• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 511 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM21212 Output Ripple

Scott McElroy1
Intellectual 2155 points

A design in webbench with the LM21212 was created and the output ripple was narrowed down to meet a 1% ripple value. My question is, Is there more to output accuracy then just the peak to peak ripple, are there other consideration in the design? How accurate does the feedback pin need to be to maintain the 1% ripple, or does that need to be considered? The main concern for this design is the output accuracy. Any help would be greatly considered.

  • 0
    TI__Intellectual 2930 points

    Hi Scott,

    Yes, there is much more to accuracy than just the ripple.
    Accuracy will be mainly determined by the IC FB tolerance and that of the Feedback Resistors. Ripple won't change the average output voltage, but if you have to stay inside of a margin it must be designed to do so. A 1% spec is hard to design for since you must take all of these into consideration. The LM21212 has a 1% FB tolerance as is, which doesn't leave you any room for your other tolerances. To truly design for a 1% spec you need an IC with better than 1% tolerance, Feedback resistors with better than 1% tolerance (unless your output is the IC reference voltage), a ripple voltage much less than 1% of the Output voltage and if there are line and load transients they might need to be considered as well.The needs will depend on the details of the specification.

    Hope this was helpful
    -Tommy

  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    TI__Intellectual 2155 points

    Thank you for the answer, "I guess the real question would be Is the LM21212-2EVM have a 1% accuracy, and if not is there a way to make it 1% by maybe change resistors or capacitors on the output?"

  • 0 in reply to Scott McElroy1
    TI__Intellectual 2930 points

    Hi Scott,
    Statistically, I think not. Since the LM21212 is already 1% any other tolerances can only make this worse. Though you could be close... I get around an RMS tolerance of 1.1% with the 1% resistors, but a worst case of 1.8%, you also need to factor in the ripple but this value you could change with the output cap.

    You could build one and measure it though and it could be 1% accurate, but that won't guarantee the next is not 1.4% off...

    -Tommy

  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    TI__Intellectual 2155 points

    This is a significant help, I greatly appreciate it. Thank you Tommy.