W5100-evb-pico only works with arduino


I have a strange problem.
I use a W5100S-EVB-Pico v.1.1 with arduino 2.0.1 with the standard example DhcpAddressPrinter and with a onboard led on addition when an ip address is aquired.

When the board is connected to my pc it works (ip aquired and led goes on) but when connected to usb power only it does not work. the lights on the ethernetconnector are on but no ip aquired or led turns on.

Anybody an idea?