Many engineers are familiar with the Bode plot as an effective stability assessment method. However, some authors suggest and even teach that the Bode plot is the only method needed. This article shows why this thinking is short-sighted. A single, low cost instrument that can produce Bode plots, as well as two other stability assessment methods is discussed providing a more comprehensive stability assessment set of guidelines.
A single word illustrates why Bode plots cannot be the only voltage regulator stability assessment method you need to use.
Preceding the words voltage regulator with“fixed” is sufficient proof that the Bode plot cannot be our only assessment method. The fixed output voltage regulator does not provide an accessible point to either inject a signal or to measure the Bode response of the control loop [1, 2]. As a result, these fixed voltage regulators are often not assessed for stability margin and, unsurprisingly, are often found to have poor stability. Fixed regulators absolutely need stability assessment just as much or more than variable output regulators. Of course the fi xed voltage regulator is not alone in this predicament. Other devices such as voltage references, integrated POLs, simple switchers, integrated class D audio amplifiers and many other devices fall into this category as well. They simply lack access to the feedback loop, prohibiting the use of the Bode plot for assessment, thereby eliminating it as potential measurement solution.
There are other conditions that can prohibit the use of Bode plots as well [3]. Some examples include opamps, which while the loop might be accessible can have bandwidths that are too high to allow injection without influencing the measurement result. Other examples include hysteretic regulators, as well as regulators with multiple control loops, which might only provide access to one of several of the control loops [4].