The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signal:

• Switching node, SW
• Inductor current, I_L
• Output ripple voltage, V_OUT(AC)

These are the basic signals that must be measured when evaluating a switching converter. When the switching waveform shows large duty cycle jitter or the output voltage or inductor current shows oscillations, the regulation loop can be unstable. This is often a result of board layout, wrong L-C output filter combinations, or both. As a next step in the evaluation of the regulation loop, test the load transient response. The results are most easily interpreted when the device operates in PWM mode at medium to high load currents.

During this recovery time, V_OUT can be monitored for settling time, overshoot, or ringing; that helps evaluate stability of the converter. Without any ringing, the loop has usually more than 45° of phase margin.