If you want to truly minimize the size of the package, the best way to do it is to
- physically build a prototype, and
- use a power meter to measure the consumption under worst-case* conditions.
*This will be a function of whatever hardware you're using and (perhaps more importantly) the computational load. You'll have to find out how much load your software suite will put on the system and ensure that your test suite meets or exceeds this load.
I would add a 50% derating factor to your measurement to account for unexpected things - you don't want your embedded system browning out after doing a software update, for instance. This also will improve the life of the power supply (less power = less heat = longer life).
Desktop PC power supplies are generally ATX-compliant, which imposes certain specification criteria on the manufacturers (regulation, overload, etc.). Desktops are inherently (dare I say infinitely) configurable, and because of this it's difficult to say how much power a typical end user will need - hence, large-ish power supplies (hundreds of watts up to a kilowatt, and beyond). Too much power capability is never a problem - too little, well, that's a totally different issue.
You are correct in that the 12V rail is generally for peripherals (hard drives, optical drives, etc.) and the lower power rails are for the 'guts' (5V as housekeeping, 3.3V to feed VRM modules to power the processor).
If your mobo is expecting multiple rails, you're obligated to provide them. If the mobo could convert 12 down to 5 and 3.3 (which often happens on laptop computers, BTW) then the power supply manufacturers wouldn't bother providing those rails and you'd only need a single rail.
There is an option to add up to 470uF on the converter's output and this will alleviate the situation in that it spreads the pulse of current you are taking over a longer time period with a smaller peak seen by the converter.
The problem is that the manufacturer states that 470uF is the max you can add and I suspect that if you added more it might prevent the converter working correctly. So this is to be avoided because, it probably needs to see a controlled amount of ripple on it's output to function properly.
A way to get round this is to use an inductor in series with the output then a decent sized capacitor to ground probably in the region of 1000uF. The inductor value depends on how long your surge lasts. The inductor "buffers" the motor output current peaks from the converter allowing it to still produce the ripple it needs to operate correctly.
The converter specifies that it's output ripple is 150mVp-p and the inductor is there to allow this to largely remain at that level. If I was at my work desk I'd simulate the effects but I'm not
Best Answer
From the TPS543x 3-A, Wide Input Range, Step-Down Converter datasheet we get the internal block diagram.
Figure 1. The block diagram shows only the MOSFET (2) in the power path.
This is a form of switching power supply. Their great efficiency comes from the fact that the switching transistor (the MOSFET at (2)) is either fully off or fully on.
Note that the supply voltage never enters the calculations. The limiting factor is the MOSFET current handling and thermal management.