Design And Research Of LED Drive Circuit In Accurate Proportional Sampling Mode

As a promising lighting application, semiconductor lighting is considered as the priority theme of industrial energy efficiency in energy field and the products to expand domestic demands by "National Long-term Science and Technology Development Plan". With the advantages of high efficiency, low pollution, long lifetime, high reliability, LED is gradually replacing incandescent lamp and fluorescent lamp to be a new lighting souce for general lighting. Along with the development of high-brightness white LED for backlight and general lighting, the research of constant-current driving IC for white LED has a broad prospect and is meaningful.In constant current driving mode LED driver IC, a sampling resistance is always used to measure the driving current directly and produce sampling voltage. This sampling voltage controls the LED driving current in constant by negative feedback. As the power of LED driving IC is increased, its driving current will reach several dozens of milliampere to several Ampere. The increase of power dissipation on sampling resistance will cause the decrease of power efficiency of this driving IC and limit the practical applications. Although reducing the value of sampling resistance can decrease the sampling power dissipation accordingly, but it will also degrade the accuracy of current sampling and controlling.In this research, an accurate proportional current sampling LED driving IC is designed based on traditional proportional current sampling mode and the equivalent circuit analysis on its constant output current characteristic is carried out. The accurate proportional current sampling mode IC can increase both the equivalent output resistance and sampling accuracy to obtain better constant output current characteristics. The integrated circuit consists of bandgap voltage reference, power MOSFET, buffer, operational amplifier, sample feedback control circuits. It realizes accurate sensing of sampling current changed with output driving current and gets better constant output current characteristics by using an amplifier to clamp both drain voltage of sampling MOSFET and power MOSFET to the same value with the feedback control. Through small signal equivalent circuit analysis, it is shown that the small signal output resistance in accurate proportional current sampling mode circuit is much larger than that in traditional proportional current sampling mode circuit and constant current characteristics will be enhanced.The chip has an area of 1.455mm×0.960mm and is fabricated in CSMC 0.5μm standard CMOS process. The testing results are shown that when the LED driving current is 350mA and the power supply is 6V with±10% variation, the stability of output constant current of accurate proportional current sampling mode LED driving IC will get 41% improvement than that of traditional proportional current sampling mode LED driving IC.