I have my own designed board. It has USB device interface circuit. The MCU is LPC1769 Cortex M3.
Basically when I use USB bus power for the board, its USB device feature is working. It means when I connect to PC, it is identified.
But when I supply the board with external power, then the usb device feature isn't working.
The external power is driven by 24V SMPS. On the board, 5V and 3.3V are regulated by regulator ICs.
Is there anyone experienced similar like this ?
The interface circuit is very simple. Vbus is float on board. GND, D+, D- are connected. For D+, D-, 33ohms resistors are connected in series.