Evaluating the stability of a Point-Of-Load (“POL”) regulator can be performed using both traditional Bode plot and output impedance measurement techniques. Measuring output impedance has an advantage over the Bode plot in some cases as the output impedance test can be non-invasive.3 With the right equipment, the non-invasive measurement does not require access to the control loop, such as an injection resistor or transformer. The test techniques used to make these measurements are generic and apply to any control loop (e.g. switching regulators, LDOs, filters, etc.).
The stability of a POL regulator is shown in the Bode plot in Fig. 1. The gain-phase information for the POL regulator is also shown, using the same data, in a Nyquist plot (Fig. 2).
Specific points along the Nyquist curve can be used to evaluate the effective circuit Q via the proximity to the singular unstable point (1,0).