The Design Of The Multi-channel Led Current Balance Circuit

LED is a novel luminescent material; it has a huge advantage compared to the traditional light sources due to its high quality and low power consumption. Energy conservation and environmental protection has become a major worldwide trend these days, our government also released a lighting development roadmap last year, which is focused on eliminating incandescent in several years. LED lighting systems can be the best substitution to the traditional lighting systems as the next generation of lighting sources, thus receive lots of attention.LED is some kind of semiconductor, its conduction current and its forward voltage has an exponential relationship; small changes of the forward voltage will cause tremendous changes in conduction current. The brightness of the LED is proportional to its current, so the LED light sources will have the most stable lighting levels when driven by a constant current circuit. The most LED drivers in the lighting market have only got a single string driving ability; the other multi-string constant current drivers could only afford dozens of mill-amperes, which is far from meeting the demand of household lighting.A four-string constant current LED driver is implemented to satisfy the demands of small and medium-sized multi-string household lighting system. A single string constant current module was implemented as a basic drive module, which included load current sampling circuit, error amplifier circuit, PWM output control circuit and over current protection circuit. The single string constant current module works with the application buck circuit to offer the LED load a constant current. The whole chip design was completed by joining the soft start circuit, the multi-string control circuit and the boost driver control circuit together with four copies of the single string constant current module.The whole chip design is based on CSMC0.5μm DPTM CMOS process, which cost much fewer than those cutting edge processes, therefore very suitable to meet the market requirements of the low-cost LED drivers. The power supply of this chip is5V; the current of the LED string is set by a sample resistant, with a max current balance capacity of1000mA; the current difference between designed value and simulated value is less than1%. The chip provides a boost mode operation and a buck mode operation, and it switches automatically according to the LED load, current balance reaches a2%accuracy between those four branches when the resistant of the maximum string is not two times larger than the resistant of the minimum one; the chip also provides a soft start function to offer a more reliable load operation, the soft start time is determined by a capacitor connected to SOFTSTART pin.After the whole system design, HSPICE was used to simulate each circuit and verify the whole chip function; the results showed that the chip can act high current constant accuracy and current balance precision, achieved the design requirements.