Design Of Boost-type LED Backlight Driver XD1430 With Constant Current Architecture

In recent years, with the continuous development of technology, white LED power is decreasing; brightness is increasing; it has become the mainstream LCD backlight. Combined with requirements of the LCD backlight, the lighting principal and driving features of white LED, 6 strings white LED drive with boost controller is designed, which can be applied to drive the backlight of big LCD.Single white LED luminous brightness is too limited to support large LCD backlight applications; so it requires multiple white LED together. Too much paralleled branches make the output current too large; too much LED in one branch make the output voltage too high. Considering these two factors, in the paper a hybrid combination connection is used, in which there are six parallel paths and the maximum number can be up to 12 in one branch. Because of the large forward drop across the white LED itself(about 3.4V), the output voltage is relatively higher. The application requires the input voltage be relatively low. Finally, a step-up type PWM switch power structure is adopted in order to meet the design requirements. This paper first introduces the basic principle of step-up type switching power supply converter; to analyze the AC characteristics average switch modeling is introduced by replacing non-linear switching elements using linear elements. Also a detailed elaboration and discussion has been carried on the peak current control mode. To guarantee the stability of the feedback loop of the peak current mode boost controller, a modeling method which consider the effects of slope compensation is utilized to build the precise ac small signal model and to calculate the transmission function.Taking into account the relationship between the current flowing through the white LED and the light emission luminance and the fact that it is not easy for the constant voltage architecture to adjust the LED current, a structure of constant current source is adopted to drive white LED. A peak current mode boost controller topology is used, the feedback loop of which can guarantee the normal operation of the driving constant current source and stabilize the voltage across the constant current source. Being used to drive the backlight of LCD, the driving chip must be able to regulate the lighting brightness automatically. Two dimming modes including analog dimming and PWM dimming are designed in the article. So the white LED current is adjustable within the range of 0 ~ 60 m A. Analog dimming means that LED current can follow the change of DC input level. Analog dimming is mostly applied to occasion which has high requirement for noise and normal requirement for the lighting quality. PWM dimming means that the LED current outputs in PWM switching signal form, and the duty changes the value of average output current. PWM dimming is usually used to cases of high brightness and low noise. The two dimming modes can satisfy most of the application requirement. In order to improve the efficiency of the LED driving power supply, the voltage feedback loop can make the voltage across the constant current source be a different value when LED current is not the same. When the current is low, the voltage across constant current source is relatively low; when the current is high, the voltage across constant current source is relatively high.Based on the analysis of the driver chip loop architecture principles, stability and white LED constant current source architecture in the first three chapters, five typical sub-circuit modules are designed in the fourth chapter, including PWM dimming signal generating circuit, LED open circuit and short to ground protection circuit, the error amplifier module, a minimum feedback voltage detection module and the inductor current sampling module.On the basis of principle analysis and sub-circuit design, EDA simulation software such as Hspice,Cadence are adopted to simulate the sub-circuits using the 0.35μm BCD process to ensure that they reach the design requirements of each index at different power supply and temperature. The overall application circuit simulations are also carried out. The simulation results show that all the electrical specifications can reach the designing requirements of the datasheet.