Voltage References Behaving Badly: Output Caps Are Key Source Of Poor Stability

by Steve Sandler, Picotest, Phoenix, Ariz., How2Power Today, Dec 15 2014

Focus:
A voltage reference is susceptible to control loop issues, due in part to the low dc output resistance of the reference and its low power circuitry, which generally results in a relatively high effective output inductance. (An example measurement in this article indicates an effective inductance of 40 uH.) Furthermore, the low dc output resistance results in this output inductor being a high Q inductor that is very sensitive to external capacitance. Yet many voltage reference manufacturers recommend the use of an output capacitor. In this article, a series of measurements are presented to highlight the negative effects of adding an output capacitor to the voltage reference circuit or, in some cases, the effect of an inappropriate capacitor value. Measurements show how the capacitor can degrade a voltage reference’s noise density, stability (phase margin as derived from output impedance), PSRR, and step-load response. Test instruments and probes used in these measurements are identified.

What you’ll learn:

• How to understand and assess the sensitivity of voltage references to output capacitors

Notes:
In a subsequent article in EDN, the author models and analyzes a voltage control loop to show where the high effective inductance of the voltage reference comes from. See "The inductive nature of voltage-control loops" at http://www.edn.com/electronics-blogs/impedance-measurement-rescues/4438578/The-inductive-nature-of-voltage-control-loops . A summary of this article also appears in this Design Guide. See Voltage Regulators.

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