Catalyzes the transfer of the AMP portion of ATP to flavin mononucleotide (FMN) to produce flavin adenine dinucleotide (FAD) coenzyme. ATP + FMN + H(+) = diphosphate + FAD Cofactor biosynthesis; FAD biosynthesis; FAD from FMN: step 1/1. Homodimer. Belongs to the archaeal FAD synthase family.