Design of valley current sensing with low output voltage ripple based on CMCOT

In conventional ripple-based constant on-time (RB-COT) architectures, output voltage ripple depended on the equivalent series resistance (ESR) of output capacitance. Smaller ESR could cause sub-harmonic oscillation to the system loop. And larger ESR could increase the output voltage ripple and reduce the conversion efficiency under heavy load. Therefore, the valley current sensing based on COT mode control is proposed, and the output voltage ripple is replaced by the sensing inductor current ripple. The current of the low-side MOSFET at the SW port is sensed by the current sensing circuit to generate a sensed voltage containing the sensed current ripple information. Then the voltage is compared with the control signal, so as to adjust the output voltage through the system loop, reduce the output voltage ripple, and improve the stability and conversion efficiency of the system under heavy load. The chip was fabricated using a 0.18μm 30V BCD process. For an input voltage of 12 V and an output voltage of 3.3V, when the load current varied from 0.6 A to 3.3A and the temperature was 25 °C, the output voltage ripple was less than 7mV,which could achieve low output voltage ripple. And the maximum conversion efficiency of the system could reach 97%, meeting the requirements of system stability.