VDD and VDDH out-of-spec consequences

[Golaz]

Hi,

our design makes use of nRF52840 MCU. Power is supplied to VDDH pin from a Li-Po battery which can range from 2.5V  to 4.2V. Internal REG0 is used to regulate voltage down to 2.7V which is then used to power some external circuitry through VDD pin.

Our product can work in two modes. Normal mode where radio is operational and ADC is active. And standby mode where both radio and ADC, along with all other peripherals are turned off to conserve power.

Our product specifications require us to enter standby mode when battery voltage reaches 3V. Having 3V on VDDH gives us 2.7V on VDD, still upholding the recommended MCU operating conditions specifying that REGOUT0 (REG0 output voltage) < (VDDH - 0.3V) where 0.3V is voltage regulator dropout voltage according to nRF datasheet.
When battery is drained more and is in the range 2.5V <= battery voltage < 3V we are violating the REGOUT0 specification.
When battery is drained even more and battery voltage < 2.5V we are additionally violating the VDDH minimum specification.

We are trying to decide if (in the rare occasions) operating MCU out of power supply specifications is something we should worry about or if it is acceptable for our use case. Our HW design is such that it is impossible to cause damage to HW through changing MCU GPIOs arbitrarily.
My questions are:
1. What are the consequences of violating REGOUT0?
2. What are the consequences of violating VDDH min?
3. What are the consequences of violating both REGOUT0 and VDDH min?
4. Can these violations permanently damage the MCU?
5. Can these violations cause MCU to get locked or be otherwise permanently inhibited prevent normal use?

Thank you,
Golaz

[ketiljo]

When VDDH is lower than the configured VDD output voltage, the VDD output voltage will be lower than VDDH by a few 100 mV. So when the battery voltage is too low for the VDD level to be what you have set it to, it will still work all the way down to the minumum VDD level. 

[Golaz]

Thank you.
And what happens when VDDH falls below minimum limit of 2.5V ?

[Golaz]

Also as a note. What we have observed in this scenario is that REG0 starts producing a very distorted output voltage on VDD. That is when radio starts transmitting the voltage dips for up to 200 mV from the baseline.