Design Of Step-down Converter XD7252 Based On ACOT Control Mode

With the rapid development of Internet technology, the portable electrical equipment has become more and more popular. As an indispensable part of electronic product, it attracts more and more people's attention on its efficiency. Since the same product in different components may require different supply voltages, so the applications of switching power supply chip are becoming increasingly widespread. At the same time, as the digital chip, microprocessor applications increasingly widespread, for the inverter, they are a class of work is very prone to changes in load, the rate of change of the load current is high, which requires the DC-DC converter chip have an extremely fast load transient response speed to meet the power requirements of the load. To achieve this purpose, constant on-time control scheme has received widely attentions and research because of it has very fast transient response.Firstly, the background knowledge of the switching power IC has been described and introduced in this paper. Secondly, the basic indispensable working principle and key technologies of buck DC-DC converter are fully analyzed. And then design of the whole structure and function division, development of the index of XD7252 through the key sub-blocks are designed in detail and simulated previously. Finally, simulations were run for the whole buck converter circuits. This paper uses an ACOT control scheme which has very fast transient response. However, the average switching frequency of ACOT control scheme still varies considerably over input voltage and load, so a block named frequency locked loop are designed in this IC. This block measure the actual switching frequency convert to voltage, and then compare it to the desired range, using the result modifying the on-time with a feedback loop to keep the average switching frequency in the desired range. This design idea can be well applied to other IC which uses constant on time control mode as well.In addition, this mode often encounters instability problem of sub-harmonic oscillation easily when ceramic capacitors are used for converter output filter capacitors. In the paper, a novel control with virtual inductor current ripple was proposed to compensate the ESR ramp normally provided by the output capacitor's ESR, the special circuit saves one pin connection and improves system stability by alleviate sub-harmonic oscillation.The designed converter XD7252 based on 0.18?m BCD process. By using the Cadence tools of Hspice S, the author has completed the circuit design and simulation. The simulation results suggest that the performance is in accord with the requirements of design.