Design Of A High Power LED Driver Based On Constant Switching Frequency DC-DC Converter

In the pressure of energy crisis and environmental crisis,LED lighting with its energy saving,long life,environmental protection and other unique advantages catch more and more people's eyes.LED driver often uses switching mode poly supply,include voltage control mode and current control mode.The voltage control mode has slow response speed.The switching frequency of hysteresis control mode and fixed on-time control mode changes with the input voltage,which will generate the frequency noise.To solve the problem,a high power LED driver based on constant switching frequency DC-DC converter has been designed.A chip used in the buck application to drive the white LED with a constant current is proposed in this paper.By using hysteretic control mode,the transient response speed of the chip was improved.Second-order curvature compensation circuit realized by voltage divided technology was employed to design a low temperature coefficient and high PSRR band-gap reference source.Moreover,the circuit of the on-time and off-time was modified.As a result,the on-time and off-time change with the input voltage but cancel out each other,so the switching frequency keeps constant and optional and can be determined by the load and the external resistor.The circuit was designed and fabricated based on ASMC 0.5?m BCD60V process.The measurement results indicate that the chip can provide a constant driving current of 350mA with current ripple of 70mA for LED and within the input voltage range of 10V?40V.The driving efficiency can reach 90%when driving three LEDs,with the output current of 350mA.And at the same load,its efficiency does not change with the output current.The chip can be widely used in low voltage lighting.