The FEEDBACK pin is used to sense output and input voltage by sensing the auxiliary winding voltage. During the primary Switch on-time, the current out of the FEEDBACK pin is sensed to detect the line voltage. During the secondary rectifier conduction time, the feedback voltage is proportional to the output voltage via the turns ratio between the bias and secondary windings.
The user can change the value of the upper feedback resistor or the under-voltage turn-on threshold. Changing the upper feedback resistor changes the under-voltage/overvoltage set-point. PIExpert will update the value of the lower feedback resistor to maintain the correct output voltage. Changing the value of the under-voltage threshold will force PIExpert to recalculate the appropriate values of the feedback resistors.
A 10pF feedback pin decoupling capacitor (CFB) is recommended to ensure proper regulation. The user is allowed to change the capacitor value. Too low a capacitor might not provide enough decoupling, while higher capacitances may result in regulation and transient response issues. The design should be validated on the bench.