The RC filter comprising RF and CF and connected between the sense resistor and the FB pin is used to generate a pole for loop stability purposes. Moving this pole can adjust loop bandwidth. The suggested frequency of the pole is 2 kHz in analog dimming mode and 4 kHz in PWM dimming mode. Use Equation 13 to choose RF and CF. Due to the dc offset current of the internal amplifier, the suggested value of RF is less than 1 kΩ to minimize the effect on LED current-regulation accuracy.
Analog dimming mode is implemented in this design. Choose the pole at around 2 kHz, with 910 Ω as the filter resistor; then the calculated filter capacitance is 87 nF. An 82 nF capacitor is chosen for this filter.