USB delivers 5V power and has 3.3V signaling levels, so any reasonable bus-powered device has to contain some kind of voltage conversion. Most common solution i

USB delivers 5V power and has 3.3V signaling levels, so any reasonable bus-powered device has to contain some kind of voltage conversion. Most common solution is LDO, voltage limits in USB specification even seem to imply that solution. Designing switched mode power supply that meets all the USB 2.0 (don't know about 3.0 but it is probably mostly similar) requirements on minimal voltage on input, current draw in various states and so on is not exactly trivial.