I want to use the LDO regulator LD39300 of ST. To buffer VOUT, I thought about buffering the output in addition with an aluminum capacitor of 6800 µF in parallel to the recommended 4.7 µF.
But in the datasheet at page 10 (figure 14 and figure 15) it is described, that the ESR of the output capaitor must be bigger than ~0.3 ohm. Here is my problem, because an aluminum capacitor of 6800 µF often has only an ESR of a few milli ohm. In parallel to a ceramic capacitor the resulting ESR will be smaller than 0.3 ohm.
Is it right, that this LDO regulator will not work if I put such a buffering capacitor in parallel with the output capacitor?
Best Answer
You are correct in assuming that going outside the stability regions is a 'bad idea'.
It's difficult to predict exactly what will happen if you operate outside the stability region of a fixed-feedback regulator like the one you're describing. The regulator may start oscillating immediately, or appear fine but start oscillating if there's a sudden input or output change.
Blindly adding capacitance here will likely make things worse. Don't add the monster cap, or if you need super smooth output, change to a different LDO.