Choose resistors R1 and R2 to set the output voltage within a range of 0.6V to 4V, according to Equation 3. To keep the feedback (FB) net robust from noise, set R2 equal to or lower than 100 kΩ to have at least 0.6 µA of current in the voltage divider. Lower values of FB resistors achieve better noise immunity, and lower light load efficiency, as explained in the Design Considerations For A Resistive Feedback Divider In A DC/DC Converter Analog Design Journal.
For devices with a fixed output voltage, the FB pin must be connected to VOUT. R1, R2, and C4 are not needed. The fixed output voltage devices have an internal feedforward capacitor.