Constant On Time Controlled Buck Converters

In order to guarantee portable electronic equipments working efficiently and reliably, the voltage regulator module (VRM) should work at light-load with high efficiency and enjoy fast transient response. However, the traditional pulse width modulation (PWM) control cannot satisfy the requirements above. The constant on time (COT) control, which is a kind of PFM control technique, compared with the traditional PWM control, has the advantage of high light-load efficiency and fast transient response.Thus,COT control technique has raised extensive focus and study. But the switching frequency of COT control technique is not fixed, the working stability can also be influenced by the output capacitor equivalent series resistance (ESR) and the steady accuracy is bad.To solve the problem that switching frequency of COT control is not fixed, quasi constant frequency turn on time (QCFOT) control technique is proposed. The working principle and characteristic of QCFOT control are also studied. In addition, the theory and simulation comparison analysis among QCFOT, COT and PWM are presented, too.To realize the working stability of COT control technique, constant on time capacitor current (CC-COT) control technique is proposed. The working principle and characteristics of CC-COT are discussed particularly. The small-signal model of CC-COT is also constructed to analyze its stability. At last, the simulation analysis of both CC-COT and COT are performed.Finally, the experimental platform of COT, QCFOT, CC-COT controlled Buck converter is established and the experiment results verify both the theory analysis and simulation analysis. The results represented that the QCFOT controlled Buck converter not only has fast transient response and small output voltage ripple but also benefits fixed working frequency. Compared with COT control technique, CC-COT controlled Buck converter keeps fast transient response as COT, has better stability, and enjoys higher steady accuracy.