An Unconditionally Stable Linear Voltage Regulator

Focus:

Contrary to popular belief, linear voltage regulators are not unconditionally stable and the ESR of the output capacitor, if not properly chosen, can induce instability as the authors discussed in a previous article. Here, the authors take the discussion a step further and ask what steps can be taken in the design of a voltage regulator to make it unconditionally stable? Their simple answer is to avoid use of an output capacitor. But recognizing that this is impractical, they proceed to analyze voltage regulator stability using the concept of Q, concluding that infinite gain bandwidth in the regulator control loop leads to unconditional stability. Practically speaking, this means that designing a voltage regulator to have very high bandwidth can lead to very stable regulator performance across load capacitance and load current ranges. An example of a custom-built regulator with 300-MHz bandwidth is presented and its performance (step-load response, output impedance, and PSRR) is measured. With the latter two measurements, performance is compared against the iconic LM317 and the superior stability and ripple rejection of the high-bandwidth design is discussed.

What you’ll learn:

• How to achieve very stable performance in linear voltage regulator designs
• How to understand the factors contributing to instability in linear voltage regulator designs

Notes:

1. This article was published in the April 2012 print edition of PET magazine. 2. This article references “A Simple Method to Determine ESR Requirements for Stable Regulators” Power Electronics Technology, August 2011. But since the discussion here revolves around the relationship between Q and stability, you may also want to read “New Technique for Non-Invasive Testing of Regulator Stability”, Power Electronics Technology, September 2011 as this gives the equation for phase margin as a function of Q.

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Author & Publication:

Steven Sandler and Charles Hymowitz, Picotest, Power Electronics Technology, Mar 23 2012

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