Yellow-orange lights, valuable in photodynamic therapies, spectroscopy, and optogenetics, are limited by the narrow bandwidth and bulky setup via the conventional Raman or optical parametric oscillation processes. Moreover, flatness in the broad-band spectrum is also important for the aforementioned applications with extended functions. In this paper, by carefully designing grating-periods of a step-chirped PPMgLN ridge waveguide for sum frequency generation (SFG), we report a compact broad-band yellow-orange light with bandwidth of 5.6 nm and an un-reported flatness (<1.5 dB). Correspondingly, the optical conversion efficiency is 232.08%/W, which is the best SFG efficiency for PPMgLN at the yellow-orange region, to the best of our knowledge. The results could also be adopted for other broad-band SFG process toward the vis-infrared region in an integrated structure.