RAMPS-FD: 5V rail needs more current

[bobc]

5V is provided by the Due, but is limited to 800mA (or less). This is not really enough for extras like LCD, SD, servos, expansion boards. RAMPS has a seperate 5V input for servos.

Need to look at providing 5V regulator on board, or at least input for external 5V (may require yet another jumper?).

It would be really nice to run off a 5V standby (ATX PSU), but I don't think 5V can be back fed safely to the Due.

[cefiar]

FWIW: RAMPS has a 5V input for running just the servos.

Moving the servos and the expansion connectors to their own dedicated +5V rail would take away most of the load. We can provide a jumper that allows people to use either an external supply, or try and run it off the Due's +5V.

Note: Do we really need +5v for SD? SD itself tends to be 3.3V now. The only reason it was used on the RAMPS adaptors was because RAMPS itself didn't have 3.3V available. It would make the board compatible with the RAMPS SD card adaptors though.

[cefiar]

This might prove useful: Arduino Due reference design - http://arduino.cc/en/uploads/Main/arduino-Due-schematic.pdf

From that, 5V is generated from VIN using an LM2734Y. 3.3V is then generated from +5V using an NCP1117ST33T.

Notes:
Using our own 7805 on board has the disadvantage that we'd be dissipating a lot of heat at >12V (if we're stepping down directly). A switch mode regulator might be worth investigating, especially if it can produce about 1-1.5A.
I think we can power the +5V line into the Due, as I think the LM2734Y handles that, as long as we don't provide power to VIN. TI spec sheet: http://www.ti.com/lit/ds/symlink/lm2734.pdf - Specifically, Figure 24 in the spec sheet matches the reference circuit.
How much current is the 5Vsb rail on ATX specced for? FWIW: 3 ATX supplies I have in the garage are specced for 2A.

[cefiar]

+5V rail needs to be thicker all round as well.