Case analysis! This solution meets the requirements of automotive USB charging ports in various environments

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—are just some of the many requirements placed on USB charging ports. Other requirements include maintaining signal integrity for high-speed USB data transfers and protecting the USB host from hazardous conditions common in automotive environments. In addition, small solution size and low electromagnetic emissions are important requirements to meet the needs of increasingly complex automotive electronics. This article demonstrates a solution that meets the requirements of modern USB charging ports in automotive environments, including design examples.

Figure 1 shows a block diagram of a typical automotive USB charger system, where a switching converter generates 5 V from the battery to power V _BUS . 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 for the connected device, enabling fast charging through charging port modes such as USB BC 1.2 CDP, DCP, and vendor proprietary charger emulation protocols. Second, the USB power switch acts as a current limiter and switch to detect and limit bus current. Finally, the port controller supports USB 2.0 high-speed data transfer between the connected device and the USB host.

Because USB ports are exposed to the harsh automotive environment, sensitive USB circuitry must be protected from real-world hazards such as electrostatic discharge (ESD) events at the receptacle and cable fault events that can subject the affected lines to voltages far in excess of 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 power, port, and protection functions into a single IC. In this example, the LT8698S integrates the functionality of a switching converter and power switch into a 4mm × 6mm package while providing robust data line protection against ESD events and cable faults.

The integrated charger solution shown contains all the necessary hardware to independently perform the USB BC 1.2 CDP negotiation sequence between the USB port and the portable device, enabling a CDP-compliant device to draw up to 1.5 A from VBUS _while simultaneously communicating at high speed with the host.

When the USB receptacle is physically far from the controller, such as when the USB receptacle is located at the rear of the vehicle and the USB host is located in the dashboard, cable drop compensation allows the V _BUS rail to maintain precise 5 V regulation. The LT8698S features programmable cable drop compensation that allows excellent regulation on the USB receptacle without the need for additional Kelvin sense lines.

Figure 3 shows how cable drop compensation works. A sense resistor, RSEN _, is connected between the OUT/ISP and BUS/ISN pins , which is in series between the regulator output and the load. The LT8698S sources a current of 46 × (VOUT/ISP – VBUS/ISN)/RCBL on its RCBL pin through the RCBL resistor to ground _. This _current is _{the same} as the current that flows into the USB5V pin through the RCDC resistor connected between the regulator output and the USB 5V pin. This creates a voltage offset across the RCDC resistor _above the ₅ V USB5V feedback pin that is proportional to the RCDC/RCBL resistor ratio. As a result, the LT8698S regulates the BUS/ISN pin to a point above the load target 5 V (limited to 6.05 V maximum) based on the load current to maintain accurate regulation of the receptacle’s VBUS pin _.

Cable drop compensation eliminates the need to connect an additional pair of Kelvin sense lines from the regulator to the remote load, but requires the system designer to know the cable resistance, R _CABLE , which the LT8698S does not sense. The components for setting the cable drop compensation can be selected using the following equation: R _CBL = 46 × R _SEN × R _CDC /R _CABLE . Cable resistance varies with temperature, and to achieve better overall output voltage accuracy over a wide temperature range, a negative temperature coefficient (NTC) resistor can be added as part of R _CBL to make the cable drop compensation vary with temperature.

Figure 2. Simplified block diagram of an automotive USB power system built around a single-IC USB controller solution

Figure 3. Cable voltage drop compensation working principle

Figure 4. Powerful protection features of the LT8698S/LT8698S-1

The automotive environment presents several hazards that the USB host must be protected from. These hazards include cable faults that expose the data lines to battery voltage or ground, and large ESD strikes at the USB receptacle. Figure 4 shows how to protect the USB host from these hazards.

The LT8698S’s HD ⁺ and HD- ^pins withstand up to 20 V _DC and block 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 withstand output voltage faults, including up to 42 V DC. In the event of an output fault, latch-off and auto-retry features accurately limit the average output current.

