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Original question:

LM3481: The flyback circuit for LM3481

LM3481: LM3481 comp circuit culculation

meng xu
Prodigy 40 points
Part Number: LM3481

Hi 

  I am looking at this design guide outlining how to design the LM3481 as a flyback.

4.4.3  fc=3kHz   Figure 11 shows that the current loop DC gain at 3 kHz is about 16.2 dB 

          The Mid-frequency gain of H EA (s) is K opto × R up × R1/(R opto × Rfb1), so R1 could be calculated  

        my questions:

                   In any case  fc=3kHz?  

                   Please check  when fc=3kHz DC gain is 16.2DB?

4.4.4   f zero = 400 Hz   C1 and C2 could be calculated as   shown in Equation 29        

        my questions:

                     how to set  fzero?

                     There is no  C1 C2  in the Equation 29    

                     in the application C1=C2=1/2πR1   where is the fzero  

                     C1=C2=1/2πR1   R1=374     the resolution is not 1uF 

       Your reply would be appreciated.

      Best regards,

       Thank you 

    

  • 0
    TI__Guru* 78400 points
    Hi Meng,

    I think I have answered most of your questions in your other thread. Here I just answer your questions about Equation (29) and C1 and C2.

    Equation (29) is used to calculate R1 for the gain compensation need such that the total loop gain curve will be unity (1, or 0dB) at fc. Once R1 is determined, you need to choose C1 and C2 to place your zeros.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 40 points
    Hi :

    Thanks for your reply very much.

    Is this design uses the "single pole compensation"?
    For my design ,do i need to clculate Mid-frequency gain of H EA (s) or find it in Figure 11 (SNVA761)?

    Dose the fc have anything to do with freq(130kHz in SNVA761)?

    The last question: If I use FB Pin how do I calculate R or C for the COMP pin ?



    Thanks and best regards.
  • 0 in reply to Youhao Xi
    Prodigy 40 points
    Hi :

    Thanks for your reply very much.

    Is this design uses the "single pole compensation"?
    For my design ,do i need to clculate Mid-frequency gain of H EA (s) or find it in Figure 11 (SNVA761)?

    Dose the fc have anything to do with freq(130kHz in SNVA761)?

    The last question: If I use FB Pin how do I calculate R or C for the COMP pin ?



    Thanks and best regards.
  • 0 in reply to meng xu
    TI__Guru* 78400 points
    Hi Meng,

    A proper design is a results of many tradeoffs. Your questions are touch the fundamentals which cannot be easily answered here. I recommend you to read the article at the following link, in which the author addresses most design considerations for loop control of various switch mode power supplies.

    www.ti.com/.../slup098.pdf

    Hope this helps.

    Thanks,
    Youhao