Editor's Note: There is a sidebar that accompanies this article. It can be found at: USB and the challenge of musical connectivity.
With the increasing availability and variety of portable electronic devices, developing an efficient and ecological battery charger strategy is becoming both an important marketing tool as well as a requirement to meet local regulations. For example, the European Commission and Chinese government have begun to mandate that all cell phones must charge through micro USB connectors to eliminate the need for consumers to purchase (and subsequently discard) a charger with each new phone they buy. With the multifunctional capabilities of most portable devices - today's cell phones also serve as a mobile Internet connection, email on-the-go, music player, and entertainment device - users need to be able to charge devices frequently. For many users, the drive to and from work provides an excellent opportunity to recharge devices.
With USB now serving as the main connector for most cell phones today, auto manufacturers around the world are increasingly integrating USB into the vehicle console to serve as both a data connection and as a universal charging interface. An integrated USB port provides many benefits directly to consumers, including allowing handsets to be connected with the low-cost USB cables they come packaged with and eliminating the expense of a $20 to $40 cigarette lighter charger for use in each car the consumer drives on a regular basis. Cigarette lighter-based chargers are also often bulky, ugly, and stick out in a way that can interfere with driving. A direct USB connection, in contrast, is small and can be flush with the dash or easily hidden under the arm of the seat or in the center console. Its streamlined design is also more aesthetically appealing to consumers, raising the overall perceived ease-of-use of the vehicle.
Introducing USB into automobiles as a battery charger technology, however, is not without challenges. Consumers are used to fast charging times when using a wall-based charger, and they expect the same performance when charging a handset in their vehicle despite the charging limitations of the USB standard. Automotive engineers must also address the compatibility issues that arise from the intelligent communication protocols today's handsets utilize to prevent overcurrent damage from "fast" chargers that provide too much current to devices and can damage both handset and USB port circuitry. Finally, given the extreme cost pressures of the automotive market, any USB charging port must be implemented at the lowest possible expense.
This article will explore the underlying considerations behind using USB as a console-based charging technology. It will cover the various types of USB connections, including how to design a Dedicated Charging Port that reduces charging time and system cost while addressing compatibility issues.
Dedicated Charging Ports
Moving to USB-based technology provides a greener approach to charging portable devices by eliminating the arbitrary hardware and form factor differences between chargers for all types of devices. Next-generation phones often require a charger that is not compatible with the chargers consumers already have and, as a result, these still-functional chargers become a part of the waste stream. The ubiquity of USB throughout the consumer electronics industry replaces the store wall of different brands and makes of chargers with a standard USB cable that guarantees charger compatibility and a robust connection to any phone or music player. The economies of scale achieved by USB also ensure a substantially lower cost compared to purchasing an individual charging unit.
While USB can supply power to devices, at its foundation it was designed as a data communications interface. Powering and charging of devices is enabled through the Standard Downstream Port (SDP) definition in the USB spec which can supply up to a maximum of 500 mA charging current per attached device. This 500 mA limitation is due in part to the need for an SDP to supply current while simultaneously supporting high bandwidth data transfers.
For those applications that require it, USB's high data rate will serve well as a fast and reliable interface between portable devices and the automotive console. However, while music playback is commonly thought of as the foremost use for USB's data capabilities in a vehicle, USB transfer is not the primary model expected for music playback in a car (see sidebar, The Challenge of Musical Connectivity). Thus, for many vehicles, the chief purpose of an in-car USB port is expected to be for charging portable devices. In these cases, only the charging capabilities of USB are required; system cost can be reduced by eliminating the interface's data capabilities.
A USB port optimized for only charging is known as a Dedicated Charging Port or DCP. A DCP reduces the cost of USB-based charging by implementing only those components required for charging devices. A DCP-based controller can be less than half the cost of an SDP-based implementation that requires a full USB controller and PHY.