While many USB port controller ICs require external clamping diodes or capacitors on the data lines for ESD protection (which adds cost and bill of materials while potentially degrading signal integrity), the LT8698S does not.

The data line switches not only withstand the DC faults and ESD events mentioned above, but also help achieve excellent signal integrity. Specifically, the –3 dB bandwidth of the HD ⁺ and HD ^– pins is 480 MHz (typical), which has been production tested. Figure 5 shows the high-speed transmission eye diagram measured on the demonstration board in Test Plane 2 according to the USB 2.0 specification. The figure shows that it meets the USB Template 1, Test Plane 2 limits with ample margin.

Figure 5. High-speed USB 2.0 eye diagram measured on a demonstration board. Mask 1 requirements are shown.

The controller IC used in this example is compatible with multiple USB connector types and charger characteristics, as shown in Table 1. Let’s look at how a single controller works in a USB Type-C 5 V, 3 A solution (15 W).

_{The schematic diagram of the USB 5 V, 3 AV BUS} regulator with cable voltage drop compensation is shown in Figure 6. The RSEN resistor value of 8 mΩ is selected for this circuit to support output currents up to 3 A, and the SYNC/MODE pin is grounded to enable pulse skipping operation, reducing the switching frequency and quiescent current at light load currents.

The LT8698S also supports USB BC 1.2 DCP mode, which can provide charging current up to 1.5 A, supporting high current charging capability. When used as a DCP port, the D+ and D– lines are shorted and no data is transmitted.

Many portable device manufacturers have developed proprietary charger protocols. Similarly, these vendor-specific charger protocols and 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 of a 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 desired charger protocol by controlling the SEL1-3 input pins. This allows the microcontroller to optimize the charging characteristics of the portable device so that it can charge safely at the highest possible current.

Table 1. LT8698S/LT8698S-1 Compatibility with Various 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 automotive electronic system power supplies is low EMI, often required to meet CISPR 25 Class 5 emissions standards. Designed with Silent Switcher ^® 2 technology, the LT8698S enables USB power supplies to meet these stringent automotive EMI standards without sacrificing solution size, efficiency, and robustness.

The Silent Switcher 2 architecture integrates bypass capacitors inside the LQFN package to minimize EMI. The integration of bypass capacitors simplifies board design, reduces the size of the overall solution, and minimizes the impact of PCB layout on EMI performance. The LT8698S-1 does not contain these internal bypass capacitors and is otherwise identical to the LT8698S. Both devices offer selectable spread spectrum modulation by applying a DC voltage greater than 3.0 V to the SYNC/MODE pin. Figure 8 shows the radiated EMI performance of the LT8698S under typical application conditions.

The LT8698S and LT8698S-1 can operate at programmable and synchronizable switching frequencies from 300 kHz to 3 MHz. Higher switching frequencies allow the use of smaller inductor and capacitor values ​​to reduce the size of the overall solution. Figure 9 shows that even at a higher switching frequency of 2 MHz, this 12 V to 5 V USB solution achieves 93% efficiency.

Figure 8. Radiated EMI performance (CISPR 25 radiated electromagnetic interference, using peak detector, category 5 peak limit)

Figure 9. Efficiency and power loss curves for a 5 V USB solution.

USB charging ports are an essential part of modern vehicle infotainment systems and must address a variety of system challenges in terms of power supply, data transfer support, and robustness in the face of various real-world hazardous events in the automotive environment. The examples presented in this article using the LT8698S USB charger IC address these challenges. They support a variety of portable device charger protocols and can provide up to 15 W of output power for USB type-C charging applications. In addition, they protect the USB host from potentially hazardous conditions, such as cable faults and severe ESD events. The LT8698S provides this protection while maintaining the signal integrity required for high-speed USB data transfer between the USB host and the portable device. Finally, the Silent Switcher 2 architecture provides excellent EMI performance without sacrificing efficiency and solution size.