Electronics > Projects, Designs, and Technical Stuff
LDO bypass capacitors
David Hess:
--- Quote from: wraper on April 24, 2019, 08:05:43 pm ---Don't confuse them with 78xx which are not LDO to begin with and recommended to have 0.1uF cap on the output.
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The emitter follower output of the 78xx series makes them very tolerant of the output capacitance and ESR. Usually 10 to 100 microfarads of output capacitance per amp was used but often no output capacitor was required. The input capacitor in one form or another was always required however.
You might still get into trouble with a lower value solo ceramic output capacitor.
The 79xx series are whole different matter because being a negative regulator with an NPN output stage, the collector is used. They *must* have a "high ESR" output capacitor and typically a 1 microfarad or larger solid tantalum or 10 microfarad or larger aluminum electrolytic was used.
The 317/337 have the same disparate requirements.
exe:
--- Quote from: David Hess on April 24, 2019, 06:58:28 pm ---I would like to see that application note. The only examples of this I have seen indicated the opposite and the only reason designers got away with it was because the large aluminum electrolytic or solid tantalum bulk decoupling capacitor damped the response of the regulator enough to tolerate the small low ESR ceramic part. (1)
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I'm afraid I won't find it. I think it was a datasheet for an LDO with high loop bandwidth. It was saying that an electrolyte may have too big impedance at high frequency (some hundreds of kHz, don't remember exact number) and may not meet requirements of maximum ESR for stability (I hope this sentence makes sense, even though I mix ESR and impedance). As I understand, capacitors are useless above their self-resonant frequency because of phase shift. May be that was also the reason. Frankly, I don't remember details.
David Hess:
--- Quote from: exe on April 25, 2019, 05:03:31 am ---I'm afraid I won't find it. I think it was a datasheet for an LDO with high loop bandwidth. It was saying that an electrolyte may have too big impedance at high frequency (some hundreds of kHz, don't remember exact number) and may not meet requirements of maximum ESR for stability (I hope this sentence makes sense, even though I mix ESR and impedance). As I understand, capacitors are useless above their self-resonant frequency because of phase shift. May be that was also the reason. Frankly, I don't remember details.
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LDOs have different and more stringent stability requirements because their output impedance is higher.
I had a thought about this today; I may have encountered the problem with sticking a ceramic capacitor on the output of a 7805/317 before.
The emitter follower output is low impedance and if driven from a low enough impedance, acts as a negative resistance when driving a low impedance load. This can cause the output transistor itself to break into oscillation if a small low loss capacitor is used.
The result would not necessarily compromise the regulation but it is a major EMI and power dissipation hazard. The resulting oscillation can be difficult to detect without a sufficient bandwidth instrument and applying a probe may stop it anyway.
I have encountered regulators which just mysteriously ran hot and heard several similar reports which may have been due to this. Of course now knowing better, this is one of the things I would immediately check for.
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