Stealing USB-port power

摘要

In addition to upgrading the data-handling capability of a desktop computer's aging serial and parallel ports, the USB also provides a voltage source that you can use to power external peripherals. Table 1 summarizes the requirements that the USB specification places on equipment (references 1 and 2). As the table shows, the host equipment provides a 5V supply capable of only 2.25W of power in the worst case. In some cases, this power is not enough for the peripheral, and you must use an alternative power source, such as a wall adapter or an offline power supply. In other cases, this power is much more than necessary, and you can use low-cost, linear regulators to generate the supply voltages for the peripherals. If you elect to use the port voltage, the USB-power limit necessitates the use of high-efficiency power-supply designs and complicates the system trade-offs of cost, efficiency, and size.