We combine measurements of the top quark pair production cross section in ppbar collisions in the l+jets, ll and tau-l final states (where l is an electron or muon) at a center of mass energy of sqrt(s)=1.96 TeV in 1fb^{-1} of data collected with the D0 detector. For a top quark mass of 170 GeV, we obtain sigma_ttbar=8.18^{+0.98}_{-0.87} pb. In addition, the ratios of ttbar cross sections in different final states are used to set upper limits on the branching fractions B(t -> H^{+}b -> tau nu b) and B(t -> H^{+}b -> c sbar b) as a function of charged Higgs boson mass. Based on predictions from higher order quantum chromodynamics, we extract a mass for the top quark from the combined ttbar cross section.