Referring now to FIG. 4, shown is a schematic block diagram of an example constant current LED controlling circuit employing a constant off time control mode. Constant current LED controlling circuit 300 can include transconductance amplifier 301 and corresponding compensation circuit, comparator 302, RS flip-flop 303, and constant time generation circuit 304. An inverting input terminal of transconductance amplifier 301 can connect to a current sensing circuit (e.g., including a sensing resistor), and may receive current feedback signal VFB. A non-inverting input terminal of transconductance amplifier 301 can receive reference voltage Vref. A current error signal can be generated at an output of transconductance amplifier 301.

Capacitor C21 and resistor R21 can connect in series between the output of transconductance amplifier 301 and ground to form the compensation circuit. In other cases, the compensation circuit may only include capacitors. The current error signal may be compensated by the compensation circuit in order to generate the compensation signal. A non-inverting input terminal of comparator 302 can receive current feedback signal VFB, and an inverting input terminal may receive the compensation signal. An output of comparator 302 can connect to a reset terminal of RS flip-flop 303, and a set terminal of RS flip-flop 303 can connect to constant time generation circuit 304 to receive a timing signal with a fixed time interval. An output terminal of RS flip-flop 303 can connect to a power switching transistor, and may provide control signal Gate to the power switching transistor.