Even for those applications requiring USB-based data transfer capabilities, it is often the case that a single USB port is not sufficient. For example, a second charging port is required either for when the driver uses a music player and wants to charge a handset or for when a passenger wants to charge a handset as well. As multiple data ports are not required in the majority of use cases, the most cost-effective approach is to implement all secondary ports as a DCP.
Charging time is dependent upon the charging current, and the more current a device can draw, the faster it can charge. When an iPad is connected to a wall charger, for example, the device can draw substantially more current compared to a USB-based charger using an SDP that is limited to at most 500 mA.
A DCP has the advantage of not only enabling device charging more cost-effectively than a SDP, it can support a higher charging current than is specified by the SDP standard to decrease charging times. By supplying up to 2.0 A charging current, a DCP port can significantly reduce device charging time compared to chargers limited to 500mA or less using SDP.
A recent addition to the USB spec, the Charging Downstream Port (CDP) offers another alternative to SDP. A CDP can supply up to 1.5A even when data is being transferred at full or low speed. If data is being transferred at high speed, however, then the current will drop to ~900 mA. A CDP may be appropriate for use with a primary USB port if the console supports applications requiring high-speed data transfer capabilities. However, CDP is not supported by all USB controllers, and therefore an external complementary circuit maybe required to implement CDP. In addition, a CDP still requires a full USB controller, resulting in a higher cost compared to a DCP for a charge-only port where a data interface is unnecessary.
SDP defines how much current each device can draw, with a set maximum limit of 500 mA per device. In addition, a hub must also manage the maximum current draw it can support among multiple devices. For example, a 2-port hub needs to support a maximum current draw of 1 A (i.e., two ports at 500mA) across all of its ports.
Many cell phone manufacturers understand the 500mA limit of SDP and design their power management blocks to draw less than 500mA to provide a margin of safety. However, this also means it takes longer to recharge the battery. Some aftermarket charger manufacturers, seeing a potential competitive advantage, design their chargers to source up to 700mA and so provide faster charging. The issue is that without a communication protocol established, the cell phone will never know the 700mA capabilities of these aftermarket chargers and could in turn only draw up to 500mA.
Exceeding USB port and hub current draw limits, however, can potentially damage the charger, portable device, and hub. Overcurrent has long been a significant problem for handset manufacturers. One of their worst return of materials (ROM) issues is that of handset PCBs which have melted from having so much current sent through them. Potential meltdown of electronic components leads to higher product return rates and increased liability (i.e., from injuries).
Even multi-port hubs are vulnerable: if two non-compliant devices are attached to a two-port hub with each device trying to draw 700mA, the resulting current draw will exceed the hub's 1.0 A maximum current. These non-compliant chargers pose a challenge to introducing USB-based charging into vehicles. Such chargers, if left to operate unmonitored, can damage internal vehicle circuitry that is difficult and expensive to replace.
To protect the USB port against shorts and non-compliant devices which draw more than the specified current, the USB port requires a power switching circuit. A power switch prevents product damage by monitoring the current being drawn as well as the dynamic temperature across the power supply path. If either an overcurrent condition or temperature threshold is reached, the power switch shuts down the port. Power switch protection or power policing capabilities, as they are also known, also protect against faulty chargers or phones which may have a short and draw a high, continuous - and dangerous - current if ignored. Power switch or power policing capabilities are available as an optional integrated feature for those applications where circuit protection is important.
To prevent cell phones from drawing too much current through "fast" chargers, handset manufacturers are adding intelligence to the charging process via communication protocols. By introducing a handshake between the charger and handset, handsets can authenticate the battery charger as a known and trusted source. If the charger is not recognized, then the handset can notify the user that the accessory is not supported. By utilizing communication protocols between chargers and batteries, manufacturers can prevent non-authorized chargers from providing too much current to devices.
