An Accurate Sense-FET-based Inductor Current Sensor with Wide Sensing Range for Buck Converters

This paper introduces an accurate sense-FET-based inductor current sensor for buck converters. The proposed sensor utilizes nonlinear adaptive biasing to maintain consistent bandwidth and phase margin in the sensor's control loop across a wide range of load currents, leading to high sensing accuracy independent of the load. Moreover, auxiliary sensing FETs are proposed to eliminate sharp transitions in the sensed high-side and low-side currents as the buck converter switches between the ON and OFF phases. Eliminating these sharp transitions enables the bandwidths of the current sensor's control loop and the nonlinear adaptive biasing generator to be greatly relaxed without compromising sensing accuracy, leading to lower power consumption. The proposed sensor is implemented as part of a 2-MHz buck converter in a 0.5-μm standard CMOS technology. Simulation results of the proposed sensor show a 10% reduction in current sensing error at light loads (~50mA) and 40 degrees of improvement in the phase margin of the sensor's control loop at heavy loads (~5A), compared to conventional sense-FET-based current sensors. Furthermore, the variation in the unity gain frequency (UGF) of the proposed sensor's control loop across the entire load range is 4.5 times lower than that of conventional designs.