I have a slightly ambitious project for my first PCB: I want to control a wall of WS2812 LEDs from an Altera CycloneIV. The LEDs are connected to a rail carrying the 5V supply, at up to 350W, and expect their control signals at 5V as well.

For the FPGA, I need 1.2V (core), 2.5V (PLL) and 3.3V (I/O).

I'm routing the I/Os (40 of them) to voltage converters to bring them up to 5V and get ESD protection.

My current approach would be a linear regulator for 3.3V, as I expect the current draw to be minimal, and there isn't much room to work with; for 2.5V and 1.2V I'd use a switching converter each and add large capacitors for stability.

For additional safety I could place reverse-facing diodes between the five voltage levels, although I'm not sure these are actually required.

Is that a viable approach, or can this be improved somehow?

  • \$\begingroup\$ 350 W / 5 V = 70 A !!! \$\endgroup\$ – Cornelius May 20 '14 at 16:40
  • \$\begingroup\$ I would expect the I/O pins controlling 350 W worth of LED's to draw more current than the PLL circuit, but you know your plans better than I do. Also, compared to 350 W worth of LED's, whatever the FPGA core draws will be insignificant, so why sweat it? Use whatever's easiest to implement. \$\endgroup\$ – The Photon May 20 '14 at 16:53
  • \$\begingroup\$ This isn't ambitious as a first project... it's madness! Seriously though, a Cyclone IV design isn't a piece of cake. For as much power as you'll be using, I would seriously consider using LDOs for the core and PLL voltages. There is A LOT of information about power and bypassing requirements for FPGAs on Altera's site, including an Excel worksheet to help you with bypassing. Quartus II should be able to help you estimate power requirements on each net too. Not saying it's easy by any means, but good luck! You've got your work cut out for you haha \$\endgroup\$ – Daniel May 20 '14 at 16:59
  • \$\begingroup\$ The actual power is not a problem -- the LEDs get 5V supply and a data line, and the data line is repeated by the first LED in each chain. Right now I have the LED hanging off a GPIO directly, with no issues. \$\endgroup\$ – Simon Richter May 20 '14 at 18:24
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    \$\begingroup\$ I'm not routing the 70A through the controller board, which would indeed be madness. :) \$\endgroup\$ – Simon Richter May 20 '14 at 18:32

You can generate all three rails (3.3, 2.5 and 1.2) from a single chip, the Alegro A4490: Triple Output Step-Down Switching Regulator. It is available from Digi-Key in a 20-VQFN package.

I have used this chip in several projects, and it works fine. You can run at least 2A through each output.

enter image description here

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  • \$\begingroup\$ I've used that in my design now. Still working on a sane routing, but once that is done and the PCBs done I'll report on the success. \$\endgroup\$ – Simon Richter Jun 13 '14 at 14:00
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    \$\begingroup\$ My test board works! \$\endgroup\$ – Simon Richter Dec 2 '14 at 17:29
  • \$\begingroup\$ @SimonRichter Glad that worked out for you. Nice looking board. \$\endgroup\$ – tcrosley Dec 2 '14 at 17:34

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