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7805 regulator stable?

Y

Yzordderrex

Jan 1, 1970
0
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
 
S

Spehro Pefhany

Jan 1, 1970
0
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

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. http://cache.national.com/ds/LM/LM340.pdf

"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
 
J

John Larkin

Jan 1, 1970
0
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. http://cache.national.com/ds/LM/LM340.pdf

"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


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

John
 
P

Pooh Bear

Jan 1, 1970
0
Yzordderrex said:
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?

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
 
O

Ol' Duffer

Jan 1, 1970
0
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'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.
 
M

mc

Jan 1, 1970
0
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.
 
F

Fred Bloggs

Jan 1, 1970
0
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?


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.
 
B

Ben Bradley

Jan 1, 1970
0
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?

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.
 
R

Rich Grise

Jan 1, 1970
0
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?

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
 
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