Well, the last change I made to the PCB killed the buck. Some pads were getting pretty frail, and some were already missing, from repeated changes and rework. I said I'd get it back to functional and then start fresh on a new board, but I guess I hosed something in the bootstrap without noticing it because it died. Nuts. So I pulled everything and put a fresh hacked '53 just sitting on top of the board and it's running 250MHz now. Not quite perfect; I'm seeing only about 90% hashrate average after six minutes or so but maybe it's still warming up. The 225MHz self-contained run overnight saw 97GH (99GH calculated) and 10 HW errors in 18 hours. Not bad.
So I guess I'll have to start fresh on a new PCB anyway.
The hacked '53 has 0.1uF bypass cap on the VREG5 and 2.2uF bypass on VDD (which is powered from 12V). I can definitely see some RC ripple on the 5V line when it's mining; it gets noticeably worse when the power kicks in and we get into full CCM. But the average is still, according to my scope, around 5.1V. The almost-idle average I was measuring on my board was 4.9V and the full-power average, I could keep it running stable with 4.1V there. There are two components different on the feedback circuit, but they should be fairly trivial - it's two of three parts across the inductor making a bit of ripple injector into the feedback to help stability with low-ESR outputs. The third component is not populated, so these parts might, I don't know, help shunt some HF around the inductor?
About the best thing I can think of is it's a layout issue. Perhaps my buck chip is too close to the inductor. Or the bootstrap circuit is hosing with me? Waveforms look pretty much the same with mine and the hacked, on the hacked it was backside components so fairly shielded from inductor noise. If I gotta change the layout in a major way like that it'll mean getting new prototype PCBs which will burn an extra week or two and probably a couple hundred bucks. I might have to raffle off some of these boards (guaranteed to 200MHz probably would be the best I could do) to help cover. If Novak's got working firmware by then they'd be self-contained, or I'd have to cook up a simple adapter you could cable into an S5 controller. Or I could keep the bucks unpopulated, fill 'em all with hacked '53 boards that let you run higher frequency. Without Novak's controls you'd have to either measure or guess the core voltage (like on the Compac) which isn't the best, but folks would know that going in. I'll see how reliably my hacked-powered board runs over the next few days and decide next week what to do.
Speaking of, now after 32 minutes or so I'm seeing about 95% expected hashrate, .0063% errors.