This pin can be noise-sensitive to capacitive coupling from the high dV/dt switch nodes, or the flux coupling from magnetic components and high di/dt switching loops.
- Minimize the loop area for the PCB traces from the opto-coupler to the FB pin in order to avoid the possible flux coupling effect. Run the opto-coupler emitter return track from AGND at the IC in parallel with the FB to collector path, to minimize loop-area.
- Keep PCB traces away from the high dv/dt signals, such as the switch node of the converter (VSW), the auxiliary winding voltage (VAUX), and the SWS-pin voltage (VSWS). If possible, it is recommended to provide shielding for the FB trace with ground planes.
- The filter capacitor between FB pin and REF pin (CFB) needs to be as close to the two IC pins as possible.
- The current-limiting resistor of FB pin (RFB) should be as close to the FB pin as possible to enhance the noise rejection of nearby capacitively-coupled noise sources.