“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.
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 common hazardous conditions 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 an 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 power to 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 supplier’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.
Figure 1. Block diagram of a car USB charger
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, which is connected in series between the regulator output and the load. LT8698S generates 46 × (VOUT/ISP C VBUS/ISN)/RCBL current source through RCBL ground resistance on its RCBL pin. 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.
Figure 2. Simplified block diagram of an automotive USB power system built around a single-IC USB controller solution
Figure 3. Working principle of cable voltage drop compensation
Figure 4. The powerful protection function of LT8698S/LT8698S-1
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 HDC 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 C3 dB bandwidth of the HD+ and HDC 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.
Figure 5. High-speed USB 2.0 eye diagram measured on the demo board. The requirements for template 1 are shown.
Compatible and support a wide range of charger features
The controller IC used in this example is compatible with a variety of USB connector types and charger characteristics, as shown in Table 1. Let’s take a look at how a single controller works in a USB Type-C 5 V, 3 A solution (15 W).
Figure 6 shows the schematic diagram of a USB 5 V, 3 A VBUS regulator with cable voltage drop compensation. This circuit selects an RSEN resistance value of 8 mΩ to support an output current up to 3 A, and the SYNC/MODE pin is grounded to enable the pulse skipping mode of operation, reducing the switching frequency and quiescent current at light load current.
LT8698S also supports USB BC 1.2 DCP mode, which can provide up to 1.5 A charging current and supports high-current charging capability. When used as a DCP port, the D+ and DC lines are short-circuited and there is no data transmission.
Many portable device manufacturers have developed proprietary charger protocols. Similarly, the proprietary charger protocols of these manufacturers and the corresponding maximum charging currents (such as 2.0 A, 2.4 A, 2.1 A, and 1.0 A) are also supported. The host microcontroller can implement these charger protocols by controlling the three SEL pins.
Figure 7 shows the schematic diagram of the 2.4 A/1.5 A USB charger. In this application, the microcontroller uses the information provided by the LT8698S STATUS pin and the IMON current monitor to select the required charger protocol by controlling the SEL1-3 input pins. In this way, the microcontroller can optimize the charging characteristics of the portable device so that it can be safely charged with as much current as possible.
Table 1. Compatibility of LT8698S/LT8698S-1 with multiple USB connector types, charger protocols and data interfaces
Figure 6.5 V, 3 A, USB type-C application
Figure 7. 2.4 A/1.5 A automatic protocol detection charger with current monitor
A key requirement for the power supply of automotive Electronic systems is low EMI, which usually needs to meet CISPR 25 Class 5 emission standards. The LT8698S is designed with Silent Switcher® 2 technology to enable USB power supplies to meet these stringent automotive EMI standards without sacrificing the size, efficiency and robustness of the solution.
The Silent Switcher 2 architecture integrates bypass capacitors inside the LQFN package to minimize EMI. The integration of bypass capacitors simplifies the design of the circuit board, reduces the size of the overall solution, and minimizes the impact of PCB layout on EMI performance. LT8698S-1 does not contain these internal bypass capacitors, and other aspects are exactly the same as LT8698S. By applying a DC voltage higher than 3.0 V to the SYNC/MODE pin, both devices can provide optional spread spectrum modulation. Figure 8 shows the radiated EMI performance of the LT8698S under typical application conditions.
The LT8698S and LT8698S-1 can operate with programmable and synchronizable switching frequencies from 300 kHz to 3 MHz. The higher switching frequency allows the use of smaller inductance and capacitance values to reduce the size of the overall solution. Figure 9 shows that this 12 V to 5 V USB solution can achieve 93% efficiency even at a higher switching frequency of 2 MHz.
Figure 8. Radiated EMI performance (CISPR 25 electromagnetic radiation interference, using peak detector, 5 types of peak limit)
Figure 9.5 The efficiency and power loss curve of the V USB solution.
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.