Simple, open loop driver board for the 12VDC to 220-380VDC Converter project.

By connecting the COMP (9) and IN- (1) pins together, the error amplifier is configured as a voltage follower.

According to the SG2525A datasheet (Electrical Characteristics, PWM Comparator):

• A typical input threshold of 0.9 volts (min 0.7 volts) will provide a minimum duty cycle of 0 per cent.
• A typical input threshold of 3.3 volts (max 3.6 volts) will provide a maximum duty cycle of 49 per cent.

Hence multi-turn trimpot R12, combined with R1 and R3 is set up to provide an adjustable voltage between 0.7 and 3.6 volts to adjust the output PWM duty cycle.