Thus, when conduction angle signal Vcd is greater than threshold DT1, switches S3 and S5 can be turned off, switch S4 can be turned on, and reference voltage REF may be switched to reference voltage REF1, which is higher than reference voltage REF2. When conduction angle signal Vcd is less than threshold DT2, switches S4 and S5 can be turned off, switch S3 may be turned on, and reference voltage REF can be switched to reference voltage REF2. With a lower reference voltage, constant current controller 34 can control transistor Q1 to generate lower drive current IQ1, thereby reducing the current ripple. When conduction angle signal Vcd is not greater than threshold DT1 and not less than threshold DT2, reference voltage REF can change along with conduction angle signal Vcd.

Referring now to FIG. 9, shown is a waveform diagram of example operation of the reference voltage adjustment circuit of FIG. 8, in accordance with embodiments of the present invention. In this example operation waveform, reference voltage REF can change with conduction angle signal Vcd in a linear manner. Reference voltage adjustment circuit 32 may define a reference voltage curve. Reference voltage adjustment circuit 32 may adjust the reference voltage based on the defined reference voltage curve and the conduction angle signal Vcd. In this example reference voltage adjustment circuit 32, capacitor CREF can cause reference voltage REF to change smoothly when being switched.