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 may be unstable. This is often a result of board layout and/or wrong L-C output filter combinations. As a next step in the evaluation of the regulation loop, the load transient response is tested. The time between the application of the load transient and the turnon of the P-channel MOSFET, the output capacitor must supply all of the current required by the load. V_OUT immediately shifts by an amount equal to Δ_I(LOAD) x ESR, where ESR is the effective series resistance of C_OUT. Δ_I(LOAD) begins to charge or discharge C_OUT generating a feedback error signal used by the regulator to return V_OUT to its steady-state value. 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.