High Precision Load Current Sensing using On-Line Calibration of Trace Resistance in VRM Applications

A method for the on-line calibration of a circuit Most of the methods described sense the inductor curboard trace resistance at the output of a buck converter is rent, which has the same DC value as the output current, described. The input current is measured with a precision and tracks it well up to the closed-loop bandwidth of the resistor and processed to obtain a DC reference for the output current. The voltage drop across a trace resistance at the output* cortr In is wh eletr l to the output is amplified with a gain that is adaptively adjusted to match capacitor's ESR is chosen to be equal to the desired output the DC reference. This method is applied to obtain an accurate impedance, as given by the load-line specification. This and high-bandwidth measurement of the load current in the allows the converter to follow the load-line ideally at an modern microprocessor voltage regulator application (VRM), arbitrary high bandwidth [2]. However, if ceramic capacitors thus enabling an accurate DC load-line regulation as well as a . . ' fast transient response. Experimental results show an accuracy are used the ESR is substanhially lower than the load-line well within the tolerance band of this application, and exceeding impedance rendering the previous design method impracall other popular methods. tical. The concept of generalized load-line was introduced [7] as a practical design objective for VRM systems with