The Design Of An AC-driven LED-IC With Flicker-free Function

LED(Light Emitting Diode)is called as the fourth generation light source. LED is solid cold light source which has several advantages such as high-lifetime, high brightness, high energy efficient, environmental friendly light, and can work in high speed switching state. It is occupying the field of lighting rapidly.Based on several design principles that are simple circuit, high reliability, high photoelectric conversion efficiency and simple application, this thesis has designed a Flicker-Free AC driven LED-IC which can avoid line frequency strobe flash under 100 Hz.This paper analyzes the electrical, optical and thermal characteristics of the LED firstly, and elaborates the performances that LED driver should fulfilled according to those characteristics. According to the requirements of LED to driver, this paper introduces three kinds of driver solutions which are R and C buck circuit, constant current switching circuit and linear constant circuit current LED. Those three driver solutions are theoretically derived and compared with each other in detail. Finally, this paper analyzes strobe theory of lighting fixture and gives the two principles of solving lighting strobe problems.The framework of Flicker-Free AC driven LED-IC is built in this paper. It includes high voltage power circuit, bandgap reference circuit, internal power supply voltage generation circuit, LDMOS driver module, the voltage bias module, the current reference module, filter module, over temperature protection module, under voltage locked out circuit and over voltage circuit, and so on. Simulation analysis and system simulation are conducted based on CSMC 0.8um BCD process. According to the results of simulation, with an 16.7Ω external resistance, four parts of LED currents are 39 mA, 47 mA,53mA and 60 mA respectively, the power of LED is 8.4W and efficiency is 87.5%. With an 7Ω external resistance and on the condition of room temperature and typical TT process corner, four parts of LED currents are 97 mA, 116 mA, 132 mA and 148 mA respectively, the power of LED is 8.4W and efficiency is 89%. During the simulation of Flicker free on the condition of room temperature and typical TT process corner, current upper limit sets at 47 m A when the LED current upper limit sets at 60 mA, the fluctuation depth is 12.5%. Current upper limit sets at 55 m A when the LED current upper limit sets at 60 mA, the fluctuation depth is barely 5%.Finally, the designed LED-IC is taped out and analyzed and tested on border level. The results of tests manifest that with an 17Ω external resistance, the power of LED is 7.5W and efficiency is 86%. With an 8Ω external resistance, the power of LED is 19.5W and efficiency is 90.3%. According to the function tests of Flicker free, the fluctuation depth decreases to 12.5%. Consequently, the simulation satisfies the design, and the design achieved the anticipated effect.