Post by thread:[EE]: Decoupling Caps

Ok, I need some help. Do decoupling caps have to be electrolytic? Is it better to have some smaller value caps, rather than none at all? Do they need to be polarized? I'm trying to design a super small circuit, and the large electrolytics I usually use are too large. I have some salvaged SMT ceramics that would be cool. Josh -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Josh Koffman wrote: >Ok, I need some help. Do decoupling caps have to be electrolytic? Is it >better to have some smaller value caps, rather than none at all? Do they >need to be polarized? I'm trying to design a super small circuit, and >the large electrolytics I usually use are too large. I have some >salvaged SMT ceramics that would be cool. > I usually have a 100n (0805) and a 47u 5mm (?) can for surface mount boards. David... -- ___________________________________________ David Duffy Audio Visual Devices P/L U8, 9-11 Trade St, Cleveland 4163 Australia Ph: +61 7 38210362 Fax: +61 7 38210281 New Web: http://www.audiovisualdevices.com.au ___________________________________________ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

Hi Josh, I'm speaking in general terms here since we don't know much about the specific application: If these capacitors will be used as local IC decoupling capacitors, it would be rare that you would see electrolytic capacitors used. Usually you will see ceramic (most likely) or tantalum capacitors for local IC decoupling capacitors. (Keep them as close to the ICs as possible -- keep the lead lengths or PCB traces as short as possible). Common capacitor values for decoupling ICs are 0.01 uF and 0.1 uF. If you do happen to use polarized capacitors, you definitely want to observe the polarity. I'm sure you will get all sorts of answers, but just a few additional things to think about: 1) If you use tantalum capacitors, just remember that there have been tantalum shortages in the past which can significantly drive up the cost of your product if you use a lot of tantalum capacitors and the industry happens to be in the middle of a tantalum shortage. Remember Horatio Alger? :) 2) Observe the working voltage rating of the capacitor -- do not violate it -- good engineering practice is to perform derating (for example: use 10V WVDC capacitors (instead of say 6.3V WVDC) if you use them for the 5V supply to ICs). 3) There will be instances in which you will see the physical size of the capacitor significantly increase as the DC working voltage rating (WVDC) increases. If you are trying to keep things small, keep this in mind: there's usually no need using a capacitor with a 100V WVDC rating if you have access to a smaller size 10V WVDC capacitor *as long as you follow #2 above*. A lot of companies should have application notes on using decoupling caps -- a few minutes web-searching should yield a treasure chest. Try this for starters (not much meat on the bones, but at least it's a start): http://www.eettaiwan.com/ARTICLES/2001MAY/PDF1/2001MAY03_AMD_DSP_AN1148.PDF Note: Cut and paste URL if it is word-wrapped. Good luck! Best regards, Ken Pergola -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

Ken Pergola wrote: > Try this for starters (not much meat on the bones, but at least it's a > start): > > http://www.eettaiwan.com/ARTICLES/2001MAY/PDF1/2001MAY03_AMD_DSP_AN1148.PDF Slightly of-topic, but.. That is an 7 page PDF file, the page contents are : page 1 : Title page and TI logotype. page 2 : Disclaimer. page 3 : Trademark notice. page 4 : Contact information (4 lines) page 5 : Contents page (3 lines) page 6 : Abstract (1 sentence of 12 words) page 7 : (called "page 8" !) Aha ! Some decoupling info ! (The info in the doc is OK, b.t.w...) :-) :-) Jan-Erik. -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

Josh Koffman wrote: > Do decoupling caps have to be electrolytic? Is it > better to have some smaller value caps, rather than none at all? > Do they need to be polarized? I'm trying to design a super small circuit, and > the large electrolytics I usually use are too large. I have some > salvaged SMT ceramics that would be cool. As an "rule of thumb", the higher the frequency the smaller the cap value. Higher value electrolytics for mains "hum", and smaller ceramics for high frequency "swithing" spikes. Electrolytics are, by design, bad decouplers for high frequency signals. Jan-Erik. -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

Decouplers.....just back from XFEST2002 Need a good variety of small to large. Most of the smaller caps are typically called out to be X7R or X5R MLC types, and the larger ones at least one ought to be a tantalum for the flatter frequency response. Just watch the total ESR and the package sizes and the layers going thru. Sometimes you will need to put the really smallers ones on the top so they are closer to the power plane. Then of course you need to watch the total ESR of the system......... Back in the day.......when it was simple.......just a decoupler on every chip. These days with muliple layers and power planes......its an art...that needs to be done right else the system might not work as well as you designed it to. {Quote hidden}>From: Josh Koffman <listsjoshspam_OUT3MTMP.COM> >Reply-To: pic microcontroller discussion list <@spam@PICLISTKILLspamMITVMA.MIT.EDU> >To: KILLspamPICLISTKILLspamMITVMA.MIT.EDU >Subject: [EE]: Decoupling Caps >Date: Sun, 12 Oct 2003 23:48:48 -0400 > >Ok, I need some help. Do decoupling caps have to be electrolytic? Is it >better to have some smaller value caps, rather than none at all? Do they >need to be polarized? I'm trying to design a super small circuit, and >the large electrolytics I usually use are too large. I have some >salvaged SMT ceramics that would be cool. > >Josh >-- >A common mistake that people make when trying to design something >completely foolproof is to underestimate the ingenuity of complete >fools. > -Douglas Adams > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body _________________________________________________________________ Instant message with integrated webcam using MSN Messenger 6.0. Try it now FREE! http://msnmessenger-download.com -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

Thanks for the advice everyone! I see that I had mixed up terminology between power supply filter caps, and decoupling caps. Thanks! Josh -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu