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.

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    \$\begingroup\$ 6V to 3.3V is half your energy wasted. You need to compare that against the quiescent consumption of your buck. Not sure where you are getting 6V though. Seems needlessly high if energy is a concern and you do not mention sleeping anything that requires 5V which implies it does not exist and therefore 6V is not actually required or you it does but don't care about power. \$\endgroup\$
    – DKNguyen
    Jun 3 at 13:49
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    \$\begingroup\$ In this type of scenario, a LDO can be more efficient than a switcher. See electronics.stackexchange.com/questions/229926/… \$\endgroup\$ Jun 3 at 14:06
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    \$\begingroup\$ The MP2331H has a quiescent current of 200 µA. That's 1700 mAh in one year. I think that'd drain your batteries too fast. \$\endgroup\$
    – JRE
    Jun 3 at 14:07
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    \$\begingroup\$ The MCU we are using requires 3.3V No it doesn't. it will work on 2.5 V also. Now prove me wrong by showing me a link to the datasheet where it says that that MCU needs 3.3 V. I ask because the requires supply is never 3.3 V, it is like 3.3 V typical and 3..0 V minimum and 5.5 V maximum for example. \$\endgroup\$ Jun 3 at 14:14
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    \$\begingroup\$ Some RF modules indeed require 3.3 V, they're more "picky" than an MCU. I also strongly advise to make a spreadsheet with the current consumption of each block/module, combine that with their Duty cycle (how long they're on) and you'll get average current consumption. From that you can estimate battery life but also an indication of where most energy goes. \$\endgroup\$ Jun 3 at 14:26

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