A convenient feature of USB is the ability to draw power from the bus. But using bus power carries the responsibility to operate within allowed limits, including reducing power in the Suspend state. This chapter will help you decide if a device can use bus power. Plus, whether your design is bus-powered or self-powered, you’ll find out how to ensure that your device follows the USB specification’s requirements for managing power.
Also covered are new power-saving options for USB 2.0 and USB 3.0. Inside a typical PC is a power supply with amperes to spare. Many hubs also have their own power supplies. Some USB devices can take advantage of these existing supplies rather than providing their own power sources. Bus power has several advantages. Users don’t need an electrical outlet near the device. A device with no internal power supply can be physically smaller, lighter in weight, and less expensive to manufacture. The device can save energy because power supplies in PCs use efficient switching regulators rather than the cheap linear regulators in the power adapters that many peripherals use. (Self-powered hubs may use inefficient supplies, however.)
Tags: A device, Power Supply, With no Internal
