A high precision constant current LED driver based on buck topology was presented. Based on the hysteresis control mode, a new type of adaptive off-time control loop was adopted in this circuit instead of valley detection feedback loop, which indirectly achieved accurate control of the inductor valley current. Therefore, the error caused by direct sampling of the valley current was avoided, and the constant current accuracy of the system was improved. The low-side sampling was adopted to this loop to reduce the loss on the sampling resistor which improved the conversion efficiency of the system. The LED driving circuit had been designed and simulated in TSMC 0.18 μm 70 V BCD process. The simulation results showed that the maximum constant current error was not more than 0.8% within the range of 20~125 mA load current. The average current change rate was less than 1% within the range of 20~100 V output voltage.