Okay, so, the BM1384 appears to really like current while initializing. Even with a fairly stable regulator, the starting problem for higher frequencies didn't go away. I once had it running at 175MHz, but I think I had the input voltage to the buck cranked up. Nothing I did within reason to the circuit could get it to handle the brief burst current required to start the chip above 150MHz. This included adding 22uF bypass capacitors to all four corners of the chip, increasing output capacitance of the regulator to 8x 47uF ceramics, and building a socket onto the regulator board so it'd plug straight into the breakout board and get rid of wiring impedance.

To make sure it wasn't the chip's fault, or the breakout board's fault or whatever, I rigged up a stupidly-overcapable regulator by cutting a chunk off a dead AMV1 Garden blade courtesy of CrazyGuy. The 53355DQP on there is rated for 30A, I figure that should be good enough. I modified the board to socket straight to the breakout, powered through a USB jack, and with a voltage range adjustment of 600-750mV and tested the chip successfully up to 250MHz (13.75GH) with that attached. The current meter on my bench PSU touched 1.8A during the chip-init burst transient, but the running current was around 1A.

So what I'm thinking is, screw that IR3899 anyways it doesn't seem to be able to handle that burst transient. If it's pushing 9W of 5V at 85% efficient down to about 630mV that's briefly touching an output current of 12A. I knew output current was the cause of my undervolt issues, but the outputs I was seeing didn't make sense for overcurrent protection when compared to the datasheet info. Given that I ruled out literally every other fault condition though, that has to be it. I'm not sure why the chip was reacting in 2.4mS instead of the 20mS listed in the datasheet, but whatever. So yeah, screw that chip anyways.

I think we're gonna go with what we're calling a "Baby Chuckwagon" strategy, which is instead of trying to jack with some unnecessarily-complex freakin' tiny-pin-pitch (or more likely, cut-up no-lead) package chip, we're gonna work out a design using a basic regulator controller with integrated FET drivers, and a couple pretty good super-low-Rdson external FETs. It'll require more board space, but the parts cost should actually be a bit cheaper, and the packages are decidedly non-evil.

While we're waiting for those parts to come in for testing, I can crank up the voltage on the schfifty-three board and start testing chip strings to prep for Amita design. How sweet would it be if I posted something tomorrow about how I had a two-chip string pushing 20GH or whatever?