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Get the most (charging) out of your USB port
Learn about the power characteristics of USB ports, and how applications can benefit from available USB power and corresponding design challenges.
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By
George Paparrizos, Summit Microelectronics
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Page 1 of 4
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Power Management DesignLine
(01/21/2007 6:12 PM EST)
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The popularity of the Universal Serial Bus (USB) in the consumer electronic space is unquestionable. Most of today's computer and other peripherals can be connected to a USB port for data transfer, including flash cards, digital cameras, printers, mice, keyboards and others. The USB 2.0 specification has expanded market opportunities since data transfer rates can reach up to 480Mbps while maintaining the attractive plug-and-play characteristics of this interface. Another major, but often forgotten, feature of USB is its advanced power management capabilities. Taking full advantage of this widely available power source requires a good understanding of the USB power specifications and its idiosyncrasies. This article discusses the power characteristics of USB ports, how applications can benefit from available USB power and corresponding design challenges.
Today's USB ports provide power and data transfer via 4-pin cables. Two of the pins are VDD (also called VBUS) and GND while the other two, D+ and D-, are used for USB communication. A USB port can provide a relatively well-regulated 5V rail that may range, at the peripheral stage, from 4.4V to 5.25V during full load (that is, 500mA). In terms of current capability, the USB 2.0 specification calls for 1 to 5 unit loads as the maximum capability of a USB port. One unit load is defined as 100mA, and whether the port can supply one or five unit loads depends on the port type. Low-power (100mA) ports are hub-based USB ports, whereas high-power (500mA) ports are host-based USB ports. Before enumeration (system assigning an "ID" to the device) is completed - and higher current is allowable - host-based ports are also required to limit current to 100mA. Ultimately, a USB port offers a great source of power that is already utilized for data transfer but is just waiting to be taken advantage of.
Figure 1: Worst-case USB Voltage Drop (Reference: Universal Serial Bus Specification rev. 2.0)
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