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USB Power Specs

The specs listed here are from Maxim Engineering Journal Volume 52 article on USB Battery Charging.

“All devices that plug into a USB port must start out drawing no more than 100mA. 
After communicating with the host , the device can determine if it can take the full 500mA.”

So, in other words each USB Host can supply about 500mA.  However, because of the ability for the USB Host to be connected to a hub which can then power more then one unit it cannot assume 500mA is initially available.

Minimum Voltage Available:
USB Host = 4.5V
USB bus-powered Hub = 4.35V