This is probably an easy question. When using an STM32 Nucleo powered by a 5V power supply, can I use its 3V3 pin to supply 50mA? Or do I need to use for instance a 3V3 linear regulator attached to my 5V power supply.
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The NUCLEO-G031K8 dev board uses a LD39050PU33R regulator to provide 3.3V rail from other high voltage sources (USB 5V for example):
You can use the onboard 3.3V rail to power other devices but make sure not to cross 500mA limit of the regulator.
Here's the NUCLEO-G031K8 schematic for reference.