ON Semiconductor emailed us and helped us identify our mystery ICs. It turns out the semiconductor IC inside our fake ATmega328s is the ON Semi NCP5318FTR2. The NCP5318 is a Two/Three/Four-Phase Buck CPU controller. Essentially it's a voltage regulator specifically designed to power high-performance Intel processors.
This application circuit is a bit daunting. We may try to wire one up, but in general, a complex buck controller is not something that really floats our boat.
From a friend at ON Semi:
"Interesting. The AG20 is called the "Hydra lite." The hydra was originally an 8 phase Vcore buck switcher for CPU/GPU. Hydra lite was reduced to 4 phase. They were built on 18 volt bipolar process. The final version was rev 13, so it looks like that die in the picture is pre-release engineering material. Where the hell did you find that?"
Funny you should ask...
Thanks ON Semi!