A method of determining a current in a switched mode power supply. The method comprises: storing values representing at least one pre-calculated matrix A; receiving measured current values; and using the stored values representing the matrix A and the received current values to calculate a respective initial value for each of the coefficients of the line and for each measured current value: calculating an estimated current using the equation of the line and the calculated initial values of the coefficients; determining a difference between the measured current value and the estimated current value to generate a difference value, and comparing the difference value against a threshold and, if the difference value is greater than the threshold, calculating updated values of the coefficients using stored values representing matrix A and difference value. A value for the current in the switched mode power supply is determined using the updated values of the coefficients