When multiple LED strings are connected in series and only one constant current LED controlling circuit is employed to provide a driving current, the amount/cost of the constant current LED controlling circuit can be decreased. If one constant current LED controlling circuit is employed to control an LED load that includes a plurality of LED strings coupled in series, when every LED string is turned on or off, the load state can be changed. The loop response speed of a typical constant current LED controlling circuit may not meet the load requirements, and possible side effects include constant current control accuracy and efficiency, as well as possible damage the LED strings due to surged driving current. Therefore, such constant current LED controlling circuits may not be suitable to achieve constant current control for an LED load that includes a plurality of LED strings coupled in series.

Referring now to FIG. 2, shown is a schematic block diagram of an example constant current LED controlling circuit employing a hysteresis control mode. Constant current LED controlling circuit 100 can include hysteresis comparator 101 and flip-flop 102. A non-inverting input terminal of hysteresis comparator 101 can connect to an external current sensing circuit (e.g., including a sensing resistor), and may receive current feedback signal VFB. An inverting input terminal of hysteresis comparator 101 can receive reference voltage Vref, and an output terminal can connect to flip-flop 102. An output terminal of flip-flop 102 can connect to a power switching transistor, and may provide a controlling signal (“Gate”) to the power switching transistor.