buck converter with a simple bypass schematic mode?
This is why the SEPIC converter was invented.


  The single-ended primary-inductor converter (SEPIC) is a type of DC/DC converter that allows the electrical potential (voltage) at its output to be greater than, less than, or equal to that at its input.


SEPIC on wikipedia

TPS40210 from TI is a SEPIC converter that'll meet your needs.