Unified constant on/off-time hybrid compensation for fast recovery in digitally current-mode controlled point-of-load converters

Constant on-time control offers fast recovery during a step-down transient; however, the step-up recovery gets penalized, which often requires extra arrangements for real-time on-time adaptation. On the other hand, constant off-time control can achieve fast step-up (transient) recovery; however, the step-down recovery is generally degraded. This paper proposes a unified constant on/off-time hybrid compensation technique in a digitally current-mode controlled (CMC) synchronous buck converter with the inductor current in the analog domain. This utilizes a single mono-shot timer and a single digital voltage controller Gc, which can be configured to either constant on-time or off-time control with a simple modification. Thus, improved step-up/down transient performance is achieved through a real-time configuration with an anti-windup controller transition because of sharing of a common voltage controller. An asynchronous error voltage sampling can achieve unconditional current-loop stability along with robust voltage-loop stability. The mono-shot counter can be updated in real-time using a time-to-digital converter for fixed-frequency operation, without the need for input voltage sensing. A buck converter prototype is made, and the proposed control is implemented using an FPGA device.