adding ceramics across power pins
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Posted by Jon Slaughter on July 25, 2008, 4:32 am
Please log in for more thread options such as tantalum. e.g., 1uF tant an 0.1uF cer? In theory they just add to 1.1uF and it shouldn't make any difference. It seems that one should then also add 0.001uF and so on? Why don't they make special "power" caps that combine tantalum and ceramics in one package just for this purpose then? Thanks, Jon | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Leon on July 25, 2008, 6:46 am
Please log in for more thread options http://www.google.co.uk/search?hl=en&q=decoupling+capacitor&btnG=Google+Search&meta= Leon | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Jon Slaughter on July 25, 2008, 8:50 am
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>> Why does one have to add ceramic's across power pins along with larger
>> caps >> such as tantalum. e.g., 1uF tant an 0.1uF cer? >> >> In theory they just add to 1.1uF and it shouldn't make any difference. It >> seems that one should then also add 0.001uF and so on? >> >> Why don't they make special "power" caps that combine tantalum and >> ceramics >> in one package just for this purpose then? >> >> Thanks, >> Jon >
http://www.google.co.uk/search?hl=en&q=decoupling+capacitor&btnG=Google+Search&meta=
> >
That doesn't answer my question. Why does the theory fail in practice? Why doesn't a capacitance add? I know that caps have a non-capacitive impedence but surely its no that bad? What I'd like to see is the frequency response of a tantalum cap with and without a ceramic to see how it actually works... it's nice to know that it should be done but I want to actually know how useful it is(so far for all my projects I have gotten away with just tant's... of course I don't do anything about 40mhz) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Posted by Guy Macon on July 26, 2008, 8:10 am
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Much depends on the application. For example: Example [1]: When I designed a ground support test rig for military cargo aircraft actuators, I was faced with a project that had no weight constraints (the fixture already weighed several tons) no size constraints (I had a full-height 19 inch rack for the electronucs and the bought gear used up less than half of it) no power constraints 15A at 120V, and I could have gotten 100A 3-phase if I had asked for it) no real per-unit cost constraints (we made up 10 copies of each board, built and tested three boxes, and put one into use with the rest stored as spares). The real constraints were minimum downtime, repeatable results, getting the fixture into service as fast as posible, and ease of use. For that application I put a ceramic cap on each device as close to the power pins as possible, used power and ground planes, and sprinkled handful of tantalum caps about. I could have gotten by with far fewer, but I didn't care. Example [2]: When I designed electronic toys for a major US toy manufacturer, the top three priorities were low unit cost, low unit cost, and low unit cost. At a production rate of over 100,000 units per hour for that one toy, a one cent reduction in unit cost equals a pure profit of $1000 per hour / $168,000 per week. That one got zero bypass capacitors. I tested the pilot run (a thousand toys) for any loss of function and I crawled through the electronics of three samples with a scope and DMM looking for any problem areas that might require a bypass cap, and would have added one if needed. The microcontrollers we use in toys nend to be designed to work well with just a battery supply and no bypass caps. -- Guy Macon <http://www.GuyMacon.com/>
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Posted by MooseFET on July 25, 2008, 9:21 am
Please log in for more thread options > Why does one have to add ceramic's across power pins along with larger ca=
ps
> such as tantalum. e.g., 1uF tant an 0.1uF cer?
> > In theory they just add to 1.1uF and it shouldn't make any difference. It > seems that one should then also add 0.001uF and so on? All real components have some amount of inductance. The best you can do on inductance is limited by the mechanical size of the part. To have a very low impedance at very high frequencies, you need the inductance to be low and hence want a mechanically small part. At lower frequencies, you need a lot of capacitance to make the impedance low. This favors a large mechanical size. It is hard to make a component that is both large and small at the same time. >
cs
> Why don't they make special "power" caps that combine tantalum and cerami= > in one package just for this purpose then?
> > Thanks, > Jon | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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> such as tantalum. e.g., 1uF tant an 0.1uF cer?
>
> In theory they just add to 1.1uF and it shouldn't make any difference. It
> seems that one should then also add 0.001uF and so on?
>
> Why don't they make special "power" caps that combine tantalum and ceramics
> in one package just for this purpose then?
>
> Thanks,
> Jon