Re power supply from 48v... my go-to device for this is one of these:



300mA '7805-style' switching regulator, 7 - 72v in, 5v out. At 20mA out it dissipates <0.1W against the 0.9W of a HV 7805. Its intended for hi-reliability 48v telecom and 36/48v e-bike/EV applications with a MTBF of 15million hours @ 25C and safe operation at full 1.5W output, 48v input to +105C.

 

I am back from my business trip. Baud rate is 9600, I would like to increase the speed a little more if possible.

Finally I found a good DCDC converter, LM5165XDRC. With this converter, the efficiency is about 69% even when the current consumption is only a few tens of mA.