My early LX150 miner prototypes had a 60A switcher for vccint for 8 chips -> 7.5A each... and that later turned out to be not enough.
So, how come the German guy (Stefan) of ZTEX gets by with a [very compact] 8 Amp converter for VCCINT?
His dynamic clocking now achieves > 200 MH/s on a -3 device.
Impressive.
(Even more impressive considering the fact that he has less than half of the Xilinx-mandated amount of bypass capacitors surrounding the Spartan-6.)
Btw, one thing that made me go WTF when looking at interior pics of copacobana a few years ago... a single massive AC->1.2V converter for vccint and really heavy cables + busbars to route it around...
Here's a picture:
http://www.copacobana.org/photos/photo_b4.jpg