In particular embodiments, a corresponding steady state holding component of constant time generation circuit 504 can be selected when the LED load varies. Therefore, the steady state signal of the current LED load can be directly employed by the control loop in order to control the system to rapidly recover (e.g., in a few switching periods) to the steady state rapidly. In addition, switching decoder 505 (see, e.g., FIG. 9) coupled to constant time generation circuit 504 may be employed to select the steady state signal of the current LED load. For example, the value of a reference voltage signal of constant time generation circuit 504 can be determined in accordance with the PWM dimming signals (e.g., PWM1-PWM3).

Switching decoder 505 can generate a selection signal in accordance with PWM dimming signals PWM1-PWM3, which can be provided to constant time generation circuit 504. Corresponding switches coupled in series with one of the voltage sources may be turned on by the selection signal control in order to select the corresponding voltage source. Thus, the off time of constant time generation circuit 504 can be determined in accordance with variable states of the LED load. The relationship of the input signals and the output signals of switching decoder 505 may be as shown below in Table 2. For example, PWM1-PWM3 can indicate the three PWM dimming signals, S1-S7 can indicate the switch controlling signals of constant time generation circuit 504, and S8 may indicate the switch coupled between the output terminal of transconductance amplifier 501 and the inverting input terminal of comparator 502. Also, ON can indicate that the PWM dimming signal is high, and OFF may indicate that the PWM dimming signal is low.