Abstract: A chip which is used in the buck application to drive white LED with a constant current is proposed in this paper.Using high-side current sensing topology, it controls the driving current for LED to be averagely constant through hysteresis-current control technique.The chip realizes constant-current driving without complicated circuit design, provides the accurate control of current and self stabilization.It is designed and fabricated with a 0.5μm 5V/18V/40V CDMOS process with input voltage ranged from 4.5V to 28V, with working temperature ranged from-40℃ to 125℃.The chip can provide a driving current of 350mA for LED, it can provide an adjustable driving current through adjusting the outside current-sensing resistor.The duty cycle of the signal DIM can be adjusted to realize lightness regulation of LED.Hspice shows that LED driving current is hysteresis, and the Precision of constant current can be controlled within the range of 6.2%.