We report on the development of a laser source in the mid-infrared spectral region based on difference-frequency generation (DFG) in a periodically poled LiNbO3 (PPLN) crystal. Continuously tunable coherent radiation from 2.75 to 4.78 ?m was produced by optical parametric interaction between a diode-pumped monolithic continuous-wave (CW) Nd:YAG laser operating at 1.064 ?m and a CW Ti:Sapphire laser tunable from 767 to 871 nm. Temperature-dependent quasi-phase-matched DFG wavelength acceptance bandwidth was studied and characterized. An empiric formula is given to estimate the phase-matched wavelength acceptance bandwidth as a function of the crystal temperature at ? = 22.5 ?m. A large frequency scan of 128 cm?1 (about 78 cm?1 above 1 ?W) near 4.2 ?m was achieved. The whole absorption spectrum of the P and R branches of the ?3 band of atmospheric carbon dioxide has been recorded with a single phase-matched frequency scan.