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7805 regulator stable?

A co-worker just asked me what capacitor he should put on the output of the regulator in order to keep it from oscillating.

I recall that many years ago the TI or National data books had some kind of curves suggesting the value had to be less than one value or greater than some other value in order to assure the thing didn't oscillate. Seems most data sheets I can find today are abbreviated and not so much info. The TI data sheet I downloaded just says all testing done with 0.33uf on input and 0.1uf on output.

Can anyone shed some light on this concern?

regards Bob

Reply to
Yzordderrex
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And 7905's will definitely scream with no output cap.

John

Reply to
John Larkin

I've never seen one oscillate with zero capacitance on the output, provided the input is reasonably bypassed. Some people claim they've seen it with some brands, but I'm very, very, very dubious. In fact some, if not most, data sheets actually say that no output cap is required for stability. Eg.

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"It is not necessary to bypass the output, although this does improve transient response."

As they say, the reason to put a cap on the output has more to do with transient response-- so 0.1uF or 1uF or 0.01uF or whatever will work okay depending on what's connected to it.

LDOs, OTOH, will merrily oscillate away if you don't get the output cap in the right range.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

I've seen them oscillate if the output cap is bigger than the input. Aside from that, they usually seem happy with anything within a couple orders of magnitude of 1uF.

Reply to
Ol' Duffer

My recollection is that it needs a capacitor on its input (which can be the big filter capacitor if necessary) but not on its output.

Reply to
mc

After having experienced occasional batch related issues with 78xx and 79xx regulator stability, I always ensure that there's decoupling close to the device on both input and output these days. A 10u electrolytic in parallel with 100n ceramic on the out and 100n close to the part on the in ( the bulk cap is rarely far away ) seems to work just fine. Never had any trouble since.

Graham

Reply to
Pooh Bear

The output capacitor is not chosen on the basis of stability. It is chosen based on maximum percentage error in output voltage tolerable due to a no load to full load instantaneous load transition and vice versa.

Reply to
Fred Bloggs

Bob Pease's "Trobleshooting Analog Circuits" says something about this, but ISTR it's with the adjustable regulator, the LM3-something-something that I can't remember right now. He says use an electrolytic on the output, it's not made to go into the very low impedance of a tantalum. Regardless, you can do Pease' 'ping' test (maybe he doesn't call it that, but I think of it that way. Maybe I've been on the Net too long), couple a square wave through a cap and a 100 ohm or so (perhaps higher to reduce damping effects) resistor into the output, and look at it on a scope. If it has overshoot or especially if it shows ringing, then you know it's near oscillation and less stable than you want it to be. Reading the other responses suggests doing this with a few off the production line too, especially after changing production runs or brand names. Just add it to the things on your "If don't check these things out it could come back to bite my ass" list.

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Reply to
Ben Bradley

Yeah. I'd put a 0.33uf on the input and 0.1uf on the output, as close to the regulator as physically possible. ;-)

Cheers! Rich

Reply to
Rich Grise

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