Lossless average inductor current sensor for CMOS integrated DC-DC converters operating at high frequencies
A novel average inductor current sensing circuit integrable in CMOS technologies is presented. It is designed for DC-DC converters using buck, boost, or buck-boost topologies and operating in continuous conduction mode at high switching frequencies. The average inductor current value is used by the DC-DC controllers to increase the light load power conversion efficiency (e. g., selection of the modulation mode, selection of the dynamic width of the transistors). It can also be used to perform the constant current charging phase when charging lithium-ion batteries, or to simply detect overcurrent faults. The proposed average inductor current sensing method is based on the lossless sensing MOSFET principle widely used in monolithic CMOS integrated DC-DC converters for measuring the current flowing through the power switches. It consists of taking a sample of the current flowing through the power switches at a specific point in time during each energizing and de-energizing cycle of the inductor. By controlling precisely the point in time at which this sample is taken, the average inductor current value can be sensed directly. The circuit simulations were done with the Cadence Spectre simulator. The improvements compared to the basic sensing MOSFET principle are a lower power consumption because no high bandwidth amplifier is required, and less noise emission because the sensing MOSFET is no more switched. Additionally, the novel average inductor current sensing circuit overcomes the low bandwidth limitation previously associated with the sensing MOSFET principle, thus enabling it to be used in DC-DC converters operating at switching frequencies up to 10 MHz and above.