Using Hysteresis Current Control, Low Emi, High-power Led Driver Design

As the application of high power LEDs are more comprehensive in decorative lights and general illuminance,the high power LED driver become more and more important.The LED driver should have a step-down circuit with dimming control and regulate the current of LEDs accurately.Furthermore, more requests as follows should be taken into account such as high conversion efficiency,smaller size,long working time,lower costs,Electro-Magnetic Interference(EMI),Power Factor,etc.To make the most of the LEDs' merits such as longevity,it is indispensable to utilize an advisable driver.Fisrt of all,the progress,structure,feature and main application area is introduced in this paper.And then conventional LED drivers are discussed about their principles, merits and drawbacks,utilizations,etc.According to crucial system design specification, the whole chip diagram was proposed.Four key sub-circuits were designed at the chip's request and consists of a LDO which supply the chip with 5V voltage,an over temperature protect(OTP) circuit,a spread spectrum circuit and a drive circuit for Power MOS.Based on the detailed analysis of the function and principle, the sub-circuits were simulated with Hspice.High voltage process is adopted in the chip with a built-in Power MOS. The system utilizes hysteresis current control method with obvious advantages: first, a rapid transient response that afford the chip a high dimming ratio;next, high stability, no compensation;last, high current accuracy over a wide input voltage range.The chip can achieve a high efficiency to 97% and a current accuracy within 5%, otherwise analog and PWM dimming can be realized by a DIM pin. Electrolytic capacitor isn't applied at the output so the chip can get a longer life time.The chip reduces Electromagnetic Interference(EMI) by spread spectrum technology. Besides, the chip has UVLO and OTP circuit to ensure the normal operation.Finally, the functions and performances of the chip are verified by simulation. Furthermore, the high voltage process was discussed and layout was designed in the end.According to simulation results, the functions and performances meeted the design requirements well.