An Output-Capacitor-Free Adaptively Biased Low-Dropout Regulator With Subthreshold Undershoot-Reduction for SoC

This paper presents an output-capacitor-free adaptively biased low-dropout regulator with subthreshold undershoot-reduction (ABSTUR LDR) for SoC power management applications. Techniques of adaptive biasing (AB) and Miller compensation with Q-reduction are employed to achieve low-voltage high-precision regulation with extended loop bandwidth while maintaining low quiescent current and high current efficiency. The pass transistor is designed to work in the linear region at heavy load to save silicon area, and a symmetrically matched current-voltage mirror is used to implement the AB scheme with accurate current sensing for the full load range. The dedicated STUR circuit, which is low-voltage compatible and consumes very low current in the steady state, is inserted to momentarily but exponentially increase the gate discharging current of the pass transistor when the LDR output has a large undershoot due to a large step up of the load current. Undershoot voltage is hence dramatically reduced. Stability of the ABSTUR LDR is thoroughly analyzed and tradeoffs between the undershoot-reduction strength and the light load stability are discussed. Features of the proposed ABSTUR LDR are experimentally verified by a prototype fabricated in a standard 0.35-μm CMOS process.