Overview of USB Battery Charging Revision 1.2 and the Important Role of Adapter Emulators

Introduction

Portable devices are ubiquitous. Whether at home, in a car, or connected to a computer, USB-powered equipment requires a smart method to determine the correct power draw for operation and charging. The vast array of portable devices, variations in USB ports, and the complexity of rechargeable batteries make the USB Battery Charging Specification Revision 1.2 (BC1.2), released in 2010, a crucial standard. It defines the proper way to charge a battery from a USB port. Despite BC1.2, some manufacturers continue to use proprietary chargers, adding complexity to USB battery charging. This article explores the reasons behind recent industry standards for USB battery charging, their specifications, and strategies for implementing USB ports capable of high-current charging for various proprietary devices.

The Need for a Battery Charging Standard—the Pre-BC1.2 Era

The widespread adoption of USB was driven by its ability to power peripheral devices. Initially conceived in the mid-1990s to connect external devices like keyboards, mice, and printers to computers, USB's utility expanded as portable products proliferated. The ability to provide power through the same connector used for data transfer gave USB a significant advantage in the portable market.

Before the first battery charging specification in 2007, charging a battery via USB was unpredictable. With USB 2.0 in 2000, a peripheral device could draw 100mA by default, or negotiate up to 500mA. If the bus was idle, it would enter a "suspend" mode, limiting current to 2.5mA. A device with a depleted battery could only reliably draw 2.5mA from a standard port.

In practice, many manufacturers did not strictly adhere to USB 2.0 current limits. Some ports allowed 100mA regardless of enumeration, while others offered 500mA without negotiation. Devices requiring more than 100mA often assumed 500mA was always available, leading to issues.

A robust charging scheme needs to signal safe power levels. The ambiguity of the previous USB "state of affairs" caused problems: some ports would shut down entirely until the device was unplugged and reattached, while others would issue a USB system reset.

BC1.2 Is Introduced

Battery charging was not an original feature of USB. BC1.2 provided the first official provision for charging devices that were powered off. By establishing a clear method for communicating USB port power capabilities, BC1.2 resolved many prior issues.

All rechargeable batteries require specific charging considerations. For instance, Li+ battery manufacturers may specify a minimum discharge level, requiring a preconditioning mode for deeply depleted batteries before a full charge. Once a nominal condition is reached, a higher charging current (constant-current mode) can reduce charge time. As the battery nears full charge, continuing in constant-current mode can be harmful; a smart solution switches to a constant-voltage mode for "top-off." Due to battery complexity, most portable devices integrate a dedicated battery-charge controller.

An advantage of BC1.2 is its provision for charging dead or weak batteries. Batteries below a "weak battery threshold" can charge with currents higher than the 2.5mA suspend current, irrespective of the port type. Once a nominal level is reached, the device must enumerate within a specific timeframe to maintain higher current draw from ports requiring enumeration.

Not All Ports Are Created Equal

BC1.2 defines three distinct USB port types and two key monikers:

• Charging Port: Delivers currents higher than 500mA.
• Downstream Port: Signals data according to USB 2.0.

The BC1.2 specification details how each port should appear to the end device and the protocol for identifying the port type. The three USB BC1.2 port types are Standard Downstream Port (SDP), Dedicated Charging Port (DCP), and Charging Downstream Port (CDP).

1. Standard Downstream Port (SDP): Features 15kΩ pulldown resistors on the D+ and D- lines. Current limits are 2.5mA when suspended, 100mA when connected, and 500mA when connected and configured for higher power.
2. Dedicated Charging Port (DCP): Does not support data transfer but can supply charge currents beyond 1.5A. It features a short between the D+ and D- lines. This port type enables wall chargers and car chargers with high-charge capability without the need for enumeration.
3. Charging Downstream Port (CDP): Supports both high-current charging and data transfer, fully compliant with USB 2.0. It includes 15kΩ pulldown resistors for D+ and D- communication and internal circuitry activated during charger detection to help portable devices distinguish it from other port types.

Diagram Description (Figure 1): A schematic illustrates the three USB BC1.2 port types: Dedicated Charging Port (DCP), Standard Downstream Port (SDP), and Charging Downstream Port (CDP). The DCP shows D+ and D- lines shorted together with a maximum impedance of 200Ω. The SDP shows 15kΩ pulldown resistors (RDP_DWN, RDM_DWN) on the D+ and D- lines, connected to VDAT_REF and VDM_SRC respectively. The CDP includes similar pulldown resistors along with internal circuitry labeled PRTBL_DET and VDM_SRC, connected to the D+ and D- lines.

