You have WIFI NET on, MOBILE NET on, and you're essentially shoveling data continuously between those two transceivers with the CPU - which I'd assume is the most significant user of available current capacity. Even though your screen is off, the CPU cannot sleep because it has to do 'the shoveling' continuously. USB port is only going to give you 500 mils to charge the battery AND do your phone's bidding. (WIFI NET, MOBILE NET, CPU) You need to have a supply source capable of giving you more current than your laptop can supply. I believe the charger that comes with the Sprint Hero can supply what you need and may be the best choice for powering a tethered handset. Good luck.