What Happens When The Plug Is Pulled

Peripherals have always had the luxury of design without concern for power. As devices become more mobile that’s changing.


By Pallab Chatterjee
The Consumer Electronics Show featured a large variety of new devices and peripherals for the mobile and portable space. In addition to the tablet, which is the fastest growing data-consumption platform, and the laptop, which is the fastest-growing data creation and modification platform, there are a large number of peripheral devices that are being released.

These devices are for both human interface and input and data capture/playback. These peripherals have long been in the marketplace, and there were questions about the replacement lifecycle of these products as new computer platforms were created. As these new computer platform roll out using new form factors and power factors, the associated peripherals are starting to shift also. Since the 1970s, peripherals for computers were either powered from a wall line cord with on-board high-voltage power supplies or they utilized a high current wall transformer that provided the stepped down operating voltage to the devices.

As the compute devices have moved from being tethered to the wall cord, the peripherals are also being forced to make this transition. These devices for the past 20 years have been connected with a data cable in addition to being powered through a wall transformer. The shift is now to a single cable connection for both power and data. This is driving the power envelope for these peripherals to get smaller. The single cable is a challenge as the devices have had the luxury of high power output from wall plug transformers, in the 1.2A-2.0A at 5V range, for their normal operation.

For things like a mouse or flash drive the new power envelopes are a big limitation for use and design. For devices with mechanical components, new architectures have to be created. Devices such as printers, scanners, video camera interface, disk writers, rotating storage and speakers all have high-current-drain mechanical components with a peak power that is task-dependent.

The two dominant power envelopes from these interfaces are both 5V with 50mA or 500mA current limits. The HDMI interface, which is targeted for video with embedded audio signals and a digital data back channel ,is the 50mA spec. Both USB 2.0 and DisplayPort are the 500mA spec. While USB 3.0 can support up to 900mA of draw, that high level of supplied output power is not compatible with the use-cycle of most portable compute devices when in mobile mode. As a result, the USB 3.0 interface is generally reserved for applications where the computer is connected to a wall socket.

The single cable interface for USB comes in several connector sizes (standard A and standard B), which feature just four pins, two for power and two for data data. This means for high-speed applications, in addition to the device power, the SerDes interface to talk to the USB port has to be powered as well. For USB2 applications, the SerDes needs to support 480Mbps data rates, and for USB3 applications it need to support 4Gbps data rates. Even in small-geometry processes, these data rates consume significant amounts of power when transferring data and also generate a lot of localized heat. The single cable interface may present packaging problem for the stacked die designs as they require the power pads in very close proximity to the high-speed data pads, so the thermal design and noise generated may impact the industrial design of the device.

A typically a design that has a 500mA peak power envelope has to consume less than 200mA on a static basis and not use a fan to dissipate heat. Designers of new peripherals need to look at MEMS devices vs. traditional electro-mechanical devices to reduce the operating power and peak current. These systems also need to look at the incorporation of RF for wireless radio communication and the impact on power and signal integrity for the function that is being performed. These single-cable systems also tend to require modifications of the ECC system to maintain the same BER and reliability of the separated power and data designs.

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