The present application provides a control device for controlling a switching converting module to generate an output signal of constant output current from a rectified AC power supply signal. The switching converting module comprises a primary winding for receiving the rectified AC power supply signal and a power switch coupled in series with the primary winding. The control device comprises: an AGC amplifier configured to receive a first sampling signal sampled from the rectified AC power supply signal, and to generate a reference signal of constant amplitude according to the first sampling signal; a signal comparator configured to compare the reference signal with a current sensing signal in proportion to a primary side current flowing through the primary side winding to generate a comparison signal; a signal generator configured to receive a second sampling signal sampled from the output signal, and to generate a clock signal according to the second sampling signal; and a control logic configured to generate a control signal for switching the power switch according to the comparison signal and the clock signal.