DCPs are electrically simple to emulate: D+ and D- are shorted together (maximum impedance of 200Ω), and the lines are left floating with respect to ground. A portable device identifies a DCP by driving one line (e.g., D+) and observing the other line (D-) for the same signal. A basic setup involves shorting the two pins and connecting to an existing wall charger capable of supplying 2A at 5V.

Proprietary Chargers and Adapter Emulators

Even with the BC1.2 specification, some manufacturers use custom protocols for their dedicated chargers. Connecting a device to a BC1.2-compliant port might result in an error message like "Charging is not supported with this accessory," although charging may still occur at very low currents. These proprietary chargers often identify themselves by setting DC levels on the D+ and D- lines via resistor-dividers between 5V and ground.

Diagram Description (Figure 2): This figure displays dedicated charging ports from various manufacturers. It shows the D+ and D- lines for a Standard USB Host Charging Downstream Port, an Apple Charger, a Sony Charger, and a Dedicated Charger. The Apple charger uses ADPPU (75.0kΩ) and ADPPD (49.9kΩ) resistors. The Sony charger uses SDPPU (5.1kΩ) and SDPPD (10kΩ) resistors. The Dedicated Charger uses 2MΩ (MIN) resistors. These are connected to VBUS (5.0V) and ground, with some variations including VLOAD PU (3.6V) and pulldown resistors (HLPU 300kΩ, HPD 14.25kΩ to 24.8kΩ).

A smart and inexpensive solution exists to optimally charge devices from different manufacturers and those compliant with BC1.2.

USB Charger Adapter Emulators: A USB charger adapter emulator is a device that allows a dedicated charger to appear as either a BC1.2 DCP or another proprietary charger. These emulators provide a dynamic element to a wall charger without needing a separate control unit to detect the attached device. Many are hardware configurable to select between different charger identification profiles, while others feature autodetection circuitry to sense the portable device and switch between manufacturer-specific voltage-dividers or the standardized BC1.2 DCP mode.

To be effective and integrate conveniently into a wall charger, USB charger adapter emulators require a small profile and a low external component count. For example, the MAX14630/MAX14632 are charger adapter emulators configurable to automatically detect USB BC1.2-compliant devices, Apple 1.0A devices, Apple 2.1A devices, or Samsung Galaxy Tablet 2A devices. Each requires only one bypass capacitor and comes in a 2.90mm x 1.60mm package. The circuit in Figure 3 demonstrates a quick implementation of a single dedicated charger system compatible with Apple 1A and USB BC1.2-compliant devices. This emulator connects a resistive divider to the data lines by default but can automatically detect a USB BC1.2 device and short D+ and D- together as per the BC1.2 specification. When used with an AC-to-DC 5V power supply, a variety of portable devices can be optimally charged by using an adapter emulator to communicate the proper current limits.

Diagram Description (Figure 3): This circuit diagram shows a DCP example for autodetection of USB BC1.2/Apple 1A devices, featuring the MAX14630/MAX14632 USB charger adapter emulators. It includes input power (Vcc), a 0.1μF capacitor, the MAX14630/MAX14632 IC with DM and DP pins, a bridge rectifier, a DC-DC converter block, a 150μF capacitor, and a USB A connector with D+, D-, and VBUS pins. The IC is connected to GND and Vcc. The DM/DP pins are routed through circuitry to the USB connector's D-/D+ pins.

Dedicated chargers are relatively simple. Charging downstream ports (CDPs) add complexity by supporting USB 2.0 data rates and handling up to 1.5A of charge current. To distinguish itself from a dedicated charger, a CDP has internal circuitry (outlined in BC1.2) that drives the D- line to a specific voltage when it senses a portable device driving D+ during port detection. This internal circuitry must only be active during port detection and contribute minimal parasitic capacitance when switched off, adhering to USB 2.0 specifications for high-speed USB communication.

After the port-detection phase, a BC1.2-compliant CDP disconnects its internal circuitry, allowing normal USB 2.0 data transfer. A challenge in CDPs, unlike DCPs, is digital noise margins. According to USB 2.0, a 100mA ground current through a USB cable can cause a 25mV difference between host and device ground. Since currents can reach 1.5A, both CDPs and compliant portable devices must resolve data with a maximum ground offset of 375mV from device to host. This highlights the need for careful design to ensure proper CDP operation.

USB Host Adapter Emulator

A USB host adapter emulator combines high-speed USB analog switches to handle full USB 2.0 traffic at 480Mbps and USB charger adapter emulator circuitry. In addition to DCP and proprietary charger profiles, host adapter emulators can be configured for SDP and CDP passthrough modes, as outlined in BC1.2. In CDP passthrough mode, the devices emulate CDP functionality upon device attachment, then hand over control of the D+ and D- lines to the USB host transceiver for normal USB 2.0 traffic after the charger detection phase.

