I'm building a standard LM317 supply for testing/general use:
Image source: ElecCircuit – Variable power supply using LM317
The datasheet states the output cap (C3) is not needed for stability, so I will omit it. I have also read low ESR caps for C3 can cause instability. So what happens if this supply is eventually connected to something with large and/or low ESR caps on its power input?
Won't those caps just act like C3 in the circuit and be liable to cause instability?
Will using a high ESR cap for C3, instead of omitting it, help prevent instability caused by whatever is connected to the supply?
If not, how is this dealt with?
Best Answer
If you can live with the poor transient response when the load changes and the inherent noise produced by the LM317 then that's fine.
Output capacitors can cause instability. After all, the regulator is a control-loop and it has a phase margin of maybe 60 degrees. Adding the wrong amount of capacitance can cause a control-loop to become unstable. Too little can cause a noisy output and too much can lead to sluggishness in response when starting. Less than enough but more than too-little can cause some regulators problems sometimes and, this is alleviated (in some designs) by having (circa) a couple of ohms in series with the capacitor (aka ESR).
On the other hand, the LM317 is pretty stable (albeit being a very noisy regulator): -
Stick to the recommendations: -