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STM32F405 not recognized by PC (Windows) on USB plugging

Question asked by frigerio.alessan.001 on Feb 22, 2016
Latest reply on Feb 24, 2016 by frigerio.alessan.001
I am trying to use my custom board with a STM32F405 chip.
I am powering the board with the USB voltage, with an LDO that brings it down to 2.8V, and programming it through ST-Link using a Discovery board.
I have set up my program using STM32cubeMX including the USB library. 

The USB pins are connected to PA12 and PA11 and I have checked their electrical connection.
I have generated the software with cube, and loaded it with ST-Link (IAR). The problem is the device is not detected at all by my computer (it does not show up in the Device Manager) and the USB initialization fails. 

I can't figure out where to start, since all I can find on the forums is about not recognized devices, with problems concerning clock speed settings.

Do you have any advice in how to fix this problem?