A State-Switching Digital LDO for PWM Thermo-Optic Tuning in Silicon Photonics

Abstract

Pulse-width-modulated (PWM) thermo-optic tuning in silicon photonics calls for a power supply featuring high-speed PWM power output with short settling time, high efficiency, and a compact size. However, the transient response of the traditional digital low-dropout regulators (DLDOs) is limited by the closed-loop response, which makes it difficult to meet the speed requirements of the PWM power output. This brief presents a State-Switching DLDO (SS-DLDO), specially optimized for PWM thermo-optic tuning. Two state selectors, controlled by a PWM signal, are inserted into the SS-DLDO structure to control the connections and operational states of the DLDO asynchronously. This enables the speed of PWM tuning to be decoupled from the feedback loop of the DLDO. The proposed design is fabricated in a 65nm CMOS process with an active area of 0.00634 mm2. Measurement results show that the rising-edge settling time and falling-edge settling time of the PWM power output are 16.3 ns and 14 ns, respectively, which effectively reduces the limit of the edge settling time to the achievable PWM duty cycle range. Under a 2 MHz PWM frequency, this design can achieve PWM duty cycles ranging from 5.92% to 97.2%, corresponding to output power ranging from 1.47 mW to 24.12 mW.