The Design Of A Step-up DC/DC Converter For White LED Driver Applications

In recent years, because of the enormous market of the consumption electronic product and development potentiality, DC/DC switch power converter of portable electronic product is a dark horse. Switch power has been widely used at communication, computer and other complete machine products in the relevant field, its development speed is surprising.Using the semiconductor power device as the switch, the main circuit which changes one kind of power form to another one form can be called switch changer circuit. The circuit which has link of protecting to automatically control the ring of closing steadily while changing can be called the switch power. It has advantages such as high efficiency, small size, light weight, high precision and responding fast. The key part of the switch power is a switch changer, and the study on switch changer becomes the focal point at home and abroad. As high-efficiency white light LED (Light Emitting Diode) with remarkable energy-conserving benefit and technological potentiality becomes the lighting mainstream in new era gradually, and the white light LED driver has already become one of the main classes in the power management products. To adopt an intact white light LED lighting scheme, the most basic request is the stability when LED is emitting light, and another request is to maintain the luminance uniformed on the whole background screen when it is lighted.However, the electrical characteristics of white light LED is dispersed, and the temperature characteristics of its electrical parameter is very unstable. Because of the characteristic of white light LED and the need for illumination, the special power management is needed for white light LED. White light LED on the market have two kinds of patternsat present, that is, charge pump and switch power driver. Because the switch power driver is small with high efficiency, it is used by most designers.Because of all the above reasons, we design a Boost DC/DC converter chip which is used to drive the white light LED using switch power technology. The operating frequency of the designed boost converter is 0.5MHz, the input voltage range is 2.7V- 5.5V, and output voltage can reach as high as 24V. The inductance of main return circuit is 4.7uH, and the input capacitance and load capacitance are all 4.7uF. We carry on Hspice simulation using CMOS 1P2M 0.6um N-WELL library.There are three kinds of controlling mode in the circuit, that is, digital mode, analog mode and PWM mode. The driving way is constant current mode, which can drive two independent branch loads, and the built-in DAC (Digital Analog Converter) module can make each branch load control the luminance independently. The Boost scheme, the linear current adjuster and PWM control method with digital characteristic make the circuit keep the efficiency as high as possible within the range of voltage of the whole lithium battery.The operation principle of boost DC/DC converter and controlling principle of the switch power system are analysed at first in this paper, and then the small signal transmitting function of the converter is given. We use Simulink to build up the simulation to boost scheme. On the basis of the theory analysis, we study the main function module of the boost scheme, including DAC, oscillator, coil driver and digital control module, and then use Hspice to simulate the main module. The simulation result shows that the designed boost DC-DC converter chip can meet the designing requirement such as high efficiency, high accuracy and low operating voltage, meanwhile, it is suitable for the drive of white light LED and the application of theportable electronic product using lithium battery as power.The structure of the paper is arranged as follows:In chapter one, the current situation of the development of the switch power is introduced, the importance of switch power chip research is pointed out, the wide application of switch power in white light LED driving is illuminated, and the advantage and disadvantage between switch power and charge pump driver are compared. In chapter two, the concept and classification of the switch power is introduced mainly, the basic topological structure of the DC/DC switch power is provided, and the operation principle of boost DC/DC converter is analysed emphatically, which offers the theoretical foundation for designed chip.In chapter three , the controlling principle of the switch power system is analysed especially, various kinds of controlling mode in system is discussed, the small signal transmitting function of the converter is given, systematic modeling to Boost is built up using Simulink, and then the simulation result is given.In chapter four, the systematic principle and the modules of the converter is given, the main module of the chip is introduced, the principles of the main module are analysed and the Hspice simulation result is given .In chapter five, by contrasting the design object, the designing result is summarized.