What is Stability (as it applies to regulators)?
A control loop is stable if, at the frequency when the gain has reduced to 0dB (unity gain), the total phase shift is less than 360. Phase shift usually includes a 180 phase shift due to use of negative feedback, leaving a maximum of 180 shift due to feedback and other elements in the control loop. The difference between 360 and the total phase shift at the unity gain frequency is the phase margin. Stability is typically considered good if the phase margin is at least 45. For regulators, we usually aim for a greater phase margin, typically 60-80, to allow for variation in component values that affect the frequency response. When a switching regulator is unstable, the average output voltage will often be between 40% and 90% of the target. This is because the switch duty cycle is swinging between 0% and nearly full cycle, at a frequency approximately the bandwidth of the regulator. The output voltage will show this cyclical pattern, with the switching frequency superimposed. The regulato
