I have a simple STM32 MCU board design that will be sleeping most of the time, waking every 30 minutes to take some measurements (which take about 20 seconds max), and then going back to sleep. It must run without intervention for 4 years (in the ground). Our sleep current usage is in the nAmps. I'm curious whether I am better off with a buck converter going from 6V -> 3.3V or use an LDO. I realize the LDO is not as efficient and every mA counts so I am leaning toward a buck converter but wonder what others think.