At 08:06 AM 11/12/99 +0000, Mike Rigby-Jones wrote: >Tantalums have their uses, but in general they aren't nice >components. They have very little, if any overload resistance. A small >voltage surge exceeding the working voltage will likely cause the device >to go short circuit, or at least reduce it's life. If you have any >requirements to build a circuit with very high MTBF's then avoid tants. This is a good practicality issue. But doesn't it depend on the likelihood of such a surge? Perhaps specific types of applications should avoid them. >Electroylitic caps are also large. When board space is important, and you >simply don't need a 1uF cap, then why use it? A 1 uF tantalum doesn't require more space than a 0.1 uF tantalum or a ceramic most of the time. This may be a lot different when surface mount technology is used. >A device running at 20MHz (usually) produces harmonics that extend far >beyond the base frequency. At 100MHz a tantalum will be as usefull as a >chocolate fireguard. I'm guesing you based your figures on leaded >components? SMD ceramic caps have a tiny inductance and good for far more >than 10MHz Again, I'm not an EE, so perhaps I'm misunderstanding this issue. But a square-wave has odd harmonics with power dropping as 1/f^2. If the impedance of a capacitor rises proportional to f, then the product still drops. The graph I found was from Rohm, who was describing the capacitor product lines they make. What types of leads were testing, I don't know, but it wouldn't seem as if they would want to portray their own products poorly. I follow the arguments on the capacitor trade-offs, but I can't see how they translate quantitatively into the right down-selection. If size is the premier issue, then small capacity surface mounts would seem a good start. But how do you prove that they will meet bypassing requirements? If over-voltage survival is the key issue, then high voltage capacitors would seem the starting point. But won't the filter caps on the regulator blow too? Won't the chip the bypass is protecting blow instead? Where are these power spikes or surges post-regulation coming from? If we've got them, doesn't it mean the onboard regulation isn't working? This simple issue is an opportunity for me to understand how engineering requirement analysis should be performed to down-select design decisions. In the past, I've just taking the word of others on what the standard practices are. However, I've noticed over the years that the 'standard' has shifted. 25 years ago, you were supposed to put a 0.01 uF ceramic across power on TTL packages that generated incredible noise and used incredible power. Then the recommendation shifted to 0.1 uF ceramic or tantalum. Now we should have a combination of a high value and a small value. People are including large filter capacitors on the outputs of regulators. If board space and cost are key issues, why include multiple capacitors? If package noise and power usage have dropped dramatically, why require more bypassing than 25 years ago? A 7805 regulator was only good to +/- 5% 25 years ago, but now its +/- 1%. Why do we then need more, not less, bypassing? Even if I wasn't a 'rocket scientist' I'd realize there's something wrong with these 'creeping requirements'. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causae scire" ================================================================