July 28, 2021
The USB charging port has become an important part of modern vehicle infotainment systems. Passengers are becoming more and more accustomed to charging smart phones (or other portable devices) through the vehicle's electrical system, and in turn use these devices to enrich the vehicle's information and entertainment functions. In order to support both power and data capabilities, and to adapt to the rapidly changing portable device market, USB charging ports must meet various system requirements related to power, data transmission, and robustness, even in the face of various dangerous situations in reality.
Portable device battery charging-including the ability to support a wide range of device charging protocols, such as USB BC 1.2 Charging Downstream Port (CDP), Dedicated Charging Port (DCP), Standard Downstream Port (SDP) and various common proprietary protocols-only It is part of the many requirements for USB charging ports. Other requirements include maintaining the signal integrity of high-speed USB data transmission and protecting the USB host from the hazards commonly found in the automotive environment. In addition, small size solutions and low electromagnetic radiation are important requirements to meet the increasingly complex automotive electronics needs. This article demonstrates a solution that meets the requirements of modern USB charging ports in the automotive environment, including design examples.
Overview of Automotive USB Power System
Figure 1 shows a block diagram of a typical automotive USB charger system, where the switching converter generates 5 V from the battery to supply VBUS. The USB charging port emulator and power switch IC shown here have three main functions. First, the USB charging port emulator determines the optimal charging current of the connected device, so as to realize fast charging through the charging port mode (such as USB BC 1.2 CDP, DCP, and vendor's proprietary charger emulation protocol, etc.). Secondly, the USB power switch is used as a current limiter and switch to detect and limit the bus current. Finally, the port controller supports USB 2.0 high-speed data transmission between the connected device and the USB host.
Because the USB port is in a harsh automotive environment, sensitive USB circuits must be protected from various real-life hazards, such as electrostatic discharge (ESD) events in sockets and cable failure events. These events may Subject the affected line to a voltage far exceeding its normal operating value.
Figure 2 shows a simplified block diagram of an automotive USB power supply system that combines many power supplies, ports, and protection functions into one IC. In this example, the LT8698S integrates the functions of the switching converter and power switch into a 4mm×6mm package, while providing strong data line protection against ESD events and cable failures.
The integrated charger solution shown in the figure contains all the necessary hardware to independently execute the USB BC 1.2 CDP negotiation sequence between the USB port and the portable device, so that the CDP-compatible device can draw up to 1.5 A from VBUS while communicating with the host High-speed communication.
Cable voltage drop compensation
When the physical distance between the USB socket and the controller is relatively long, for example, the USB socket is located at the rear of the vehicle and the USB host is located in the dashboard, the cable voltage drop compensation can keep the VBUS rail in a precise 5 V regulated state. The LT8698S has a programmable cable voltage drop compensation function, which can perform excellent adjustments on the USB socket without the need for additional Kelvin detection lines.
Figure 3 shows the working principle of cable voltage drop compensation. A sense resistor RSEN is connected between the OUT/ISP and BUS/ISN pins, and the resistor is connected in series between the regulator output and the load. LT8698S generates 46 × (VOUT/ISP-VBUS/ISN)/RCBL current source on its RCBL pin through RCBL grounding resistance. This current is the same as the current flowing into the USB5V pin through the RCDC resistor connected between the regulator output and the USB5V pin. This will produce a voltage offset higher than the 5 V USB5V feedback pin across the RCDC resistor, which is proportional to the RCDC/RCBL resistance ratio. As a result, the LT8698S adjusts the BUS/ISN pin to a point 5V higher than the load target according to the load current (the maximum limit is 6.05V) to maintain the accurate adjustment of the socket VBUS pin.
Cable voltage drop compensation eliminates the need to connect an extra pair of Kelvin detection lines from the regulator to the remote load, but the system designer is required to know the cable resistance RCABLE, which is not detected by the LT8698S. The components for setting the cable voltage drop compensation can be selected by the following formula: RCBL = 46 × RSEN × RCDC/RCABLE. The cable resistance will change with temperature. To obtain better overall output voltage accuracy in a wide temperature range, a negative temperature coefficient (NTC) resistor can be added as part of the RCBL, so that the cable voltage drop compensation varies with temperature. Variety.
Provide strong protection for the automotive environment
There are many hazards in the automotive environment, and the USB host must be protected for this. These hazards include cable faults causing the data lines to withstand battery voltage or grounding, and large ESD impacts at the USB socket. Figure 4 shows how to protect the USB host from these hazards.
The HD+ and HD– pins of the LT8698S can withstand up to 20 VDC, and prevent up to 8 kV contact discharge and 15 kV air discharge IEC 61000-4-2 ESD events, while also protecting the host from these harsh conditions. In addition, the USB5V, OUT/ISP, and BUS/ISN pins can withstand output voltage failures, including DC voltages up to 42 V. In the event of an output failure, the latch and automatic retry functions can accurately limit the average output current.
Although many USB port controller ICs require external clamping diodes or capacitors on the data line to provide ESD protection (which increases cost and materials, and may reduce signal integrity), the LT8698S does not.
The data line switch is not only able to withstand the aforementioned DC faults and ESD events, but also helps to achieve excellent signal integrity. Specifically, the –3 dB bandwidth of the HD+ and HD– pins is 480 MHz (typical value), which has been tested in production. Figure 5 shows the high-speed transmission eye diagram measured on the demo board of Test Plane 2 according to the USB 2.0 specification. The figure shows that it meets the limits of USB template 1, test plane 2, and has sufficient margin.
The USB charging port is an important part of the modern vehicle infotainment system. In the face of various realistic and dangerous events in the automotive environment, it is necessary to deal with various system challenges in terms of power supply, data transmission support and robustness. The example that this text introduces adopts LT8698S USB charger IC to solve these challenges. They support various portable device charger protocols and can provide up to 15 W of output power for USB type-C charging applications. In addition, they can protect the USB host from potentially dangerous situations, such as cable failures and severe ESD events. The LT8698S provides this protection while maintaining the signal integrity required for high-speed USB data transmission between the USB host and the portable device. Finally, the Silent Switcher 2 architecture provides excellent EMI performance without sacrificing efficiency and solution size.