Proton latchup was investigated for several CMOS integrated circuits, including a modern microprocessor. The proton latchup cross sections of these devices differed by more than two orders of magnitude. A modeling approach that takes differences in charge collection processes for longand short-range particles into account was effective in comparing latchup cross sections in heavy-ion and proton environments, as well as explaining why the proton cross sections were so different among the device types.