UCC2818: designing a PKLIMIT peripheral

Hello Nishie-san, 

Thank you for bringing the SLUA321 app-note to my attention.  

I don't know of any documentation for formulas and selection criteria to design the PKLMT peripheral function, and I don't believe that there is any documentation.
Instead, I think the author expects that designers can develop the circuit by inspection and testing.  The timing is not critical, so a crude estimate should be sufficient. 
Here is a sample statement from the app-note about timing: 

"The selection of C100 and R100 should be such that the time constant is less than any soft start circuits on either the PFC or the down stream converter."

In other words, the time constant (C100 x R100) should be fast enough that PKLMT does not interfere with the soft-start timing on the SS pin or on the DC/DC converter soft-start.
In the app-note, this time constant = 1uF x 15kR = 15ms.  PKLMT threshold will be normal after about 1 time constant.
The small-signal PNP transistor should have high hfe (beta) so that R100 current dominates the charging of C100, with very little contribution from PNP base current. 
System evaluation testing can determine if this R-C time constant must be increased or decreased to optimize its performance in the system. 

 Regards,
Ulrich