8.4.4 PWM Dimming-Mode Operation

Once the PWM dimming mode is chosen, the internal voltage reference for the FB pin is fixed at 100 mV. The LED current is on or off corresponding to the PWM state, see Figure 19. Due to the limited control-loop response, to get a relatively linear dimming performance, the suggested PWM signal frequency should be less than 1 kHz.

Figure 19. PWM Dimming Operation

In some application where dimming is not needed, one can just connect a resistor divider from V_VIN to the PWM pin as Figure 20 shows.

Figure 20. Application Without Dimming

R_TOP and R_BOT should be sized to make sure the PWM pin voltage is higher than 1 V when V_VIN reaches its steady voltage. It is best to make sure the PWM pin voltage is less than 2 V, thus one can have 100 mV at the FB pin for better efficiency. Use 10 kΩ as a good starting point for R_BOT, then choose R_TOP according to Equation 2:

Equation 2.