A Wide Load Range Buck Converter with Dual-Mode Zero-Current Detection Using ZCD-N Acquisition and ZCD-F Tracking

This paper presents an adaptive on-time (AOT) buck converter with a dual-mode zero-current detection (DM-ZCD) scheme for high efficiency over a wide load range. NMOS-switch-based ZCD (ZCD-N) determines whether a transition to discontinuous conduction mode (DCM) is required without periodically activating the freewheeling switch during continuous conduction mode (CCM), and rapidly detects the vicinity of the zero-current crossing (ZCC) during the initial transition from CCM to DCM. The ZCD scheme is then transferred to freewheeling switch-based ZCD (ZCD-F) for accurate ZCC tracking and suppression of switching node ringing in steady-state DCM. During the DCM-to-CCM transition, the absence of a ZCD-F event automatically re-enables ZCD-N circuit. The AOT controller provides near-constant frequency operation in CCM and load-dependent frequency operation in DCM. Dynamic biasing and a subthreshold-biased differential stage reduce control-circuit power consumption and short-circuit current in the comparator output buffer, respectively. Designed in a 0.18-μm CMOS process, the converter operates from a 1.8 V input with a 0.8–1.5 V output and a 10 μA to 700 mA load range. Post-layout simulations predict 98.9% peak efficiency at VOUT = 1.5 V and ILOAD = 10 mA. At a 0.9 V output, peak efficiency is 97.1% and efficiency remains above 90% from 30 μA to 300 mA. The proposed event-based handoff between ZCD schemes enables efficient operation across a wide load range without explicit load current sensing.