A Study Of High Efficiency High Power Driver XD1420 With Four Strings WLED

LED is developing gradually into the most ideal light source for the backlight of LCD. During the development of backlight LED drivers, the method of boost converter, based on switching power supply, with LEDs in series becomes more suitable for today’s LCD backlight applications, compared with methods of traditional charge pump. Firstly the structure of boost converter ensures the design can be applied in the low-voltage system whose power is supplied by a Li-battery, and the output voltage can be auto-adjusted according to the number of the LED in series. Then the output of constant current can keep the brightness of LEDs consistent. As a contrast, charge pump often results in a small amount of LED and uneven brightness for its fixed output voltage and limited load capacity, which means charge pump can only be applied in such low cost applications with a small dimension of LCD.With the background of “Theoretical research and design of key techniques for power management IC”, a high power white LED drive controller of high efficiency is designed in this paper. It adopts the structure of boost switching power converter, and constant output current. The design is based on the 0.18μm BCD process of a company and simulated by software of Cadence and Hspice.LED current of the XD1420 is designed up to be 350 m A which makes it a typical high power LED driver. In addition LED current is programmable to improve the flexibility. To meet the requirement of different dimensions of LCD, LED current can be extended to four strings, and the maxim output current can be 1.4A.By selecting peripheral components of proper values, the system can operate with good performance at different loads in a wide range. The chip is integrated with general protections.When a channel is failure, it will be removed from the control loop so that other channels won’t be affected. Each channel’s current is good matched and the error is below 1%. The feedback voltage that should be regulated varies with LED current, so the extra power loss caused by fixed regulated voltage can be effectively avoided and a high efficiency is thus achieved even at light load. For example the efficiency in application of 60 m A LED current is above 90%. In order to meet the general requirement of brightness control, both analog and PWM dimming are integrated with brightness adjustment ranging from 0 to 100% of the maximum value.System application methods and critical circuit functions are studied in detail in this paper, including the calculation of the peripheral circuit design, the analysis of the stability of the control loop, and the achievement of dimming circuits.It starts with the peripheral circuit design, in which the basis for the selection of the devices is discussed and the process of parameter calculation is shown. Then the stability of the control loop is studied. The power stage is modeled based on the idea of “averaged switch modeling”, and the LED is modeled to get its small signal model, according to which a compensating method is proposed and the detailed compensating step and relative calculation is shown. As a result, the stability of the control loop is guaranteed. In addition, the realization of dimming function is discussed and the influence of PWM dimming on LED current is analyzed based on which a method for improving LED current is proposed. At last, a method is proposed to relief the problem of LED current overshoot occurs when the system starts. All blocks are simulated and verified using Hspice software, and the relative waveforms are shown which show a good performance of the chip.