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TINA/Spice/LM3481: LM3481 Flyback How to calculate the comp circuit

meng xu
Prodigy 40 points
Part Number: LM3481
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hi :

  I am using lm3481 to cret a circuit  input 6-16V  output1 15V 1A output  2 5V 1A 

  I am tyring follow the compensation approach that is suggested in the application note.   SNVA761A

 4.4.3 step 3:culculate R1 R2    

 my questions: 

   Why set the fc=3kHz

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

4.4.4 culculate C1 C2 

 Why set fzero=400Hz 

 Thank you 

B&R

 

  • 0
    TI__Guru* 76005 points
    Hi Meng,

    The author of the app note has other considerations which are not explicitly expressed. The flyback has right half plane zero pole Frhpz, which is the lowest at min Vin, and you usually place your fc not higher than 3xFrhpz. This example the Frhpz can be as low as ~10kHz, so the 3kHz fc is not low.

    Regarding the placement of Fzero, you can place it at the power plant pole (100Hz in this example), or like the author at 400Hz while is well below the fc to be able to boost the phase. I believe the author try to limit C1/C2 not greater than 1uF.

    Please note that a proper design is always the results of tradeoffs between various factors.

    Thanks,
    Youhao Xi, Applications Engineering