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LM5166: Able to used fixed-voltage versions as adjustable regulator?

David Dean
Expert 2305 points
Part Number: LM5166

I am using the LM5166 (adjustable version) to generate a 6.5V output.  Would it be possible to use either 3.3V or 5V fixed versions to generate the 6.5V output?  Basically, can I simply recalculate the feedback resistors as if the FB node is 3.3V or 5V instead of 1.223V?  Or is there something "special" about the fixed versions that require them to be tied directly to Vout, or can only generate their fixed voltage, or anything else that would ruin the ability to use these as a 6.5V regulator?

  • 0
    TI__Mastermind 30660 points

    Hello David,

    The fixed output devices have an internal voltage divider, that would be the major difference for COT based devices.

    In practice, I could see it being done, but I would not recommend as you are now introducing additional error to output thru the voltage divider, as well, it will convolute the ripple injection design.

  • 0 in reply to Marshall_Beck
    Expert 2305 points

    Thanks Marshall.  I am using the LM5166 is PFM mode, so ripple should not be a factor (correct?).  I also do not require a highly accurate output - a little additional error won't be an issue.

    Considering these two points, would you consider it safe to do this in my particular application?

  • 0 in reply to David Dean
    TI__Mastermind 30660 points

    Hello David,

    I can not think of any additional issues that would arise.

    PFM behavior is dictate by FB ripple. In this case, FB (internal) ripple. You are now additional additional gain to the 10mV typical ripple.

    I would ensure that is acceptable per your application requirements.

    Nevertheless, this is not standard application circuit for this product, so I would ensure you perform appropriate product validation with this circuit.

    If you have circuit results that you would like analyzed, feel free to post back in a new thread.

  • 0 in reply to Marshall_Beck
    Expert 2305 points

    Do you think there are any risks of instability or damage to the regulator?  My application will be ok with additional ripple or inaccuracy.

    In terms of how the fixed version is made, is it identical to the adjustable version but with an internal resistor ladder?  From the Vout input current listed in the datasheet I can determine the internal resistor values, and then the affect of an external ladder to increase the output to 6.5V.

  • 0 in reply to David Dean
    TI__Mastermind 30660 points

    Hello David, I do not. With COT, you ensuring appropriate ripple amplitude and phase and you have a stable converter.

    Besides, integrated divider, the devices are identical.