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Forum Index : Microcontroller and PC projects : Vcap for surface mount
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| viscomjim Guru  Joined: 08/01/2014 Location: United StatesPosts: 925 |
I am attempting my first surface mount PCB using the uMite. I always use a 47uf tantalum when I use the DIP version of the uMite, however, I noticed that the folks here that have made surface mount boards use a 10uf (???) surface mount cap. Could you please let me know the part you use for this? I will be using the PIC32mx170f256b (28 pin) in the SOIC package. Thanks!!!! |
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Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 767 |
Hummm... Maybe something in a 1206... Digikey Part This one is a 5% part.. There is cheaper, but this is a important cap... There is also stuff in 805... |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
Holy Smokes thats expensive! But you are correct, it is an important part for sure. 1206 is about the smallest I want to start with. Thanks for the info. Is this the one you use on your 28 pin board? |
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| akashh Senior Member  Joined: 19/01/2014 Location: IndiaPosts: 115 |
Also make sure it is X7R rated. I pulled some hair out trying to figure this out, most boards would boot fine but a few didn't and the cap was the culprit. I think the normal caps vary a lot in their values and probably operate near the limits of what the chip can handle. |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9593 |
The ones I use are X5R type, and this is the minimum spec. X7R are fine too, as mentioned by akashh, and X7R actually have a higher stability at high temperatures then X5R, but for use with the MM, X5R is fine - either will do. DO NOT use cheap ceramic caps(X7S, Y5V, Z5U) for Vcap, as this cap is critical to the stability of the PIC32 core. Here are the ones I use. This is a NZ website, but the part # should be the same for the American Element14 website. By my calculations, it would be US$2.90 for ten, or 29c each. With this cap from Element14, you cannot buy less then multipules of 10. EDIT: Here is a chart to help you choose the right cap characteristics: 
Smoke makes things work. When the smoke gets out, it stops! |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
Wow, that is about 10% of the cost of the X7R. I think I will try these. At least they have a reliable track record with you Grogster. Thanks for the info. I was getting worried that the Vcap would cost more than the PCB... |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9593 |
Don't get me wrong - nothing bad about akashh's suggest of X7R. It would work fine, but as it is rated for a higher temperature, which you won't be exposing the cap to when used with the MM, then paying the extra for X7R is, IMHO, a waste of cash. Note how if you were using something like Y5V, then the cap can vary in capacitence up to 82%!!! That is a hell of a lot!!!  Yes, it is proportional to the ambient temperature the cap is exposed to, but that still represents a huge amount of change.  Smoke makes things work. When the smoke gets out, it stops! |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
Cap university. Thanks for that info Grogster. Very informative. |
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| Zonker Guru Joined: 18/08/2012 Location: United StatesPosts: 767 |
Sorry Boy's.. I didn't notice the price tag...!  |
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Lou Senior Member Joined: 01/02/2014 Location: United StatesPosts: 229 |
JIM, The part we use on the LCDuMite and most Zonker boards is from Digi-Key, part number 1276-2891-1-ND. This is a 10uf 25v 10% X5R 0805 part made by Samsung. We buy 1000 parts for $60.16 USD. The 1206 version of this Samsung part is 1276-1075-1-ND from Digi-Key, 1000 parts for $59.93 USD (100 parts for $10.20 USD). We have used the 0805 part for 44 pin TQFP and 28 pin SOIC parts without problems. Lou Microcontrollers - the other white meat |
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| srnet Senior Member Joined: 08/08/2014 Location: United KingdomPosts: 164 |
I understand its important, but why exactly is the choice of ceramic type critical ? The data sheet spec calls for a minimium of 8uF, so say a 22uF ceraimic ought to be well within limit at all temperatures. The only other requirement I can see is for ESR to be lower than 1R. $50SAT is Silent but probably still working. For information on LoRa visit http://www.loratracker.uk/ |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9593 |
Ceramic type is NOT critical. You could also use a tantalum. The general rule of thumb I have been seeing, is that you use 10uF X5R ceramic, or 47uF tantalum. The main point of interest, is that if you do use ceramic, make sure it is at least X5R. From the PIC32MX470 PDF(just cos that is the PDF I had open, but they are all the same as far as Vcap is concerned): Smoke makes things work. When the smoke gets out, it stops! |
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bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Hi ViscomJim, I have always used a 47uF SMD either 1206 or 3225 size for my boards.. I know they are dearer than some of the ceramic 10uF but I have been scared off the 10uF part since the UBW-32 days... I have had quite a few of them `play up' because they had only a 10uF VCap.. They did some crazy things that wasn't immediately attributable to the cap... All that aside WhiteWizard uses 10uf Ceramics (he sent me a link a long time ago but I have since misplaced that one) with no issues.. I guess the story is if you use a 10uF ceramic make certain it is a good brand and either X5R or X7R. At this stage I will keep using my 47uF tantalums.. Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
Mick, are you using a surface mount 47uf tantalum and if so, could you post a link? Thanks!!!!! |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2949 |
Hi Jim, These are the ones I prefer but they are not super cheap at about 80c (AUS) each.. Tantalum However they are often, as is the case now, out of stock.. Then I get another similar type, there is quite a variety out there.. I look for either one of the following sizes 1206 3525 (some, as is this one, are 3528) and 6v3 and low ESR (Equivalent series resistance) I believe Microchip state below 2 Ohm ESR but I look for less than 1 Ohm and these are 250 mOhm.. I like RS Components because they offer free postage in AUSTRALIA.. I am not sure they have a store in the US. Regards, Mick EDIT *** I just checked their web page and their USA rep is Allied... they are dearer over there than here.. No wonder they are often sold out, you Americans are buying our stock... 
Anyway here is the Allied link Allied Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
Mick, Thanks for the info and link!!! |
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OA47 Guru Joined: 11/04/2012 Location: AustraliaPosts: 982 |
Just ordered and received a few hundred 10uf X7R ceramic caps from Element14 and having been tearing what is left of my hair out trying to work out why the Micromite II units wont boot. It seems that these units are not compatible with the PIC chips. Part Number 165-7936 AVX 08056C106KAT2A SMD Multilayer Ceramic Capacitor, 10 µF, ± 10%, X7R, 6.3 V, 0805 [2012 Metric] Element14 link to 165-7936 I thought that with the earlier posts I would order this type due to their good price and what I thought was good specs. Bigmik it looks like I will be back to the tantalums.
GM |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9593 |
I would have thought those would work OK, as X7R is actually better spec then X5R(X7R upper temperature is +125'C vs X5R's +85'C) The X7R's should be working fine. Interesting..... Smoke makes things work. When the smoke gets out, it stops! |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
They should but somehow they don't. I also bought a few hundred of them. Not worked. tried it with MX150 and MX170's Then i tried a X5R also 10uF and they did work. Why, i have no clue! I thought the '7' stood for higher temperature range, but somehow there is something else which causes the MX150/MX170 not to start up right. Sometimes they work for a few seconds and once a pin is toggled (low to high) the chip resets and hangs. Microblocks. Build with logic. |
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| isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689 |
Some ceramic smds have a high-ish (low frequency) resistance so the esr is also a no-go. The good-spec ones are more expensive, I have found the Murata's always good but are a bit hard to find. You need to check the spec at a few Mhz. ed - In fact some of the usual ceramic through hole bypass 0.1uf's actually act as a open circuit at a few Mhz... also some of the push-the-envelope smd ceramics are altered by soldering conditions. And to make it more confusing - I think some tantalum's (resin dipped) are actually electrolytic and the others are ceramic ! I just though that would help |
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