We propose and analyze theoretically a trap-door optical buffer based on a coupled microrings flat-top add/drop filter (ADF). By tuning one of the microrings into and out of resonance we can effectively open and close the buffer trap door and, consequently, trap and release optical pulses. To attain a maximally flat filter we present a new design approach utilizing the concept of a white light cavity to attain an ADF that resonates over a wide spectral band. We show that the resulting ADF exhibits superior performance in terms of bandwidth and flatness compared to previous design approaches. We also present a realistic silicon-on-insulator-based design and a performance analysis, taking into consideration the realistic properties and limitations of the materials and the fabrication process, leading to delays exceeding 5 ns for an 80 GHz bandwidth and a corresponding delay-bandwidth product of approximately 400.