Raspberry Pi "Hat" powersupply question & eagle component - Page 1
Forum section: electronics / projects

author: Toffe
author_rank: Newbie
timestamp: October 27, 2018, 09:26:20 pm
content: Hello.I'm trying to make so I can power a RPI from my hat with a LiPo Battery.So I have two question regarding that.Question 1. How to know what kinda capacitor to use and how to distinguish them on schematic?Using a TPS54202 as a power supply on this hatNow they say at the datasheet to use a "A ceramic capacitor over 10 µF"!and ". An additional 0.1 µF capacitor (C2) from VIN to GND is optional to provide additional high frequency filtering"So basically I'm wondering is both ceramic? Does it matter? and how do i "seperate" them from symbolics? And what decides what kinda cap I can use for them? Want to use 0603 caps (easier to handsolder and not that big).Question 2. Trying to recreate a eagle component, not sure I'm on the right path?The component and datasheet is: PCA9306DCTR (http://www.ti.com/lit/ds/symlink/pca9306.pdf)On the schematic for DCTR version it says Package Type SM8 and Package Drawing DCT. Going down to the DCT drawing I my outline. Then i found ref-packages.lbr which supposedly had both SM8 and R-PDSO-G8 (defined after the title DCT on the datasheet).So I've laid the SM8, and R-PDSO-G8 and my own created one side by side on a 0.1mm grid and there are big differences, which worries me a bit. I trought I managed to create a perfect package following the datasheet reccomandation but seems like I am waaay of? Any tips? See attached image where SM8 is left, mine is middle and the R-PDSO is right side.

author: Buriedcode
author_rank: Super Contributor
timestamp: October 27, 2018, 11:15:01 pm
content: Quote from: Toffe on October 27, 2018, 09:26:20 pmI'm not sure what you mean. Are you referring to package sizes? A 10uF will most likely be larger than a 100nF one. Whilst you can get 10uF in 0603, please be aware of voltage derrating - their capacitance can drop significantly with a DC bias even half of its rated value. This effect gets worse with smaller packages. Example, a 0603 10V 10uF cap, with 5V across it might only be ~2uF. For this reason I tend to pick the largest package, within reason. 1210 or something for a 10uF ceramic, rated at 25V. Also a 100nF in parallel with this - I'll let you google why that is recommended.If in doubt - look at the evaluation board for the device in question:http://www.ti.com/tool/TPS54202EVM-716I'll show you what parts they used, and include a BOM so you can check for yourself what caps (capacitance, voltage, and type, like X5R, X7R etc..) they recommend.Quote from: Toffe on October 27, 2018, 09:26:20 pmUnfortunately eagle can have odd naming convension with libraries. To muddy the waters different manufacturers can used different names as well. Often parts are available in multiple packages, so double check the part number you order - usually the suffix for that package will be in the datasheet. This device is a good (well, bad) example. "SM8" could mean anything but I'm assuming its SOIC8 which is a standard 0.05" pitch part, but according to the datasheet its 0.65mmThen there's TSSOP8, or SSOP8, MSOP8 VSOP8 which can have 0.65mm pitch or 0.5mm pitch, with different body lengths (the middle one in your pick is shorter than the TSOP8 on the right). "R-PDSO-G8" I don't think is a package name but rather a datasheet reference.As you've already make the package in eagle, I would go with that, as it is based off measurements in the datasheet, rather than relying on an existing eagle library to have the correct name. If you want to double check - print out the layout on paper and use a real part to double check the foot print - and to make sure the pads are long enough for hand soldering. Whilst it is nice to just use an existing library (saves time) I always have to double check the dimensions.