The Design Of Constant Current LED Drive Circuit With Thermal-shutdown Function And Low-power

As the Energy Star of the21st century, LED (Light Emitting Diode) has thefollowing advantages: high efficiency, high lifetime, high reliability. Furthermore, LEDwas listed as the key promotion product by National Long-term Science and TechnologyDevelopment Plan. LED will be gradually taking the place of incandescent lamp andfluorescent lamp. Now, the high-brightness white LED will be widely used as all kinds oflight sources, such as the projection lamp, automotive lighting, LCD backlight, decorativelight. Researches show that small change in LED forward voltage causes dramatic changein the current. This phenomenon leads to rapid increase of heat production in the LED,even burns up the LED in a worst case. Therefore, constant current driver is generallyused for LED. However, in order to explore the advantages of high efficiency of LED, itis necessary to coordinate LED with a specialized and high performance drive circuit.Especially in the field of high power LED lighting, high integration, high lifetime, highefficiency, high reliability constant current driver chip of research has practicalsignificance to LED s driving and controlling.In the view of requirements of low powerrequirements, high reliability, constantcurrent output and over-temperature protection about LED drive circuits, this paperintroduces a LED constant current drive chip from the following three aspects:(1) By describing the common constant current driving methods for high-powerLED driver chips, the relative merits of each drive mode has been analyzed, and thefeasibility of improving the chip feature on the basis of the existing driven circuit hasbeen explored.(2) In order to obtain the adjustable of constant current output and the low-power,this paper proposed a new constant current drive program. Our program used theclamping action of the power NMOS tube s drain with the operational amplifier andoperating in the linear region of the NMOS tube common-source and common-gatecharacteristics to get constant current output, achieving the goal of low powerconsumption.(3) An over-temperature protection circuit which will automatically shut off whenthe chip temperature exceeds the set value was proposed. The solution use the negativetemperature characteristic of the carrier mobility μ to generate PTAT voltage proportionalto absolute temperature, and to control comparator output voltage level by comparing with the reference voltage, and to shut off or shut on the power NMOS transistor for theconstant current module, it can realize overtemperature control of the chip.