A search for a light charged Higgs boson decaying into cs is presented using data recorded in pp collisions at √s = 7 TeV. The analysis uses a data sample corresponding to an integrated luminosity of 35.3 pb⁻¹ collected by the ATLAS detector at the LHC between June and November 2010. The search is based on the semi-leptonic tt channel searching for the process t → H⁺b where H⁺ → cs⁻. The invariant mass distribution of the dijet system consistent with the hadronic decay of a W⁺ is used to search for a secondary bump from hadronic H⁺ decays. With no observation of the charged Higgs signal, 95% confidence level upper limits on the decay branching ratio of top quarks to charged Higgs bosons are set as a function of the charged Higgs boson mass.