Eric Pittana, Director of Marketing, Power Products Line at Exar, explores the power features of the new USB interface standard and how it is keeping pace with the demands of modern application performance requirements
If you’ve ever sat and impatiently drummed your fingers waiting for video, music or high def images to open from a removable media, USB3.0 is for you. If you’re a power engineer, USB3.0 is for you also, because the new standard can, not only deliver 80 percent more power from the port than USB2.0 but also comes with a range of power management features to reduce power ¬consumption to one-third.
The inferface is backward-compatible so that USB3.0 devices will work with a USB2.0 host and vice versa, though obviously the benefits are only realised if the standard is fully ¬supported at both ends.
More power less waste
The power architecture offers power designers the best of both worlds and is adaptable to offer more or less power on a scale dependant upon need.
The bus power spec has been increased so that a unit load is 150 mA, which is the maximum that can be drawn by an unconfigured device.
This is half as much again as is available from USB2.0. A configured device can now draw up to six unit loads or 900 mA, compared to a 500 mA maximum for USB2.0. Minimum device operating voltage is dropped from 4.4 V to 4.0 V.
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 devices that need it, allowing other devices that can, to go into 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 a device and individual function level to 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 (Figure 1).
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.
USB3.0 power architecture
In general, the power scheme for the port or hub needs to be designed from scratch, as there are too many changes from USB2.0 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 series of single and two dual-channel power distribution switches support the latest USB 3.0 and all ¬earlier USB specifications.
The 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 continuous current ¬ratings of 1A per channel accept input voltages as low as 1.8V and are suited for generic power distribution applications. The switches also provide enhanced power port protection by an externally programmable current limit providing better than eight percent accuracy at full current range. A simple ¬setting optimises these devices for USB2.0 or 3.0 applications.
With a capability of up to 1A continuous current per channel, the series of power distribution switches feature an independent port, enable, independent 10ms filtered error flag signals, controlled start-up time in hot-plug events, short circuit protection and a precision current limit.
Additionally, the power protection scheme includes under voltage lockout, reverse output voltage blocking and thermal shutdown.
The devices come 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.
A major electronics OEM has announced a USB-powered 14-inch mobile LCD monitor, illustrating the extent to which the enhanced power design extends the possibilities for this hugely popular peripheral interface.
Early press reports suggest that USB3.0 is going to be highly popular in the market. According to the USB Implementors Forum, over 200 devices were certified at the time of CeBIT last March.
Second only to its higher speed, low power consumption and the fact that more devices can now be charged from a USB port are amongst the most ¬frequently mentioned features of the new standard in the consumer technology press.
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.
