155.  I would normally expect to see a specification regarding the FB pin input bias current, but I have not found this in the datasheet.  This current level may determine my maximum resistor values for the FB divider.  With 80V maximum output and limited space and input power available, I do not want to burn too much power in my FB divider.  Is there any guidance regarding limits on the FB pin input current, or at least a maximum recommended value for the lower divider resistor?

Thanks.

  • 0
    TI__Guru 61535 points

    Hello Bob,

    For the lower feedback resistor we are using 2 kohm on the EVM, so if possible stay around this value.

    Let me try to get the typical bias current, but this might take some time.

  • 0 in reply to Brigitte
    Prodigy 210 points

    Thank you, Brigitte. 

    In my thinking, this is a fundamental parameter which should be in the datasheet; particularly in a boost (converter or controller).  A boost can produce high voltage, and that means more power in the FB divider for a given divider current.

  • 0 in reply to Bob Zwicker
    TI__Mastermind 21720 points

    Bob,

    Typically the bias current of the FB pin is in the nA range. Compared to the current flowing through the feedback resistors this the current is negligible. 

    Please let me know if you have any questions.

    Thanks

    Garrett 

  • 0 in reply to Garrett Roecker
    Prodigy 210 points

    Thank you Garrett. 

    Anything under 1 uA sounds OK.  I think my question is likely to crop up for any experienced designer using this chip and I feel like it should be in the datasheet.  Of course the design examples imply that a 2K bottom FB resistor is OK, but that means 500 uA divider current.  I can see wanting to go at least 5X or 10X that resistance value in some cases.