I purchased a Nucleo-L476RG eval board and downloaded the PWR STOP2 RTC example code to test for a low power application.
After following the instructions for the jumper settings outlined in UM1724 for external power, I built and tried the example. The user LED blinked for five seconds then stopped as the board went into power-down/STOP2 mode for several seconds.
This all appeared correct but instead of the expected STOP2 current of about 1.5uA, the current was about 450uA.
I searched the STMicroelectronics Community and tried several different fixes for similar problems on other boards. But nothing worked.
The provided PWR_STOP_RTC code is good and correct. So, I examined the schematic more closely and found extra required jumper settings and power connections not mentioned in the UM1724 user manual.
UM1724 (7.3.3) states to apply +3.3V to CN6-4, CN7-12, 16. And SB2 and SB12 are removed.
The problem is applying external +3.3V to the +3V3 net at CN6-4 which draws excess current into R23 and R27 of the debug MCU U2 (STM32F103CBT6). Instead, external power should be applied to CN7-5 (VDD). VDD is the power net used only by the target MCU U5.
Additionally, the following changes were made:
Remove jumpers: CN2 x 2
Remove CB15 (and SB2 & SB12)
Remove JP6
First program the board before making all these changes, otherwise the program section and USB interface won't work anymore.
After programming the board and making these changes, the USB connector is removed, and the board is externally powered with +3.3V via CN7-5.
The normal LED blink current is about 12mA and the STOP2 mode is now 1.5uA.
