It’s impossible to directly measure the stability (gain/phase) characteristics of a device if the device’s control loop lacks an access point to apply a test signal or can’t be broken. For many of today’s linear, switching and point-of-load regulators, the part is literally a closed system; even a hybrid regulator might not have an exposed control loop.
Fortunately, a new technique permits non-invasive stability testing of such devices.1 Designers can evaluate stability by measuring (on the bench) or simulating (on a computer) the device’s output impedance. The bandwidth and phase margin are calculated by analyzing the impedance’s group delay.