A novel ripple-based constant on-time control with virtual inductor current ripple for Buck converter with ceramic output capacitors

Ripple-based constant on-time (RBCOT) control has found applications because of its fast load transient response, small component count, and good light-load efficiency. However, this control scheme often encounters instability issue of subharmonic oscillation when ceramic capacitors are used for converter output filter capacitors. In the paper, a novel RBCOT control with virtual inductor current ripple was proposed to alleviate this problem. The novel control scheme improves system stability of RBCOT control without adding extra pins and converter components in IC implementation. Thus resulting in less component count and controller cost. Describing function method was used to derive the model and stability criterion of the proposed scheme, including the effect of delay-time. The scheme was implemented in an IC. Simulation and experimental results also confirm the proposed concept and the accuracy of the stability criterion developed.

I. Introduction

In recent years, ripple-based constant on-time (RBCOT) control scheme has been widely reported to possess the advantages of fast transient response, small component count requirement, and high light-load efficiency [1]–[6]. Fig. 1 shows the circuit diagram of the conventional RBCOT control scheme for Buck converter. and represent the switches, is the output inductor, is the equivalent series resistance (ESR) of the output capacitor , and current source represents the load. It can be seen that the only feedback signal, , is directly connected to the modulator input without compensation; therefore, this scheme provides simple implementation with less component count. Fig. 1. Circuit diagram of a RBCOT control for Buck converter Fig. 2. Modulation waveforms and ripple ingredients of RBCOT control