TPS1663: Rlim selection

Part Number: TPS1663

Tool/software:

Hi TI Team,

i am want to use tps1663 to protect my 24v Flyback converter.

looking at the DS to calculate Rlim i noticed that the equation been used is i_lim=18/R_lim

my question is - where did the number 18 come from? 

looking at the block diagram there is no such internal voltage...