Self-Tuning Digital Current Estimator for Low-Power Switching Converters

An inductor current estimator suitable for low-power digitally controlled switch-mode power supplies (SMPS) is introduced. The estimation of the average current value over one switching cycle is based on the analog-to-digital conversion of the inductor voltage and consequent adaptive signal filtering. The adaptive filter is used to compensate for variations of the inductance and series equivalent resistance affecting accuracy of the estimation. Based on the response to an intentionally introduced and known current step, the filter tunes its own parameters such that a fast and accurate estimation is obtained. A practical realization of the estimator resulting in a modest increase in digital controller complexity is shown. Besides a simple digital IIR filter and a load step circuit, it only requires a slow analog-to-digital converter for the input voltage measurement. The estimator is tested on a 6.5 V to 1.5 V, 15 W, digitally-controlled buck converter prototype. The results show that between 20 % and 100 % of the maximum output load the estimator has accuracy better than 10 % and one switching cycle response time.