| Low power consumption and high efficiency are the tendency of green power design nowadays. As large as electrical power system module, as slight as single power chip are all designed for high performance and efficiency. Based on buck DC-DC converters, this paper research the control loop scheme of low power dissipation, high efficiency and performance.In order to analysis and contrast, ZCS (Zero Current Switch) mode is discussed firstly, which combined with zero current switch and burst mode and widely used in light-load high efficiency systems. Systems enter in light-load mode when inductor current drops to zero continuously. When detects output voltage dropped, systems exit light-load mode. Based on0.5-μm BCD process, under the condition of12V input and3.3V output, converters access ZCS mode when load current drops to400mA, and then the switching efficiency is signific-antly higher than PWM mode at the same load. Especially at10mA load, the efficiency in PWM is only45.6%, ZCS can achieve for81.8%. But load transient form light load to heavy load, the response of ZCS is slow and causes large undershoot of output voltage.LAM (Load Adaptive Mode) is also light-load and high efficiency mode. Unlike ZCS, the voltage loop with error amplifier is always working in LAM. So load transient for mode switching is faster than ZCS. At LAM, systems adjust the light-load threshold voltage and inductor peak current according to the load conditions. Through analysis, derivation and circuit design, the ripple control of output voltage can be achieved. The disadvantage of LAM is the high threshold of light-load mode. From400mA to1A load current, systems enter in light-load mode lead to performance reduction.PCS (Peak Current Switch) mode is proposed with peak current switch and sample hold circuits. Conbined with the advantages of ZCS and LAM, PCS solves the load transient pro-blem of ZCS and the high mode threshold of LAM. In the typical application, the mode threshold of PCS is200mA loading, and the efficiency of full load range (10mA to3A) is above82%, the peak efficiency can be up to95%. Under the condition of3.3V output, from50mA to2A load transient, the output voltage only has56mV (1.7%) undershoot.At last, three light-load modes are compared and summarized. From the full input and output range, the optimal light-load mode is PCS, which has lower output voltage ripple and faster load transient, only the efficiency at several loadings are slightly lower than ZCS. |
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