Design Of Buck DC/DC Converter XD2676 With Adaptive On-Time Control

The arrival of the information age, has driven the the rapid development of the computer industry and the electronics industry, the rise of these industries greatly expands the application scope of the electronic technology, which leads to the appearance of electronic devices in large numbers, that has provided a great convenience for people’s life and work. and power acts as the supply source of all electronic devices, its position is more and more important. Today’s electronic devices tend to be more compact, lower cost, and to promote the constant breaking in the direction of lightening, thinning and high frequency. In this paper, take the research project of the Key Laboratory of High Speed Circuit Design and EMC of the Ministry of Education of Xidian University as the background, an adaptive on-time control mode DC/DC synchronous Buck converter XD2676 is designed.Firstly, some common topologies of DC/DC converter are introduced, specially the operating principle, modulation mode and control mode of Buck converter; Secondly, the electrical characteristics and working principle of the chip are described briefly; Then, the important sub modules of the chip are analyzed and simulated; Finally, the overall function of the chip is simulated, the results are analyzed, and a brief introduction to the design process of the layout is given.The chip XD2676 uses an adaptive on-time control mode, which changes the conduction time of the main switch adaptively by detecting signals of the input and the output, so the chip can keep the frequency constant basically. The control mode can respond the change of the output signal directly to the feedback circuit to keep it stable almost without outside compensation devices, thus the chip has a fast transient response. At the same time, due to the control mode needs to sample the change of output voltage directly, a ripple compensation circuit has also been integrated to make the loop work properly, which ensures that the chip can work normally. XD2676 uses the PFM modulation method to adjust the circuit. When signals of the input and the output are fixed, the conduction time of the main switch is constant, chip’s operating frequency can be kept constant by adjusting the off time of the main switch. In addition, the chip XD2676 uses a valley current detection mode, the current through the inductor is detected when the down tube is opened, when the current through the inductor is too large, the down tube continues to be opened to make the current drop down continually until to the threshold which has been set, and then the main switch is turned on again, the next working cycle will come, and the current through the inductor is detected cycle-by-cycle. In addition, the chip also integrates a series of protection circuits, such as input under-voltage lock-out circuit, over-temperature protection circuit, over-current detection circuit, so as to ensure the chip can be more reliable and stable.Based on the simulation process of 0.18 μm BCD, the sub modules and the whole system of the chip are simulated and verified with Cadence and Hspice S, and the simulation results show that the chip meets the circuit parameters and works well.