On Mar 23, 2010, at 7:08 PM, Xiaofan Chen wrote:

> Very few capacitors will have ESR of more than 2 Ohms

That's what I would have thought, but apparently it's not as true as  
all that.
Almost half of digikey's 10uF tantalum caps have an ESR over 2 ohms.   
The percentage is lower for Aluminum (surprisingly?) and not listed in  
the parametric search for ceramics (although most ceramic caps have  
very low ESRs.)  It seems that these moderately low-value  
electrolytics are the sore point for ESR.  I found this tables:

http://www.your-book.co.uk/design/esrchart.htm

The regulators that have both min and max ESR requirements make me  
particularly nervous.  You can always change to ceramic and add  
parallel caps to lower ESR, but ensuring a minimum ESR in an actual  
circuit with who knows what additional bypass caps "nearby" seems  
fraught with peril...

BillW

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