USB has established itself as the world’s principal data interconnect technology. Widely deployed, within the region of 10 billion ports now in operation, its incredible prevalence could be further exploited if it became the primary method for powering equipment too.
Legislation in both Europe and China mandates USB charging capability for all new smart phones. Consumers will benefit since they no longer will have to worry about being forced to use a proprietary cable that’s incompatible with other portable products.
The USB interface has the potential to provide the necessary charge for many different devices via a solitary unified interconnect. But certain performance issues must be addressed first.
Until now, the level of charging supported by USB has been relatively low. For example, USB 2.0 offers a mere 500 mA of charging current. As a result, the time needed to complete the charging process has limited USB’s use.
Over the years, consumers have become used to relatively short recharge times. The idea of having to return to anything that is notably lengthier is not going to help advance the USB-based charging cause. However, this is all about to change.
Version 1.2 of the USB Charging Specification presents the industry with a foundation that can be used to standardize equipment charging. The inclusion of new powering modes makes it better optimised for charging the batteries of portable devices.
The Dedicated Charging Port (DCP) detailed in this revised specification offers the potential to deliver 10 W by supporting current levels of up to 1.8 A. This is a considerable increase over what was previously possible, resulting in shorter charging periods.
Basic Principles Of The DCP
USB interfaces consist of four shielded wires. Two of the wires address data transfer (D–, D+) and two address power (the GND connection, the VBUS connection). Unlike the normal Standard Downstream Port (SDP), the D+ and D– wires in the DCP both have been shorted together to prevent data transfer from occurring.
By shorting these lines together, the DCP can inform a portable electronics product that the port it is connected to is fully focussed on charging and does not provide host functionality. By being able to demarcate between DCP and SDP, the connected product can benefit from the DCP’s increased charging capacity.
Identifying A DCP
Charging circuits that comply with USB Charging Specification Version 1.2 employ a large number of discrete components (Fig. 1). In addition, the microcontroller at the heart of the system has to carry out DCP identification with some processing power, diverting it from its main function and hampering performance. This inefficient approach calls for many hours of code creation and circuit design implementation as well.
Through the recent introduction of its X-CHIP series of USB controller chips, FTDI aims to make it easier for OEMs within the portable electronics sector to add advanced USB-based battery charging to their products (Fig. 2).
This IC integrates functionality that automatically detects whether it is connected to a DCP or SDP, without the additional software and hardware overhead of the circuit in Figure 1. When a DCP has been detected, the IC asserts a signal on one of its CBUS pins to tell the portable device employing it. Battery charging circuits using the X-CHIP can charge a battery when they’re connected to a conventional USB host port or a DCP (Fig. 3).
When designing a peripheral that can be charged over USB, it is important to consider the amount of current available. This depends on whether the peripheral is currently connected to an SDP or a DCP. It also depends on the enumeration state (when connected to an SDP). The charge rate of the battery is in turn defined by the resistance connected to the PROG pin of an LTC4053 battery charging controller IC from Linear Technology.
The CBUS pins are:
- BCD#: This open-drain active-low output signal is asserted to indicate that the X-CHIP has been connected to a DCP
- PWREN#: This open-drain active-low output signal indicates that the USB host controller has enumerated the X-CHIP has been enumerated.
- SLEEP#: This push-pull active-low output signal indicates when the X-CHIP has been put into USB suspend mode. It can shut down the LTC4053 when the device is powered from a USB host and the host has put the X-CHIP into suspend mode.
The PWREN# and SLEEP# outputs indicate the enumeration status and allow the charging circuit to adjust the charging rate of the battery based on the amount of current available. This enables reduced charging time when connected to hosts that can provide higher currents.
In Figure 3, resistors R12, R13, and R14 set the resistance of the LTC4053 controller’s PROG pin. The BCD# configures the resistor network on the PROG pin to set a charging current. The table summarizes the charging current using the values shown.
The maximum current allowed is for the overall peripheral, which also includes the current drawn by the X-CHIP (only 8 mA) and other circuits on the board. The charging current in this example has therefore been set slightly below the maximum allowed.
Outputs BCD#, PWREN#, and SLEEP# have each been designed to minimize the external circuitry needed for charging applications. External MOSFET devices normally are required to select the range.
For example, as the X-CHIP asserts its BCD# signal to indicate detection of the DCP, this open-drain output short circuits R14 to ground. The LTC4053’s PROG pin, then, will have a resistance of 16.5 kΩ in parallel with 1.5 kΩ to ground. This leads to a 1-A charging current being initiated.
When the X-CHIP is connected to a normal USB host controller, this pin is not asserted, so the device just operates as a conventional USB interface device. Because the BCD# signal is open-drain without an internal pull-up, it can be used to pull resistor R14 to ground without using any external MOSFETs.
The CBUS pins will default to an input with weak pull-up, during device start-up, until the MTP ROM is read. The CBUS pins will then take their selected function.
Summary
Because of its huge popularity, USB is a highly flexible and convenient conduit for charging next-generation portable electronic products. Implementation of the updated USB charging specification will decrease the time it takes to complete the charging process, as charging levels can be increased fourfold.
Furthermore, its implementation might significantly reduce the electronic waste entering the environment each year, as fewer adapters would need to be produced and subsequently thrown away.
By using a highly integrated interface solution, rather than one based on more cumbersome discrete components, it is possible to create far more streamlined and effective charging circuits for USB interfaces to supply power to portable goods. It also means that OEMs can keep bills of materials, development times, and engineering overheads in check.