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Forum Index : Microcontroller and PC projects : Micromite Vcap Capacitor
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Geoffg Guru  Joined: 06/06/2011 Location: AustraliaPosts: 3167 |
A few designs for the Micromite are using an SMD capacitor for Vcap (pin 20 on the 44-pin chip or pin 7 on the 44-pin chip) and I thought that a couple of observations would be in order. If you are using an SMD capacitor you then have the choice of using a multilayer ceramic (MLCC) which is only available in SMD packages. A multilayer ceramic is MUCH better than Tantalum and is also cheaper. I would recommend a 10uF ceramic with a X5R or Y5V dielectric, 6.3V minimum. Because the PIC32MX150/250 is so forgiving you can easily get away with a 10uf Y5V which are quite cheap (eg, Element14 1759485 which are about 25 cents). If you are stuck with a through hole component your only choice is Tantalum and in that case I would recommend a 47uF 16V Tantalum. I tested a number of cheap 10uF Tantalums (from Futurlec) and their ESR was greater than 1 ohm (which is not good) but all 47uF 16VW Tantalums have been OK. This is because the higher the capacitance and working voltage the lower the ESR will be. If the capacitor is not right you will find that the Micromite will be sensitive to glitches on the power or ground lines (ie, will randomly reboot) or in severe cases will not start up at all. Happy Micromiting !! Geoff Geoff Graham - http://geoffg.net |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
Thanks Geoff, this is just in time as i wanted to order some parts and send my PCB design. Will check the footprint first to make sure. Good to know that the MLCC are better AND cheaper. What would you want more from a component. :) Microblocks. Build with logic. |
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| WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2794 |
Hi Geoff, I notice that the SMD MLCC you recommend (Farnell 1759485) is +80%/-20% tolerant. This seems a bit excessive. Have you actually used these within a MicroMite or are you just suggesting it based on it looks ok? I will order some regardless - just wanted to know if you've experienced them . . . . WW For everything Micromite visit micromite.org Direct Email: whitewizzard@micromite.o |
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| Geoffg Guru Joined: 06/06/2011 Location: AustraliaPosts: 3167 |
All ceramics have a wide tolerance range but the exact value is not so important as the ESR and how it changes with temperature and that is dependent on the dielectric. The part number that I quoted was an example of a suitable part as I cannot remember exactly where my current stock came from. I used a 10uF ceramic with Y5V dielectric in the GPS Tracker and that has worked well. That is what Jaycar ship in their kits and hundreds of them have been built. For the Colour Maximite I recommend a 10uF ceramic with a X5R dielectric because the faster processor is more sensitive to the capacitor's characteristics and thousands of them have been built. The reality is that any 10uF multilayer ceramic will work just fine with the PIC32MX150/250. I just quoted the Y5V dielectric as an additional safety factor. Geoff Geoff Graham - http://geoffg.net |
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| WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2794 |
Ok- have ordered a few hundred of two types. One X5R, the other Y5V. Much cheaper than the Tants like you say. They will arrive within 12hrs so will build a few 44-pin Modules and see how they go. Thanks for the initial post - could help shave a bit off the total cost of the module 
WW For everything Micromite visit micromite.org Direct Email: whitewizzard@micromite.o |
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