In this example, the open loop response on the left indicates that there is no ESR zero influencing the loop in the output stage. Power XR products have two zeros allowing the designer to compensate for the power stage LC double pole. This ensures that the gain crosses zero dB (or crossover frequency) with sufficient phase margin. The red line on the plot above is the closed loop gain of the system. The vertical dotted line is placed at the crossover frequency, and the dotted line intersects the red phase plot with better than 45 degrees of phase margin. To better understand Nyquist, consider the example of a CD player. A CD player samples at 44.1kHz so that it can reproduce frequencies up to 20kHz in the analog domain. Since a switching regulator samples at the switching frequency, the maximum bandwidth is between 20 and 33% of the switching frequency. Power Architect limits the bandwidth to 20% of the switching frequency.