Electrical – Exceeding voltage on STM32 Vbat pin

If your goal is the longest runtime, use a switching buck regulator to efficiently drop the battery voltage to 1.8V (or 2.4V if you require the full radio power output).

If your goal is decently long runtime without the complexity of a switch-mode supply, use an LDO regulator to drop the voltage to the same level. The microcontroller and regulator will most likely draw less current at the lower operating voltage, compared to running them directly on the supply.

If you want minimal extra components and consider an LDO + input & output capacitors to be too many, then you could place a 3.6V Zener diode in parallel with the supply. That would ensure your ICs don't get exposed to out-of-spec voltages. When the battery is really full and has an open-circuit voltage above 3.6V, the Zener will conduct and the "excess" voltage will be effectively dissipated in the battery's internal resistance.

No, a Low Drop Out Linear Regulator cannot be used to boost a voltage up. That specific one has a Drop out of 0.1 ~ 0.25 Volts, so the maximum V OUT is VIN - 0.25 Volts.

You need a Boost circuit. Depending on your specific current needs, there are different types of boost regulators that will work. A charge pump uses a few capacitors and will be the simplest for under 200 mA. Otherwise there are plenty of normal regulators that use an inductor and diode pair, with internal mosfet, that can be used.

TI and every other manufacturer have tools that basically design it for you.

The three other options, since you have a 5V power source in the first place, are:

• Parallel the 5V to 3.3V PMIC with a 5V to 3.8V~4.0V LDO. Less complex, and no need to double up on 5V down to 3.3V, then up to 3.8V.

• Change the PMIC to be 3.8V output, and use an LDO to get 3.3V from that.

• See if you can just change everything to 3.6~3.8V. No more multiple regulators.