The configurability of these devices makes host adapter emulators particularly suitable for computers. For example, when a computer is connected to its power supply, it can enable a high-current USB charging port by configuring its host adapter emulator as a CDP. When running on battery power, the computer can switch the adapter emulator to a standard USB port configuration to limit the current draw to 500mA maximum. By reconfiguring its adapter emulator as a dedicated charger, a laptop can still offer high-current charging from its USB port while powered down but plugged in.

An important consideration for host adapter emulators is how they handle a USB bus during reconfiguration. Improper switching can cause faults in downstream USB devices. Therefore, a bus reset or current-limiting switch is often included. For instance, in addition to supporting SDP passthrough, CDP passthrough, DCP, Apple 2.1A, and Samsung 2A modes, devices like the MAX14640, MAX14641, MAX14642, MAX14643, and MAX14644 issue a bus reset to ensure downstream devices are aware of changes. They also feature an automatic current-limit switch-control output that resets portable devices when the host transceiver reconfigures the emulator.

The charger and host adapter emulators mentioned are designed to be coupled with a separate power supply, either from a wall adapter or a computer's supply. Beyond home and computer use, the third most common place to charge a USB device is in an automobile. Auxiliary power outlets in vehicles typically provide 12V DC, and sometimes 24V DC, but this voltage can fluctuate widely (from 9V to 28V, with temporary surges up to 40V). USB port applications like car chargers and navigation systems require automotive-qualified USB charger/host adapter emulators and rugged DC-DC converters to generate the necessary 5V for USB power.

Using an adapter emulator offers the advantage of isolating the host USB transceiver from potential ESD damage, USB line shorts, or battery short circuits. The DC-DC converter must guarantee proper operation over a wide range of input voltages. Preserving the car battery is critical, so a well-designed converter includes an adjustable current-limiting circuit. Additionally, car charging systems need to sense output current and communicate this data to the control unit, requiring a current-sense amplifier. The MAX16984 is an example of a product class designed for integrating the DC-DC converter, host adapter emulator circuitry, ESD protection, and current-sense amplifier into a single IC. The MAX16984 supports 4.5V to 28V inputs (with protection up to 42V transients), has built-in support for a resistive feedback network to monitor current, features USB BC1.2 compliance, supports Hi-Speed (480Mbps) USB data, and can emulate Apple 1A/2.1A chargers.

Diagram Description (Figure 4): This is a block diagram of a highly integrated DC-DC step-down converter with USB host charger adapter circuitry using the MAX16984. It shows input connections (IN, VBUS, GND, D+, D-) to a USB connector. The MAX16984 IC block encompasses sections for I/O Control and Diagnostics, USB Auto DCP/CDP/Apple/iPhone/iPad Charger Detection, a DC-DC converter (with associated control pins like BST, SUP, LX, PGND, SYNC, FOSC, BIAS, ENBUCK, REF, I LIMIT), and a Current-Sense Amp (with sensing pins SENSP, SENSN, feedback pins FBCAP, CFBCAP, and output pins RSENSO, RFBMAX, FBMAX, FBPER). It also includes connections for fault output and I/O voltage (3.3V USB I/O VOLTAGE) to a Low-Voltage µC or ASIC with Integrated USB Transceiver.

Summary

The USB BC1.2 standard, defined in 2010, enabled battery charging for powered-off devices and provided for charging dead or weak batteries. It defined three distinct USB port types, allowing USB ports to communicate their power capabilities to USB-powered equipment, ensuring safe charging for a wide range of portable devices. The development of USB adapter emulators adds significant configurability to USB port design.

For charging-focused USB ports, new charger adapter emulators increase the number of compatible pluggable devices. In dynamic applications, such as those involving computers, host adapter emulators simplify design. In harsh automotive environments, integrated DC-DC converters and adapter emulators can manage fluctuating voltage levels, reduce costs, and save space.

General References

1. The USB-IF. (2010, December 7). Battery Charging Specification Revision 1.2.
2. Sherman, L., Maxim Integrated application note 4803, "The Basics of USB Battery Charging: A Survival Guide."
3. Sherman, L., Maxim Integrated application note 3241, "Charging Batteries Using USB Power."
4. channelE MAGAZINE FOR ELECTRONICS, 25-10-11, Simion, D., "Emulators and Detectors for USB Battery Charging: Industry Migrates to a Global Standard."

Trademarks

Apple is a registered trademark of Apple Inc.

Samsung is a registered trademark of Samsung Electronics Co., Ltd.

Sony is a registered trademark and registered service mark of Kabushiki Kaisha TA Sony Corporation.

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