Current status

I have some basic firmware written for this board that correctly drives a sensored brushless motor, either with locked-antiphase or sign/magnitude style PWM. Originally, I had intended to use locked-antiphase exclusively, since it allows for regenerative braking. Unfortunately, when designing the board, I hadn’t fully appreciated just how low-inductance most brushless R/C car motors are. The low inductance results in very quick current changes in the phase windings. Since locked-antiphase depends on the winding inductance to smooth out and reduce circulating/ripple current, this was a major problem; at reasonable PWM frequencies, the ripple current was very high – while, at high PWM frequencies, the MOSFET switching losses were unacceptable.
Switching to sign/magnitude resolves most of those issues, but introduces some of its own. Most importantly, the body-diode on the MOSFETs must now come into play during PWM off-cycles. The body diode is a lot higher loss than the MOSFET, so it results in more power dissipation. Synchronous rectification would help with this, but would be tricky to implement in firmware (especially given the rapid current decay rate). Many dedicated motor controller ICs have integrated hardware for implementing synchronous rectification.
Despite these potential inefficiencies, the board still works fine. Now it’s just waiting on me to finish the rest of the robot, so it actually has something to do!

Downloads

This board was designed before I standardized my part libraries, so you won’t find the parts in my EAGLE parts library. In any case, the EAGLE schematic and PCB layout can still be downloaded via Mercurial, in my eagle_mtrdrv1 repository (or via a direct ZIP download).
For those without EAGLE, you can download PDF versions of the schematic and layout.