Wednesday, July 16, 2014

Pi Power BOM details

If anyone is interested in building your own Pi Power (it is open hardware, after all), there are some details of the BOM (bill of materials - fancy name for a parts list) that I thought I'd share.

R1, R2, R4, C2 and C3 are fairly ordinary. I use 0805 parts. For the resistors, I buy 1%, simply because when you buy a reel of them, the price difference for 5% or worse isn't worth it. For the caps, 50 volts is fine. I forget which dielectric (X7R or NP0) I used. It hardly matters.

C4 is a crucial component. It's an OS-CON electrolytic. It needs to have minimal ESR and has to be able to tolerate the ripple currents - which won't be insignificant at 2 amps of output current. It's 100 µF at 20V.

C1 is a bit of an oddball. I'm used to input filter caps being electrolytic, but TI's design tool spat out a huge ceramic one. It's a 1210 X5R 25V. I currently use a Murata GRM32ER61E226KE15L.

R3 is another weird one. It's 25 milliohms and a half watt. They're in a 1206 form factor. I buy them from Stackpole.

L1is a 15 µH inductor rated at 5A of current. I'm using a Bournes SRP1040-150M. Large surface mount inductors are always fiddly to deal with because there are generally no standard footprints for them. I happened to find an Eagle library that lined up with the inductor I needed.

D1 was chosen for its forward current capacity. Not much else is special about it.

D2's purpose is both as a reverse polarity protection and to absorb voltage surges. Its breakdown voltage is a bit higher than the maximum rated input voltage for the circuit. If you're going to use Pi Power in a more challenging environment - in particular, a car - then you probably should augment the input protection - perhaps with an MOV and a fuse.

Q1 had to be upgraded. The original SOT-23-3 MOSFET that I tried got too hot at the maximum rated current. It got hot enough, in fact, that on my prototypes you can't read the markings on the top anymore. The replacement going forward will be a Vishay SI3443CDV-T1-GE3. It's going to have a nice, big heat spreader under it on both sides of the board with 3 chimneys underneath the case. If that isn't good enough, the next step would have to either be a D2PAK part or a TO-92. I seriously doubt that will be necessary. We'll know for sure in a couple weeks.

IC1 is an LM3485. There are two basic variants - the LM3485MM and LM3485MMX. I can't tell what's different about them, but they're the same price (at least for cut-tape) at DigiKey. I've been buying the MM one.

The power jack is the one I've been using ever since the very first SMD thing I've made. It's from CUI, and the only downside is that you can only conveniently get them from DigiKey, so when DigiKey is out of stock... poof. :(

That, and the board, are all you need to make your own Pi Power.

Or you can save yourself the bother and just buy one from me!

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