Absolute Value, 1% Linear and Lossless Current-Sensing Circuit for the Step-Down DC-DC Converters With Integrated Power Stage

A circuit allowing to accurately measure the average output current of the step-down DC-DC converter operating in PWM mode is presented. It relies on the measurement of the average voltage drop across the PMOS-NMOS or dual-NMOS power stage. The average voltage drop is measured by a simple RC filter, as a difference between the power-stage average output voltage and the average output voltage of an auxiliary "ideal" power stage. Obtained voltage drop is applied to an element labeled as "composite load". This approach allows to remove the dominant errors originating from the use of a standard sense-FET circuit. Obtained output current image exhibits very low linearity error <1% in 10 mA to 2.1 A range, and high +/- 4% accuracy of the conversion gain over supply voltage, temperature and statistical chip-by-chip variations. The performances of integrated current sensor are demonstrated by measurements on 3.2 MHz step down DC-DC converter integrated in 0.5 mu m CMOS process.