| Due to its obvious advantages like high efficiency, long-term reliability and life, white LED is more and more widely applied to lighting field. And it is expectable to replace traditional lighting implement by long life white LED. As the applications of white LED is becoming wider and wider and its power is becoming larger and larger, new types of driver IC with flexible function are urgently demanded. This paper proposed a complete design of a whit LED driver IC for Buck switch circuit featuring high digital dimming ratio, wide supply voltage range and subsection linear slope compensation.In the first part, applications of modern white LED and the requirement of the its driver system were discussed. The reasons why high dimming ratio and wide supply voltage range are requested were introduced. And the corresponding solution was given.The second chapter introduced the system structure of the proposed driver chip. It is a constant current mode driver used in a Buck switch circuit. Through holding the status in power stage and the control loop when the digital dimming signal comes in, the high digital dimming ratio was successfully archived. To get balanced performance, the technologies such as driving circuit boost, subsection linear slope compensation, front end pulse skip, frequency fold back and soft/force startup were applied. At last, the paper shows the simulation results including the frequency response of a typical application.Chapter three focused on module circuits design. The circuit design was described in detail, which includes a LDO work under supply voltage from 7.5V to 36 V with 100mA peak current, a two stage reference which have good supply voltage rejection and temperature performance, a oscillator realize the subsection linear slope compensation, a error amplifier which could work on common mode input voltage up to 36V while keeps good common mode rejection, a driver circuit to driver up to 3nF parasite capacitor and a EMI-reduce circuit. |
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