摘要
白光LED具有发光强度大、效率高、体积小、寿命长等优点,被认为是当前最具潜力的光源之一。为了充分发挥其优势,良好的驱动是不可或缺的。白光LED驱动芯片已成为目前电源管理芯片市场的研究热点。
驱动白光LED,关键是要提供恒定的电流,以保证发光强度的稳定和均匀。利用升压式DC/DC转换器来驱动白光LED具有高效率、高精度、高匹配度、应用灵活等优点,是便携式应用中首选的方法。本文主要研究的就是用于白光LED驱动的升压式DC/DC芯片的设计。
本文首先搭建了芯片的整体构架,芯片采用了电流模式PWM控制方式和同步整流技术,从而提高了瞬态响应速度和转换效率。接着,设计了芯片中的带隙基准电路、电流采样及斜率补偿电路、功率管栅极驱动电路、软启动电路、反转保护电路等,并给出了仿真结果。其中,电流采样电路采用无损采样SENSEFET方法替代传统的直接串联采样电阻的方法,降低了损耗;软启动电路采用脉冲充电方式,降低了充电占空比,在不用外接电容的情况下可以实现700μs以上的软启动时间;在整流管栅极驱动电路中,通过引入负跳沿延时单元,消除了CMOS瞬态短路导通现象,降低了功耗,保护了输出级。
最后,用HSPICE对芯片进行了全局仿真:芯片启动时间达到800μs,输出电压纹波系数仅为1.64%,中等负载下转换效率在70%以上,输出电压达到15.5V时可以实现有效的过压保护,能够实现PWM亮度调节功能,仿真结果表明设计的芯片达到了预期的要求。
As one of the most promising light sources, White LED has many advantages, such as high luminous efficiency, high luminous intensity, long life period etc. The White LED driver's main function is providing constant current, and now it has become the star of power management field. Using step-up DC/DC converter to drive White LED is a major method for portable applications. The design of the step-up DC/DC converter Chip, with the current mode PWM control and synchronous rectification, is introduced in this thesis.
In the main part of this paper, firstly, the whole chip’s structure is constructed. And then, the subcircuits such as bandgap, current-sensing and slope compensation circuit, gate driver for power MOSFETs, soft-start circuit and IR comparator are introduced in details. Instead of the traditional series resistor technique, the lossless SENSEFET technique is utilized, which reduces the power dissipation. Without the external capacitor, the soft-start circuit can realize 700μs start-up delay by charging the small on-chip capacitor at long intervals. An improved gate-driver for the synchronous rectifier, which is not only reducing the power dissipation, but also protecting output stage, is proposed. Finally, the whole chip simulation results by HSPICE are given: the start-up delay attains 800μs, the output voltage ripple coefficient is only 1.64%, the efficiency is above 70% under moderate load, and the over-voltage potection and PWM luminance modulation are effective.
引文
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