Portable device battery charging—including the ability to support a wide variety of device charger profiles such as USB BC 1.2 charging downstream port (CDP), dedicated charging port (DCP), standard downstream port (SDP), and common proprietary profiles—is only one part of a wide range of demands placed on USB charging ports. Other requirements include maintenance of signal integrity for high speed USB data transmission, and USB host protection from hazardous conditions commonly found in the automotive environment. In addition, small solution size and low electromagnetic emissions are important requirements for meeting the demands of increasingly complex automotive electronics. This article demonstrates a solution that satisfies the requirements of modern USB charging ports in the automotive environment, including design examples.
Overview of an Automotive USB Power System
Figure 1 shows a block diagram of a typical automotive USB charger system, in which a switching regulator generates 5 V from the battery to power V BUS. The USB charging port emulator plus power switch IC shown here has three main functions. First, the USB charging port emulator determines an attached device’s optimal charging current, enabling fast charging via charging port modes such as USB BC 1.2 CDP, DCP, and vendor-proprietary charger emulation profiles. Second, the USB power switch acts as a current limiter and switch, sensing and limiting the bus current. Finally, the port controller supports USB 2.0 high speed data transfer between an attached device and the USB host.
Since USB ports exist in a harsh automotive environment, sensitive USB circuits must be protected from a host of real-world hazards, such as electrostatic discharge (ESD) events at the socket and cable fault events, which can expose affected wiring to voltages well beyond their normal operating values.
Figure 1. Automotive USB charger block diagram.
Figure 2 shows a simplified block diagram of an automotive USB power system that combines many of the power, port, and protection functions into a single IC.