How To Verify Control Loop Design

The non-invasive stability assessment is a method that uses an output impedance measurement to accurately determine stability without access to the control loop. The non-invasive stability assessment, whether performed as a physical test or with a circuit simulation, is a fast, simple, and inexpensive means to verify or optimize any control loop design.  This measurement is useful in almost all systems, though especially in high-speed, instrumentation, and RF systems.  Improving stability reduces noise in a system, allowing better SNR, dynamic range, clock jitter and many other performance characteristics to be enhanced.  Issues related to noise can be very difficult to trace and fix. Non-invasive testing is sometimes the only way to single out and eliminate potential stability problems.

As discussed in reference 1, the non-invasive stability assessment involves measuring the output impedance at the output of the regulator or switching converter. The phase margin is then determined mathematically from the characteristics of the output impedance. Following are four items you should know about the non-invasive phase margin measurement.

#1: Sometime Bode Plots Aren’t a Choice Many circuits do not have access to the loop which would allow a traditional stability assessment via a Bode plot.  Some common examples where the control loop may not be accessible include voltage references, fixed voltage regulators and integrated audio amplifiers to name just a few.  In other cases, you may have access to the feedback loop; however, components have to be lifted or traces cut. This is not usually acceptable in high reliability systems or in production hardware. In all of these cases, the non-invasive test provides a path for assessing the stability of the loop and optimizing the system performance.  In addition, the non-invasive test is not limited by order of the system.

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