PICOTEST Application Note | ESR Requirements for Stable Regulators

Application Note | ESR Requirements for Stable Regulators
Application Note Picotest ::  ESR Requirements for Stable Regulators. A Simple Method to...


Application Note Picotest :: ESR Requirements for Stable Regulators.

Application Note Picotest :: ESR Requirements for Stable Regulators.

A Simple Method to Determine ESR Requirements for Stable Regulators.

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. This article discusses a method for the determination of 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.

The majority of linear regulators, regardless of the internal topology, reflect an output impedance that provides all of the information necessary to determine the ESR required to achieve a particular degree of stability.

Without performing an analysis or derivation of the shape of the impedance response, which is beyond the scope of this article, it is possible to measure the output impedance of the voltage regulator without any output capacitors connected to it. Not all regulators are stable in this condition, though most are. The impedance result can be segmented into three distinctive regions. At DC and low frequencies, the output impedance is resistive, with the resistance being related to the load regulation of the regulator and circuit trace resistances. In the second region, the impedance is inductive, with the inductance being dependent on the load current and the bandwidth of the regulator. In the third region, it is possible that the output impedance is again resistive, or not, depending on the regulator........

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