Constant on/off-Time Hybrid Modulation in Digital Current-Mode Control Using Event-Based Sampling

Ripple-based constant on-time and off-time control techniques can achieve fast step-down and step-up transient performance along with an improved light-load efficiency. However, they suffer from varying switching frequency at the steady state and often require a phase-locked loop for frequency regulation. This paper proposes a constant on/off-time hybrid modulation technique in a digitally current-mode controlled synchronous buck converter. The proposed technique considers an event-based sampling mechanism using one sample per switching cycle, which makes it useful for high-frequency implementation. This requires a monoshot timer and a digital voltage controller G
_c
(z). The proposed technique can be configured to a constant on-time or off-time modulator with seamless transition because of sharing G
_c
(z); thus, improved step-up/down transient performance can be retained. Furthermore, a discrete-time framework is proposed for fast-scale stability analysis, and the effects of finite sampling and practical parasitics are discussed. Discrete-time small-signal models are derived for the direct digital control design with enhanced stability and performance. A frequency adaptation method is discussed to customize the switching frequency in real time. A buck converter prototype is tested, and the usefulness of the proposed modulation technique is verified experimentally.