Using external power supply and USB at the same time

[rdmisc]

Posted on February 16, 2012 at 15:27

I'm wondering if it is safe to have a 5V power supply connected to the EXT_5V pin at the same time as the board is connected to the computer via USB (from which the board also derives a 5V supply). The Discovery user's manual is a bit unclear to me on this point.

#usb #external-power-supply

[donald2]

Posted on February 18, 2012 at 01:41

Which Discovery board?

(Yes, the forum reorganization led to exactly the result Clive suggested.  Didn't anyone at ST think about this first?) The boards I've looked at have a Schottky diode feeding the USB 5V to the ''5V'' pin and the 3.3V regulator.  It's an excellent diode, with less than 0.3V drop under the light load of running typical firmware.

[rdmisc]

Posted on February 18, 2012 at 05:57

Thank you for the reply. I have the STM32L Discovery board. It does have a diode between USB5V and the EXT_5V pin but  I wasn't sure what that meant. I have tried it now and it appears to work well. My circuit pulls around 500mA. While the board was connected to the USB port, I set an adjustable power supply to 5.0V and connected it to the EXT_5V pin. This caused all the current to be drawn from the power supply. When I slowly reduced the power supply voltage, the current drawn from the power supply fell gradually until it got to 0A at 4.2V, at the same time as the current drawn by my circuit stayed constant. So the USB power seamlessly picked up the ''slack''. Fortunately, my computer was able to supply 500mA from a single USB port during the one minute it took to do this experiment.