Charismatic Armor

Your charm and natural affability make others hesitant to strike you.  While not wearing armor, a protective magical force protects you, and anyone able to see invisible objects sees a swirling vortex of sparkles surrounding you. Your base AC becomes (10 [or whatever it might be naturally while unarmored] + your Dexterity modifier + your Charisma modifier), and can only be improved beyond that by using  protective magic other than armor or shields.  The spell ends after 8 hours or if you are rendered unconscious or otherwise unable to exert at least minimal concentration while the spell is active.