As always, Henry's advice is most useful. An additional process for making splinter-free cuts across plywood grain is to carpetape a piece of thin (1/4") plywood to the bottom of the portable circular saw. With the blade elevated to maximum up, and the lock off, plunge it down, while running, through the plywood shoe. This creates a zero-clearence throatplate, and will resist splintering even if using a coarser blade. The only caveat is that this overrides the blade guard, and so the blade remains exposed, but you should be able to do a clean job with a sharp stock blade.