Digital auto-tuning system for inductor current sensing in VRM applications

Inductor current sensing is becoming widely used in current programmed controllers for microprocessor applications. This method exploits a low-pass filter in parallel with the inductor to provide lossless current sense. A major drawback of inductor current sensing is that accurate sense the DC and AC components of the current signal requires precise matching between the low-pass filter time constant and the inductor time constant (L/R_L). However, matching accuracy depends on the tolerance of the components and on the operating conditions; therefore it can hardly be guaranteed. To overcome this problem, a novel digital auto-tuning system is proposed that automatically compensates any time constant mismatch. This auto-tuning system has been developed for VRM current programmed controllers. It makes it possible to meet the adaptive voltage positioning requirements using conventional and low cost components, and to solve problems such as aging effects, temperature variations and process tolerances as well. A prototype of the auto-tuning system based on an FPGA and a commercial DC/DC controller has been designed and tested. The experimental results fully confirmed the effectiveness of the proposed method, showing an improvement of the current sense precision from about 30% up to 4%. This innovative solution is suitable to fulfill the challenging accuracy specifications required by the future VRM applications