Plug-and-Play USB Charging Controller Makes Charging of Battery-Powered Devices Seamless
By Johnny Lu, PTC Marketing Manager
In the not-too-distant past, it seemed like every electronic device had its own custom power supply, cable, and interface (Figure 1). USB Type-C® (or USB-C®), which implements the USB power delivery (USB-PD) specification, was supposed to solve this problem by introducing a single standard interface that all devices could use. Unfortunately, while solving one problem, another was created—nowadays the choice of cable (rather than the connector interface) is the major concern when charging a device. Some cables allow faster charging than others and unless the correct (usually vendor-specific) cable type is chosen, some devices will charge slowly or not at all (and legacy devices may even be damaged). There exists a large variation between standard USB ports rated for 500mA and dedicated charging ports delivering up to 3A of current. It is important for a charger to be able to determine the optimal current to use when charging a downstream device.
Figure 1. Assortment of power cables for different electronic devices
The USB Implementers Forum (USB-IF) defines the standard USB-A cable to have four wires and a grounded case (Figure 2). These include a 5VDC, ground, and a data pair for Data+ (D+) and Data- (D-). Data is transmitted on D+/- while the charging current is provided on the 5-volt wires. D+/- are also used to determine the appropriate charging profiles for a downstream port device or client device. This allows the charger to determine how much current to source when charging the client device. This type of smart charging is essential for high capacity (larger current) battery devices like tablets and laptops, and helps to reduce the charging time of on-the-go devices.
Figure 2. Pin configuration for standard USB-A connector
The data pair D+/- is also preserved in the USB-C standard and is mandatory for all USB-C devices to support, ensuring backward compatibility.
Figure 3. Pin configuration for standard USB-C male connector
A USB Type-C to USB Type-A cable is defined to ensure maximum connectivity between new USB-C hosts and legacy USB-A devices. The end of the C-to-A charging cable that is terminated by a USB Type-C connector (Figure 3) is connected to the device being charged, while the end terminated by a standard USB-A (Figure 2) connects to the charger.
Along with the introduction of USB Type-C, the USB Power Delivery (USB-PD) specification was introduced. The USB-PD specification offers the following features:
- Up to 100W of power
- Bidirectional power transfer (host or peripheral can provide power)
- Optimize power management across multiple peripherals by allowing each device to take only the power it requires, but can draw more power if required for a given application
- Intelligent and flexible system-level management of power via optional hub communication with the PC
- Allows low-power peripherals (such as headsets) to negotiate for only the power they require
USB-PD grew from its predecessor, the USB Battery Charging Specification v1.2 (BC1.2), which identified three charging profiles: the standard downstream port (SDP), the charging downstream port (CDP), and the dedicated charging port (DCP) whose features are summarized in Table 1. Note that USB-PD is not mandatory for all USB-C hosts and devices. In contrast, virtually all USB2.0 hosts of a laptop or desktop PC support BC1.2 charging profiles. It is also common to implement BC1.2 support on D+/- in a USB-C port. Implementing BC1.2 on both USB-A and USB-C ports ensures that user requirements are met; fast charging is experienced no matter the port the client device connects to.
Table 1. Device charging profiles from BC1.2 specification
A USB charging controller, such as the PI5USB2546A DIODESTM (Figure 4), is required to identify devices with a charging profile other than SDP. This IC is inserted in the data path between the USB host controller and the connector where the USB cable is plugged into. If the bus is configured and not suspended (e.g., a PC in S0 “on” mode), the SDP can only charge with a maximum current of 500mA for USB 2.0, or 900mA for USB 3.x. This is insufficient in meeting the charging requirements of most modern electronic devices.
Figure 4. A USB charging controller IC is located between the USB host controller and the USB port
USB-IF charging profiles are well defined, but those of proprietary profiles are not as straightforward to identify. Identifying the correct charging profile, called “profile enumeration”, has two stages. For simplicity, it is assumed the charging port is on a laptop computer with its CPU in standby or sleep mode (or completely powered off). Even though the device is powered off, BC1.2 stipulates that this USB port can still be used for charging.
Stage 1: Primary Detection
This stage determines if the port is an SDP, CDP, or DCP.
- The client device sends out a 0.5 – 0.7V pulse on D+ and waits for a similar pulse from the USB port on D-. If there is no response on D-, the profile is SDP.
- If the client device receives a response on D-, secondary detection begins.
Stage 2: Secondary detection
This stage determines if the profile is CDP or DCP.
- The client device then sends out a similar pulse on D- and waits for a reply on D+
- If there is no reply on D+ then the profile is CDP.
- If a 0.5 – 0.7V signal is detected on D+ then the profile is DCP. This is commonly done by the charging IC by simply shorting D+/D- together.
The enumeration sequence for auto-DCP mode with proprietary profiles is more complex and uses different voltage levels (Figure 5). The PI5USB2546A USB charging controller contains logic and voltage dividers to properly apply a dedicated fast charge. The sensing control and voltage divider circuits are necessary to match the proprietary profiles. The steps in this enumeration sequence, patented by Diodes Incorporated (US 8,237,414 B1; 2012), are:
- Identify the client device
- Select the correct communication protocol
- Identify the profile required (including non-BC1.2/proprietary profiles)
- Adjust the voltages to match the profile identified
Charging of the client device then begins and is managed by the PI5USB2546A.
Figure 5. Enumeration of proprietary charging profiles
Apart from being able to enumerate USB-IF and proprietary charging profiles, the PI5USB2546A brings several additional features and benefits to smart charging applications. These include:
- Adjustable current limit: This protects the USB port, the host, and the load if a short circuit occurs or if a faulty battery draws excessive current. Having a programmable current maximum provides design flexibility and allows for future proofing.
- Undervoltage lockout: This feature disables the charge IC if the system input voltage is too low. This can occur when the host is battery-powered, the CPU and multiple peripherals are operating, or for any other reason that causes the supply voltage to unexpectedly drop. By disabling charging in this condition, damage is less likely to be caused to the PD or its battery.
- Sleep-mode charging: This pertains to the common S3/S4/S5 modes. An intelligent charge controller such as the PI5USB2546, PI5USB2546A, PI5USB2546J, and the PI5USB2546H will still enumerate the PD and determine the proper BC1.2 or proprietary charging profile.
- Keyboard/mouse wake-up: This is a feature of most USB host controllers but only works if the USB charger between the host and the port properly conducts this signal.
USB-C is quickly becoming the industry-standard interface for charging battery-powered devices. However, advances in battery technology mean that the charging profiles of portable devices are constantly evolving. These devices require a USB charge controller that can adapt to these changes. To maintain the best user experience, more and more host devices (such as laptop PCs and desktop PCs) implement BC1.2 charging capability on USB-C ports. This is even more important for those host devices without USB-PD support. In this article, we identified that the features of the PI5USB2546x charging controller make it an ideal plug-and-play solution for both USB-A and USB-C charging applications.
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