BOOST Topology Dual-mode PWM LED Driver

As a hopeful solid state lighting source, white LED has some outstanding advantages like non-pollution, high efficient, long life, etc. and thus it is widely adopted by modern electronic lighting system. Because its color and brightness are seriously affected by the forward current, the high performance driver integrated circuits (IC) are demanded. Besides, some rigorous applications such as industrial control system or automotive lighting system bring more difficulties to the design of drivers due to their high input voltage and wide input voltage range. Therefore, a lot of efforts were put into the research on these drivers.To meet the requirements of such drivers, this thesis proposed a solution of white LED driver, in which the BOOST topology and peak current control mode were employed to keep LED load current constant. To provide the maximum application flexibility of driver and reduce its cost, a solution with off-chip power MOS transistors and off-chip switch diodes was chosen. The driving current can be fixed by system and users could conveniently select periphery elements according to their requirements. Since the load LEDs are in series, the output voltage depends on the number of LEDs, and the output voltage could be higher or lower than the input voltage. To meet the different requirements requested by different operation states, one solution which provides both constant current mode and constant current/constant voltage mode was introduced. Considering that one of the important functions of LED driver chip is dimming, PWM dimming was employed to keep the true color, and to facilitate digital use. Meanwhile, the analog dimming was reserved in this design to extend dimming range. The input voltage of the chip ranges from 3 V to 36V. The number of LEDs that could be driven is 1 to 10. And the operating frequency up to 1MHz could be set by an external resistor.The circuit design of main modules in the driver such as error amplifier, oscillator, ramp compensation circuits and voltage regulator were completed. To get precise voltage reference under very high input voltage, the structure with two stages bandgap voltage reference was employed. All circuit modules were carefullydesigned to ensure the high performances of the system. The circuits of soft-start, over voltage protection, over temperature protection and system shutdown were integrated into the chip to protect it from damage and make it convenient to use.The LED driver presented in this paper was designed in 1.5um BiCMOS process, and simulated with Cadence Spectre. Simulation results show that it works well and all expected functions were achieved successfully..