Efficient USB port battery charging design principle

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USB provides a convenient way to charge a variety of portable devices with a single "universal" power supply. But there are also many challenges in this regard. Previous USB charging capabilities were quite limited; the current on a standard USB host port was really only suitable for powering computer peripherals such as keyboards, mice, and card readers. USB 2.0 supports a supply current of up to 500mA, which can slowly charge the battery. At the same time, what everyone is looking forward to is how to significantly shorten the charging time.

In this environment, the 1.2 version of the USB charging specification introduced in early 2011 proposed a new power supply mode, which increased the flexibility of charging the device battery. This article describes the concept of a dedicated charging interface (DCP) that can support up to 1.8A. This is almost four times the current carrying capacity of the standard downstream port (SDP) USB 2.0 connection, which greatly speeds up the charging time.

About DCP
The USB interface has 4 shielded wires. They are: VBUS for powering the connected peripherals, negative data terminal D-, positive data terminal D+, and ground GND (Figure 1). In DCP, D+ and D- are shorted together by a 200Ω resistor to prevent data transfer. It indicates to the connected peripheral that the port is fully focused on the charging function and does not provide a master function. Any portable device connected via a USB port can distinguish whether it is in DCP or SDP. This allows it to take advantage of the larger available charging current.

Figure 1: Differences between DCP and SDP.

DCP Identification Figure 2 shows an example of a charging circuit that complies with the 1.2 USB charging specification. It includes a large number of discrete devices that require the system microprocessor to process the recognition program. This means that part of the microprocessor's valuable processing power can't be put into the core application, which affects the overall performance of the system. In addition, a considerable amount of devices are required, which takes more time to write code and achieve a fully efficient overall circuit design.

Figure 2: Traditional USB battery charging line.
FTDI is committed to simplifying USB charging to minimize development time, system complexity and engineering resources for maximum return on investment. This led to the X-CHIP series of USB controller ICs, the main feature is to support the new concept of USB battery charging. The internal circuitry of each IC enables a portable device designed with this chip to detect when the device is connected to the DCP. Once the DCP is detected, X-CHIP sends a signal on one of its CBUS pins to initiate charging.
Figure 3 depicts an X-CHIP based battery charging application. The circuit charges the battery when it is connected to a USB host port or DCP. The CBUS pin can be used to control the battery charge rate (depending on the power supply detected and the corresponding current limit). The battery charge rate is determined by the resistance of the PROG pin connected to the Linear Battery Charge Controller LTC4053.

Figure 3: Simplified USB charging line.
The CBUS pins include BCD#, PWREN#, and SLEEP#.BCD# are open-drain active low output signals. It is used to indicate when X-CHIP is connected to DCP. PWREN# is an open-drain active low output signal. It shows that X-CHIP has been enumerated by the USB host controller. This signal is used to select the value of the resistor on the PROG pin to drive a 500mA charge. SLEEP# is a push-pull active low output signal. It shows when X-CHIP enters USB suspend mode. This signal is used to power off the LTC4053 when the device is powered by the USB host and the host sets X-CHIP to suspend mode.
In Figure 3, the impedance of the PROG pin of the LT4053 is set by resistors R12, R13 and R14. BCD# is used to configure the resistor network of the PROG pin to achieve a charging current of approximately 1A.
The BCD#, PWREN# and SLEEP# outputs are designed to minimize the external circuitry of the charging application. Typically, external MOSFET devices require a range of choices. As mentioned earlier, X-CHIP drives the BCD# signal to indicate the detection of the DCP. The open-drain output shorts R14 to ground, so the impedance of the LTC4053 PROG pin will be 16.5kΩ in parallel with the 1.5kΩ resistor to ground. This can trigger a charging current of nearly 1A. When X-CHIP is connected to a standard USB host controller, this pin is not driven and the device operates as a traditional USB interface chip. Since the BCD# signal is an open-drain output with no internal pull-up, it can be used to pull resistor R14 down to ground without the need to use any external MOSFET.
At device startup, the CBUS pin defaults to an input and has a weak pull-up until the MTP ROM is read. This will take approximately 14ms, after which the CBUS pin will take their selected function, as described in this article. That works like that.

This summary is due to the wide application of USB. The USB port is obviously a convenient place for portable devices to obtain power. By separate interconnection, it can provide 10W power and supply it to various devices. Moreover, the implementation of USB charging standardization in countries (including China and EU member states) enables consumers to use fewer charging cables. This will increase the amount of e-waste entering the environment each year while increasing convenience (this is mainly due to custom adapters). By using an integrated approach rather than relying on discrete devices, you can create better charging circuits for space, device, and engineering resources. USB is not only the most popular digital interconnect, but also a key system component for electronics power and battery charging.