UCC28C53-Q1: Webench design query

Hi,

D21 is to help soft start. Initially C21 has zero voltage, so D21 will clamp the opto diode to sink max current then opto emitter will source max current to pull up FB to as high as possible so to make COMP as low as possible which will limit the duty cycle to the minimum. Then as C21 gradually charging up, the duty cycle increases from min to its target. This helps soft start.