USB first changed the electronics world by enabling near-universal connectivity among the myriad of computer and portable devices. Now, in response to a rising need for ecological charging strategies, USB is once again being called upon to unify the consumer electronics industry as a universal charging technology.

The arbitrary hardware and connector differences between chargers for all sorts of consumer electronics devices require users to discard fully functional chargers, simply because next-generation devices often aren’t compatible with them. To eliminate this waste stream, governments worldwide have begun to introduce local regulations that impact how portable devices can be charged. For example, the Chinese government and the European Commission require all new handsets to charge through a micro USB connector.

The near-complete penetration of USB into computers and electronic devices positions it well as a platform upon which to replace the plethora of brands of chargers. In addition, USB’s economies of scale combined with a device’s ability to connect to a charger with a standard USB cable, which most consumers already own, means that USB provides a greener charging alternative compared to individual charging units.

The most effective green strategies, however, are those that not only reduce the waste stream, but also lower cost. Moving to USB-based charging technology allows devices to be charged with the data cable they’re shipped with, eliminating the need for a home wall-based charger and separate aftermarket cigarette lighter/car charger, each costing $20 to $40. USB also uses a sleek, aesthetically pleasing connector that can sit flush with the casing of a device or the dash of a vehicle, compared to bulky standalone chargers.

Making the shift to USB-based charging poses a variety of design challenges, though. The current draw limitations of Standard Downstream USB ports can significantly increase charging time compared to the chargers they’re intended to replace, causing them to fall short of consumer expectations.

Universal chargers must also address compatibility issues arising from the use of intelligent communication protocols. Such protocols authenticate chargers to prevent non-compliant chargers from providing too much current and, thus, damaging devices. Finally, given the cost-conscious mindset of the consumer electronics market, any USB-based charger must be implemented at the lowest possible expense.

Dedicated Charging Ports

For all of its advantages as a charging technology, USB is first and foremost a data-communications interface. As such, a full USB controller comprises complex circuitry to support high-speed data transfers. The Standard Downstream Port (SDP) definition within the USB specification also limits powering and charging device—it only supplies up to 500-mA charging current per attached device. This 500-mA limitation is due, in part, to the SDP’s need to simultaneously support high-bandwidth data transfers.

For applications that require data transfers, a full USB controller provides a reliable, ubiquitous interface as well as charging capabilities. For many applications, however, data-transfer capabilities are unnecessary. For example, a wall charger only supplies power and has no need to receive data. In these cases, a USB port optimized solely for charging will provide a more efficient and cost-effective approach.

Such dedicated charging ports (DCPs) reduce the cost and complexity of USB-based chargers by implementing only those components required for charging. The savings are significant: a DCP-based charger controller can be less than half the cost of an SDP-based implementation requiring a full USB controller and physical layer (PHY).

DCPs are appropriate even for applications requiring USB-based data-transfer capabilities that, depending on the application, may need several USB ports. In a car, for example, the driver and passengers may wish to charge multiple devices. Developers should consider the true value of implementing each port as a full USB port since only one device connects to the console at a time.

While the primary USB port can be used to connect devices, implementing additional ports with full USB capabilities is unnecessary given that they only will be used to charge devices. Secondary ports, then, are better implemented as DCPs offering fast charging capabilities, since DCPs can increase the current draw limit from 500 mA to 5 A.

Is Data Transfer Always Necessary?

The need for data-transfer capabilities is worth a closer look. For example, streaming audio from a portable device to an entertainment system may be better implemented using a standard audio jack than a USB port. From a system perspective, using a full USB port to charge both a device and stream audio seems to be the most cost-effective approach.

However, the architecture required to support USB-based playback is fairly complex. Not only must the system include a USB port with full functionality, it also must support a wide range of audio codecs to ensure compatibility (Fig. 1a).

The issue of universal audio compatibility should not be overlooked. Home-entertainment equipment and automotive manufacturers don’t want to lose customers because a prospective customer’s digital device isn’t supported.

Further complicating compatibility is the proprietary codec employed by Apple devices, which prevents devices from streaming unprotected content over USB. No hardware-based codecs currently exist for Apple’s content format. Therefore, supporting playback from Apple devices requires a subsystem that can run iTunes (Fig. 1b).

The cost of components, e.g., a high-performance processor, operating system, and additional memory, clearly exceed the cost of the alternative—a simple audio jack for playback and a low-cost USB charger controller for power (Fig. 1c). Stereo and car console manufacturers worldwide are using this approach, as it guarantees 100% connectivity with all digital devices while minimizing cost and system complexity.

Charging Time

Since devices must be tethered when charging, the more consumers find a device useful, the more inconvenient the device seemingly becomes to charge. Thus, charging time is a high-profile feature of any portable consumer-electronics device. Ideally, a device can be completely charged within a short period, such as during the drive home.

Charging time is a function of charging current—the more current a device can draw, the faster it can charge. Charge current depends on the power source and the method of power supply. For example, devices can draw substantially more current from a traditional wall charger than a USB-based wall charger using an SDP, which is limited to 500 mA maximum.

By supporting a higher charging current than that specified by the SDP standard, a DCP combines the cost advantage of a universal USB-based charger with the higher current of a wall-based charger. With the ability to supply above 1.8-A charging current, a DCP can dramatically reduce device charging time compared to SDP-based ports.

Recently, the drafters of the USB spec introduced the Charging Downstream Port (CDP) as an alternative to the SDP. A CDP can supply higher current—1.5 A to 5 A—even when transferring data through the USB2.0 protocol.

However, not all USB controllers support CDPs, which creates significant compatibility issues because a CDP may require external circuitry to operate with legacy devices. In any case, a CDP still requires a full USB controller, resulting in higher system cost compared to a DCP for a charge-only port in applications that don’t require data transfers.