A Winthrop man who allegedly robbed a Dunkin’ Donuts on Cambridge Street was caught after he left his cellphone at the scene, prosecutors said. Dennis Stack, 41, was arraigned Tuesday in Charlestown District Court in the Dec. 10 robbery, during which he dropped a tire iron and an iPhone while struggling with a customer, Suffolk District Attorney Daniel F. Conley’s office said. A phone contact identified Stack as the owner of the cellphone. Stack is charged with additional robberies in Suffolk and Middlesex counties, Conley’s office said. A not-guilty plea was entered on Stack’s behalf, Conley’s office said.Jennifer Smith can be reached at email@example.com.