I believe the Rbst resistor is there to limit the inrush current into the Cbst capacitor as it charges. It’s good practice to put a placeholder resistor next to the Cbst, even if you plan to place a 0 Ohm resistor during your build.
That way, you can stuff a Rbst if you need to slow down the ramp as Cbst charges.
Thank you for your answer so quickly.
I will suggest customer to use Rbst=22ohm to limit in-rush current
, Cbst=1uF for proper operation when duty is so low, and have a placeholder at least
for Rbst.