Low drop-out COT buck converter without maximum duty cycle limitation due to minimum Off-Time

Abstract

Constant On-Time (COT) buck DC-DC converters are widely used due to high responsiveness, reliability and simple design. However, maximum duty cycle and inductor charging slew rate are limited by minimum Off-Time [1]. The proposed solution overcomes these limitations, providing a wide output range converter with enhanced transient response. A constant on time buck converter, where On-Time is extended beyond the internal timer if output voltage is lower than reference, is proposed. As a benefit, there is no limitation to maximum duty cycle due to minimum off time, just allowing low drop-out operation. This also improves inductor current slew rate during transients. An additional peak current comparator is needed to limit current during On-Time, as the classic valley limitation is not enough.