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Question on UCC38500 Datasheet

Alberto Jadan
Prodigy 20 points
Other Parts Discussed in Thread: UCC38500

Hi,

I need help about an equation found in the datasheet for the UCC38500,  (SLUS419C-August 1999 -REVISED November 2001).

The equation # is 47, and it is used to calculate the value of R35.

R35 = R31 * 10^(-Gco(s) + Gopto(s) + H(s)))

I plugged the values and there is no way I get 38.2KOhms for R35. 

Could somebody please explain me how the document get 18.2 KOhms for R35?

Also, is there a excel file/document that helps with the calculations?

 

Thank you. 

  • 0
    TI__Guru* 86286 points

    I reviewed the calculation and believe it is in error.  You need to convert the dB gain back to standard absolute gain.   Also H(S) should not have an effect on the loop gain. The equation should be as follows.  R35=R31*10^(- ((Gco(S)dB+Gopto(S)dB)/20).  I have also attached a marked up equation 47 as well. I will work on getting the data sheet corrected.

    This device does not have an excel design calculator.  It is an older device and at this time there are no plans to create one.

    Regards

     

     

    Corrected Equation 47.pdf
  • 0 in reply to Mike O'
    Prodigy 20 points

    Hi Mike,

    Thank you for the reply...

    I used your formula, but I still do not get the value of 18.2 KOhms for R35.  What values are you inputting in the formula?

    Thank you.