LoRa node buck step-up/down converter advice

Hi everyone,

I'm building Lora sensor node that incorporates RTC (DS3231), external eeprom memory (24LC256) and timer (TPL5111) that works on it's own cr2032 battery. Timer wakes up Atmega328P-AU along with eeprom and rtc.

MUC checks battery level and time/date. If the time is right it will wake up rest of the circuit and collect data from BME280 and send data with sx1276 module to Lora gateway.

I'm having issues selecting the right component from Digi-Key and co. The whole thing has to work
with voltages ranging between 3,5V (Lora voltage max. is ~3,7) and 3V where battery cuts of. Battery output voltage is 3.7V.

To do that with lowest possible losses I need Buck (step-down) converter that is easy to solder on PCB (SOT and similar) and does not require a lot of external passive components. It's a requirement for a project.

For example, TPS61291 and TPS62136 are great candidates but they are hard to solder so no go. MT3608 is excellent candidate but I can't get it anywhere except AliExpress, Ebay etc.

What is the main power source, the battery which you have quoted a voltage of 3.7volts. If it is a lipo, have you considered the fully charged voltage ?

If the lora module is this or similar, https://www.makerfabs.com/desfile/files/RFM95_96_97_98_DataSheet.pdf , which can operate down to 1.8 volts, I’d be tempted to use a schottky diode in series with it to knock off ~0.4 volts which should ensure it is always within tolerance.

How are you measuring the battery voltage in this scenario ? I guess you’d need a mosfet switched potential divider and use the 1.1 reference voltage of the Atmega328P.

Which 'LoRa Gateway' are you using ?

Which Arduino are you using ?

I dont quite see why you have a TPL5111 as a timer (max 2 hours) power up a 'MUC' which then checks an RTC to see if its time to do something, the DS3231 can do that on its own.

I would not be assuming that the 'lowest possible losses' will be achieved with a switched power converter. Standard low dropout regulators can do a good job and are a common choice on nodes that need to run for extended periods on batteries.

A circuit diagram would help.

A CR2032 lithium battery is nominal 3V, and pretty stable at that. Your 3.7V (nominal?) and 3V cut-off are more typical for rechargable LiPo batteries.

If using a LiPo battery - which should be cut off at about 3.2-3.3V, not 3.0V - you're usually best off using an LDO regulator such as the RT9013 or MIC5225-3.3 regulator. Power savings from the more efficient buck conversion are usually negligible at these small differences, and made up big time by the lower quiescent current of a linear regulator.