STM32WL - VDD & VDDRF different power supply ?

[StéphaneZaack]

Hello everyone!

MCU : STM32WLE5CC

I am reaching out for help to validate an architectural choice! Here is the design part of the MCU datasheet : 

In our design concept, we planned to power VDD directly from our 2x alkaline batteries and the VDDRF part from a 3.3V DC/DC converter that we can turn ON/OFF. Thus, in stop mode 2 (sleep), we can cut off the VDDRF part (3.3V from the DC/DC) to reduce overall consumption to 3uA. Here is the simplified diagram :

We have implemented a modified board, and our results show that 1 out of 5 times upon waking up, we have no signal modulation. No LoRa messages are sent. If we wait for a few minutes with everything powered, the LoRa transmissions start working again. The behavior is random each time.

We found this line in the datasheet : 

Has anyone tried to achieve this scenario before ?
Can we have two different voltage sources between VDDRF and VDD? Or is it mandatory for both to be at the same voltage level ?

Thans for your help guys !

Stéphane

[STTwo-32]

Hello @StéphaneZaack 

As mentioned on the DS13105, VDDRF is between 1.8V and 3.6V and VDDRF pin and must be connected to the same supply as VDD. In any other case, we can't guarantee anything for your system.

Best Regards.

STTwo-32