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Conventionally, inductor current is sensed and quantized before it can be used by digitally controlled DC-DC converters. These are intuitively done by using two separated functional blocks-an analog current sensor and an ADC. A few research works have investigated other ways of obtaining the inductor current information in the digital domain. This paper proposes an on-chip digital inductor current sensor that combines both the inductor current-sensing and quantization into a single functional block. In this way, the redundancies found in the conventional approach can be reduced and optimizations can be made to save chip area and power consumption. An 8-bit digital inductor current sensor has been designed and fabricated with a UMC 0.13 μm CMOS process. The measurement results show that the digital sensor can provide digital inductor current information with a conversion time of 225 ns. This can be used by a buck converter with a switching frequency up to 4 MHz. The digital sensor has linear and monotonic input-output transfer curve properties with an LSB of 6.79 mA. It consumes current of 700 μA at 1.2 V supply voltage.