I have seen some development boards (for example. BL652 dev kit) for low power chips have battery power connected directly to the MCU without a regulator.
For the example case, the battery used is a 3V CR2032. The datasheet for the MCU defines the following parameters:
datasheet page 16. Absolute Maximum Ratings Min Max Voltage at VDD_nRF pin -0.3 3.9 datasheet page 17. Recommended Operating Parameters Min Typ Max VDD_nRF 1.8 3.3 3.6
I'm interpreting this as
"If your battery voltage drops to a value between 0-1.7 it isn't defined what will happen".
Why this worries me is because I've seen regulators having the Power Good pins and have found no explicit statements in the datasheet that the MCU from the example won't be damaged by the undervoltage.
How can I decide if a regulator is needed between a battery and a load, to guarantee there are no damages when the battery voltage starts dropping?