External LED/USB

I’ve got a strange problem which I’m not sure how to solve.
I’ve got a Pixhawk clone with an external LED/USB board. Two things:
If I connect just this board (without power from power module), pixhawk loads apparently but does not light up onboard LED, nor external LED. I can flash it, but I cannot connect Mavlink (no hearbeat message). If I connect power first, LED works and Mavlink works also, even if I disconnect main power and just leave USB.

What am I doing wrong?