Re: Wanna see what happens when you don't read the data ***?

Forgive my stupidity, but why should I bother with a low esr cap if I am adding .33ohm series resistance? The difference in ESR between say low ESR tant and say a generic electrolytic will be minimal compared to the ..33ohm resistor you mention.
For that matter a real cheap cap, say 22uF at 16V is going to have an ESR around around that point anyway. A Nichicon PW series (my cheap client loves these for cost) is 22uF 16V = 0.60 ohms @100k @ 20c

Hawker


On 2/28/2007 2:24 AM, The digits of John Popelish's hands composed the following:
Christopher Ott wrote:
Making a long story short, the LM2940 does not like running with a 1uf output cap. It needs at least 22uf. This was a bugger to track down (I thought it was related to the new switcher circuit I had designed, and incorrectly focused my efforts there.)

http://www.ottelectronics.com/photos/LM2940.JPG

Yellow trace is input voltage (from the switcher), blue trace is output. The oscillation was making my A/D's go haywire. It seems such a simple thing to miss.
(snip)

Based on the capacitor ESR stability graph on page 10 of:
http://cache.national.com/ds/LM/LM2940.pdf
I would use a low ESR 22 uF capacitor in series with a .33 or .47 ohm resistor to make sure. If you can generate a pulse load for your regulator, you can easily see the improvement in stability the series resistor gives you.
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