The addition of such intelligence, however, adds complexity to the overall USB charger design and introduces interoperability issues as well. Different devices can handle varying amounts of charge current safely, and even though a DCP may be able to supply more current, that current must be supplied in a way that the handset can draw in a reliable and safe manner.
Charging protocols vary from phone to phone. For example, Blackberry follows the USB Battery Charging 1.0 spec while Nokia and Motorola follow the 1.1 specification. Phones to be sold in China must follow the YDT-1591 spec, as mandated by the government, and Apple uses its own proprietary protocol. In addition, version 1.2 of the USB Battery Charging specification is under development.
To serve as a charger, the DCP must support the charging protocol required for the handset. To serve as a universal charger, the DCP must support each of the charging protocols. Therefore, before charging can begin, the DCP must identify the handset brand and type to determine the charging protocol in use. Next the DCP notifies the handset what current draw it supports. The handset acknowledges the amount of current it wants to draw and then starts to draw that current.
Figure 1. This figure shows a typical architecture for implementing a Dedicated Charging Port (DCP). Since DCP controllers do not need to support data transfer applications, their internal circuitry is significantly less complex than that of a full USB controller. This results in not only substantially lower system cost but also the ability to supply a higher current for faster charging.
By implementing device identification mechanisms, a DCP can automatically select the appropriate charging protocol based on the device it is connected to. When these protocols are integrated into the USB charger controller, the overall system architecture is compact, self-contained, and straightforward to implement.
Implementing a DCP or CDP
There are a number of different ways to implement a Dedicated Charging Port. Pericom, for example, offers a family of USB charger controllers with optional power policing capabilities. These self-contained controllers also provide full support for the various charging communications protocols in use today so that a universal handset charger can be implemented using a single chip. In addition, since charger controllers do not need to support data transfer applications, their internal circuitry is significantly less complex than that of a full USB controller. This results in not only a substantially lower system cost, but also the ability to supply a higher current - and therefore faster charging - than is supplied by an SDP from a standard USB controller. Figure 1 shows the architecture of a typical DCP.
For applications requiring data transfer capabilities, a USB charger controller can be used to increase the maximum charging current above the 500 mA limitation of an SDP. As shown in Figure 2, the USB charger controller is placed inline between the standard USB controller and USB port connector. The USB charger controller passes all data through transparently while boosting the charging current up to 1.5 A by converting the port into a CDP. The CDP controller will manage the charging protocol communication and therefore avoids any need to alter the USB protocol stack on the host. This approach is useful for increasing the charging current in applications where the USB controller is already integrated into the main application processor but a higher charging current is desired.
Figure 2: A Dedicated Charging Port (DCP) controller can be placed inline with a processor that has an integrated USB controller to boost the charging current up to 2.0 A and support high speed data transfer. Since the DCP controller also manages all charging protocol communications, no modification is necessary to the USB protocol stack on the host [confirm].
Note: can it support simultaneous high-speed data transfer and fast charger?
USB-based charging through a Dedicated Charging Port brings many benefits to both automotive manufacturers and consumers. The ability to connect a handset using a standard USB cable eliminates the need for a specialized charger while providing a more aesthetic placement of charger ports. DCPs also overcome the charge current limitations of SDPs, enabling significantly faster charging with currents up to 2.0 A per device. With full support for charging communication protocols, a DCP can serve as a universal charger with interoperability across all handsets. Optional power switch capabilities also ensure safe and reliable operation when unexpected shorts occur to prevent damage to the console.
USB-based battery charging technology is already changing the automotive industry worldwide. By offering reliability, universal compatibility, and faster charging times through a low-cost DCP controller, auto manufacturers have a simple yet powerful way to differentiate their consoles from the competition.
About the Author
Abdullah Raouf is a senior product marketing manager for Pericom Semiconductor. With a BSEE from the University of California Davis, has worked for over ten years in Business Development and Product Marketing, mainly focusing on the semiconductor industry. Abdullah also serves as a member of the VESA, HDMI, and USB organizations.