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How to Solve 8 of 10 Design Issues

The vast majority of design issues are directly or indirectly attributable to control loop stability. The most common offenders:

  1. Linear regulators

  2. Voltage references

  3. Op-amps

The control loop stability of these devices can propagate through an entire system. Symptoms include:

  1. EMI

  2. Increased circuit noise

  3. Clock jitter

These are all simple circuits, so why is this a common issue?

We are not clear on the definition of stability

In the high-reliability world, including satellite systems, where I do most of my troubleshooting, stability means a minimum phase margin of 30 degrees and a gain margin of 6dB at the end of life, including all component variations and environmental factors. There are guidelines that define the stability margin limits, and one of our tasks is to ensure that margin.

This requirement is not well aligned with the information given out by manufacturers. They generally do not provide quantifiable metrics for the stability of their devices. In many cases, the devices will not meet a 30-degree phase margin even typically when coupled with low-ESR capacitors. The definition of stability in the semiconductor world seems to be whether the circuit has a stability margin that is greater than zero.

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