NUCLEO-64-G474RE (MB1367C) External 3V3 Source Usage and ST-Link.

[hmcb]

Hi there all,

I have been looking to power some LEDs which require 25mA supply at 3.3V. This is above the capabilities of this Nucleo-64 board as it is stated that the maximum GPIO supplied current is 20mA per pin (100mA total across all pins).

For reference, this is the circuit I would like to connect which is part of the Bourns PEL12T rotary encoder. Pin 5 is supplied a 3v3 source and all other LED pins are held high. When the LEDs need to light, the GPIO pins should go low allowing current to flow.

Would it be better to use an external 3V3 power supply to power pin 5 only, or desolder SB5 (which disconnects internal 3V3 regulator) and power the whole board by plugging the external 3V3 supply to the 3V3 pin on the arduino or morpho header.
I have several questions:

• Would the above described method of using two 3V3 supplies be safe for the Nucleo board?
• If plugging in an external 3V3 supply and desoldering SB5 would it be possible to still use ST-Link if a USB cable is plugged in?
• Does using an external 3V3 supply allow you to draw more current from each GPIO pin or simply increase the maximum draw of all pins at once (retaining the 20mA per pin limit)?

Please let me know if I can give any further information to help an answer be made.

[Andrew Neil]

---

@hmcb wrote:

This is above the capabilities of this Nucleo-64 board as it is stated that the maximum GPIO supplied current is 20mA per pin (100mA total across all pins).

---

Are you sure that's a Nucleo board specification ?

It sounds like a specification (limit) for the STM32G474RE chip ?

EDIT:

Here:

https://www.st.com/resource/en/datasheet/stm32g474re.pdf 

[Tesla DeLorean · Accepted solution]

It means you need a DRIVER that can conduct 25 mA, and NOT THROUGH the transistor gates on the STM32 itself.

You don't need another supply, you've probably got 3.3V @ >= 500mA via the USB and regulator, less draw of STM32, ST-LINK, etc.

Perhaps something akin to a 74LS06 or a small darlington pair

https://www.build-electronic-circuits.com/7400-series-integrated-circuits/74hc06-74ls06/

[hmcb]

Thank you for the speedy reply!
It looks like you're right, so I guess this means that no matter the supply, no more than 20mA can be supplied by or to the GPIO pins?
Meaning I need to use a transistor based solution to drive and switch the LEDs.

[Andrew Neil]

---

@hmcb wrote:

 this means that no matter the supply, no more than 20mA can be supplied by or to the GPIO pins?

---

Correct.

---

@hmcb wrote:

Meaning I need to use a transistor based solution to drive and switch the LEDs.

---

There are many choices - transistors are but one.

There are dedicated LED driver chips, generic load-switch chips, and more ...