From: Winfield Hill
Subject: Re: LM317 or 7808 is better performance for 9V output ?
Date: 11 Dec 2002 12:01:43 -0800
Organization: Rowland Institute
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>In article , Winfield Hill
>> A small capacitor on the controlled node is often helpful with
>> these circuits, improving the regulator's transient response by
>> counteracting the rising Zout of the opamp at high frequencies.
> 100% feedback and a capacitor on the output... Hmm, isn't a touch
> of over-compensation needed to keep the op-amp phase margin nice?
Yes indeed, either a very small cap (100pF), or a rather large cap
(brute-force compensation), or a series resistor would seem to be
wise. This whole business has a rather emphirical aspect to it, so
one would want to test the overall transient stability at different
Looking at a recent design (using a LM317 as it happens), I see a
1000pF cap directly on the opamp's output. It's an OPA2251, which
is a dual micropower opamp (25uA supply), operating with a gain of
+2.7 at a current of -5mA. The opamp's data sheet curves show this
condition to be nicely within a large stable region. Even so, I'm
surprised to see I didn't add a series resistor.
The 2nd opamp half is operating at a gain of -1.0 and a current of
+65uA, which the curves show to be marginal with 1000pF. I see that
here my design has a 1.5k series resistor.
As for a LM358, I'd probably add a 100-ohm resistor, etc. However
that would drop another 450mV, further muddying the design waters.
Again illustrating the appeal of using a LM317 in the first place.