The vast majority of design issues are directly or indirectly attributable to control
loop stability. The most common offenders:
- Linear regulators
- Voltage references
The control loop stability of these devices can propagate through an entire system.
- Increased circuit noise
- 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
Component manufacturers drive us to it through recommendations
In some cases, manufacturers provide recommendations. One example I often use is
the REF02 voltage reference. The manufacturer's datasheet highly recommends the
addition of a 0.1uF output capacitor on the voltage reference. Read