Hello James,
The MCU KL25 is intended for the VDD voltage range from 1.71 to 3.6V max. The Li-based battery has voltage range from 2.8V (totally discharged) to 4.2V (fully charged), thus it is not possible to connect this battery directly to 3.3V power supply line. It is LDO NCP1117 on the board which provides posibility to use 5V (from USB) as the power supply voltage. The voltage drop of this LDO is 0.95V/100mA, thus it is OK for 5V input. In case you will use the Li-Ion battery connected to the 5V supply line, the MCU VDD will be not stabilized to 3.3V, but it will fly down as the battery discharge. The MCU can work on this "flying" supply voltage, because it is in range of 1.71 to 3.6V. In this case you need consider, if it is usable for your application. For the accurate analog measurements you can use the internal 1.00V reference and recalculate all measured ADC values to this reference.
Next option if you want to make own design with KL25 MCU is to use the VREGIN pin as the supply pin for the MCU. The voltage drop on this internal LDO is only 0.3V. It is not possible to use this LDO for the external circuitry, because of low power. The external circuitry has to be powered by separate LDO if needed. In case you will use the VREGIN pin for the battery connection, please pay attention to the layout design of the PCB. It has to be not voltage spikes on this pin when switched ON/OFF.
I think it could help you, but you are welcome for next questions.
Best Regards,
Stano.
