On Wed, Mar 24, 2010 at 11:38 AM, Richard Prosser wro= te: >> Very few capacitors will have ESR of more than 2 Ohms, so actually you >> can normally ignore the high limit (but may not be true if you use wrong >> type of capacitors). The lower limit is the key here, especially >> for older LDOs. Newer LDOs work with ceramic capacitors with very >> low ESR (mili Ohm range). >> > > Just be a bit careful here as I got caught by it. The small cheap, > electrolytic caps can have marginal ESR. And if they get cold they can > get very much worse. And iIf they get hot they dry out and the ESR > increases also. So if you are going to be working over an extended > temperature range it would pay to have a close look at the specs or > use an alternative type. As per the article from National Semi, do not use Electrolytic for LDO and I have never used Electrolytic for LDO. I used Tantalum capacitor last time. For new designs, typically I use ceramic but occasionally also Tantalum. http://www.national.com/nationaledge/jul02/article.html "The biggest reasons many LDO's oscillate are: a) Using an aluminum electrolytic output capacitor in a design that operates at cold temperatures. Aluminum capacitors may have an ESR in the =93stable=94 range at room temperature, but their ESR increases exponentially as the temperature goes below about 10 degrees Centigrade. These capacitors must never be used with LDO's if cold temperatures can occur in the application. b) Using a ceramic output capacitor on an LDO not designed for it. The typical 2.2 - 4.7 uF ceramic capacitor will have an ESR of about 5 milli Ohms. This puts the ESR zero somewhere around 6 MHz where it clearly won't help compensate the loop. Using ceramics on the output of LDO's which are not designed to work with them is presently the #1 reason for unstable LDO operation. ... Most LDO's designed in the late 1980's and early 1990's were made assuming a Tantalum capacitor would be used for the output capacitor, and so they don't tolerate ceramics very well." > Or if you are happy to pay top dollar for good quality components, > then it's less of an issue. > I am in this market segment where quality is more important in most cases than cost. ;-) -- = Xiaofan http://mcuee.blogspot.com -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist