Wintel support drives demand for USB 3.0

USB 3.0 has transmission speeds of up to 5Gbit/s, which is 10 times faster than USB 2.0 (480Mbit/s). It also comes with a range of power management features to reduce power consumption to one-third, while still being able to deliver up to 80% more power to a peripheral; three compelling reasons to adopt the new implementation of the world’s most popular PC interface.

Inevitably, this means that demand for USB 3.0 will spread to the industrial market. New peripherals and embedded computing platforms should be engineered to support the new version of the standard. Even though USB 3.0 is backward-compatible the benefits are only realised if the standard is fully supported at both ends, so consideration should be given to upgrading established products too. How hard is this to do?

Inevitably, this means that demand for USB 3.0 will spread to the industrial market. New peripherals and embedded computing platforms should be engineered to support the new version of the standard. Even though USB 3.0 is backward-compatible the benefits are only realised if the standard is fully supported at both ends, so consideration should be given to upgrading established products too. How hard is this to do?

More power, less waste

The USB 3.0 power architecture offers the power designer the best of both worlds – more power when it’s needed, less power when it isn’t needed. The bus power specification has been increased so that a unit load is 150mA, which is the maximum that can be drawn by an unconfigured device. This is half as much again as is available from USB 2.0. A configured device can now draw up to six unit loads or 900mA, compared to a 500mA maximum for USB 2.0. Minimum device operating voltage is dropped from 4.4V to 4.0V.

Power efficiency was a key objective in the move to USB 3.0. To reduce power consumption, a range of new power management features have been introduced, including support of idle, sleep and suspend states, as well as link-, device-, and function-level power management. Improvements include the elimination of continuous device polling and of broadcast packet transmission through hubs. The interface now only transmits data to the link and device that need it, allowing other devices to go into a low power state and remain there until needed. Power management is at a link level, so that either the host or the device can initiate a low power state when idle. Links can progressively enter lower power management states when the link partners are idle. The power management features also include device and individual function level suspend capabilities to allow devices to power down any function not in use.

These new features are available in part because USB 3.0 has two extra differential pairs of wire over which full-duplex signalling occurs (see figure below). This results in a USB 3.0 cable having a total of 8 wires: one power, one ground, two for non-SuperSpeed data (as one differential pair), four wires for SuperSpeed data (as two differential pairs), and a shield that was not required in previous specifications.

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USB 3.0 Power Architecture

In general, the power scheme for a USB 3.0 port or hub needs to be designed from scratch, as there are too many changes from USB 2.0 to allow significant design reuse. On the one hand, the architecture needs to handle the higher load switching while providing enhanced protection for hubs and peripherals. Standby power consumption in general and battery life in particular has become a significant differentiator in the market, so designers will also want to make best use of the new power management features available in the new interface standard.

Exar’s XRP252x series of single and two dual-channel power distribution switches support the latest 3.0 and all earlier USB specifications. The XRP2525 and XRP2526 devices are optimised for V(BUS) power distribution applications with a guaranteed continuous 900mA per channel current capability, controlled rise and fall time and enhanced port protection.

The XRP2527 and XRP2528, with continuous current ratings of 1A per channel accept input voltages as low as 1.8V and are perfectly suited for generic power distribution applications. The XRP2527 and XRP2528 also provide enhanced power port protection by an externally programmable current limit providing better than 8% accuracy at full current range. A simple setting optimises these devices for USB 2.0 or 3.0 applications.

With a capability of up to 1A continuous current per channel, the XRP252x series of power distribution switches features independent port enables, independent 10ms filtered error flag signals, controlled start-up time in hot-plug events, short circuit protection and a precision current limit in the XRP2527 and XRP2528. Additionally, the power protection scheme includes under voltage lockout, reverse output voltage blocking and thermal shutdown. These devices are offered in a single or dual channel configuration and accept a wide input voltage range of 1.75V to 5.5V suitable to operate from industry standard 1.8V, 3.3V and 5V power rails.

Power potential

By 2015, all new PCs are expected to offer USB 3.0 ports, and over 2 billion new ‘SuperSpeed’ USB devices will be sold in that year alone. The new standard will not only allow the performance of existing devices to be improved, but will enable new applications. To fully realise this potential, power designers need to ensure that they specify best in class switching components and make full and intelligent use of the power management features that they offer to maximise the power when it is needed, and minimise it when it isn’t.