The above thread references further discussion of this in:
The consequence of this design error is that, on some Raspberry Pi boards, the LAN9512 chip runs hot because its regulator outputs slightly more than the set-point voltage of RG1, therefore turning off RG1 and supplying the entire board's demand for 1.8V. On those boards, the LAN chip runs very hot (too hot to touch, according to some reports). On other boards, RG1 outputs slightly more than the set-point of the built-in regulator on the LAN chip, therefore turning off that regulator and letting the LAN chip run cold. In fact, of course, these two 1.8V subsystems should be isolated from each other.
A brief aside: For practical purposes, it is unlikely to the point of impossibility that any two regulators will have exactly the same output, so when two regulators are put in parallel, unless there is enough resistance in the circuit to divide the load between them, one will always win over the other and do all of the work.
Response 14 by tmackay, Jul 27, 2012 11:28 PM, in this thread
proposes a solution. There's a photo in the thread, but here's the text:
Carefully, I cut the circuit traces from pins 15 and 38 on LAN9512 and the 1.8V side of C29 to the vias. I then very carefully soldered some wire between these three points. This more closely resembles how the circuit should be (minus the 100nF caps). (The 9512 reference design says 4u7 should be by pin 38 btw)
I've reworked boards with surface-mount components before, and judging by tmackay's photo and my reading of the schematics, his rework is doable (for someone with a very steady hand). So, to my questions:
A: Has anyone else tried this to cure a Raspberry where the LAN chip runs hot?
B: Is there a better solution? This is a question that someone with access to the layout of the inner board layers must answer.
My examination of the board photos (my board has yet to arrive) shows vias from the 1.8V pins on the LAN chip to capacitors on the flip side, and to nowhere else. Obviously, the 1.8V power connection is somewhere in an inner layer of the board. That's why the most useful answers to question B will come from Raspberry Insiders. If only I knew the layout of the inner layers, I'd know if there was some spot where a hole could be drilled through the board, cutting the 1.8V power between RG1 and the LAN chip while not cutting anything else.
Another aside: Drilling a multilayer board is a fairly standard way to cut a trace in an inner layer -- so long as the trace is small enough to cut with a drill without cutting any other trace. If the hole goes through a ground or power plane, you should scrape the inside of the hole with a dental tool to prevent short circuits. Even with the scraping, it's practical to modify hundreds of boards per hour this way, once you set up your drill press with a board alignment jig.
-- Doug Jones