A single, simple measurement is described that allows the determination of the ESR required to achieve a desired phase margin, using a particular value of output capacitor. Solving the ESR requirement at the lowest operating current provides a stable solution for higher operating currents as well.
Engineers often have trouble analyzing and achieving stability for the linear regulators they employ. Given how little stability information is published by the manufacturers of these devices, this is not surprising. In most linear regulators, the output capacitors’ ESR provides the control loop zero, stabilizing the regulator. The datasheet generally offers little information regarding the stability of the regulator as a function of the load current, output capacitance and output capacitor ESR, which are the external parameters which impact the stability of the regulator. Additionally, the latter two parameters are often uncontrolled by the manufacturer.
Past articles on this topic have focused on the criticality and impact of phase margin and overall stability, as well as how to measure the stability using either invasive or non-invasive techniques, therefore, this article provides a different perspective. We discuss a method here to determine the capacitor ESR required to achieve a particular phase margin for any output capacitance value, based on a single simple measurement. That is, if you know the output capacitance and output load current, you can tell what value of ESR will lead to a configuration with the desired phase margin.