The inductive nature of voltage-control loops

by Steve Sandler, Picotest, EDN, Feb 05 2015

Focus:
In a recent article, the author noted the presence of a high effective inductance in the output impedance of a voltage reference. One example showed 40 uH, which lead to questions about where this inductance came from. Here, the author answers this question analytically, starting with a model of a voltage feedback loop (as would be found in voltage references, voltage regulators, and op-amps) and the key equations for analyzing its output impedance. In this analysis the inductive component of the output impedance is isolated. Simulations of voltage reference output impedance show its three different regions and the sensitivity of the output impedance to load current and operating current. Two forms of equation for output impedance are derived (partial fraction and exact solution), which are shown to be equivalent in simulation.

What you’ll learn:

• How to understand the origins of the effective inductance seen in the output impedance characteristic of voltage references

Notes:
To understand the impact of the output inductance term on voltage reference performance, see "Voltage References Behaving Badly: Output Caps Are Key Source Of Poor Stability" at http://www.how2power.com/pdf_view.php?url=/newsletters/1412/articles/H2PToday1412_design_Picotest.pdf. A summary of that article appears in this Design Guide under "Voltage Regulators".

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