Protect USB devices in automotive infotainment applications

With industry standards and other elements yet to follow, selecting proper protection devices can be challenging, but with the right guidelines it can easily enhance module reliability.

USB basics
The Universal Serial Bus or USB as it is commonly known has become a widely used interface to connect various electronic devices together. One family of devices where this interface is common is portable music and video players. Car manufacturers have begun to include ports in vehicles that permit the connection of these devices to the in-vehicle entertainment system to allow the audio to be played using the vehicle's own system—and in some cases to include control and display functions, too. (Take Ford's SYNC system, for example.)

Briefly, the USB interface is a master/slave architecture except for USB On-the-Go (OTG) which allows a remote device to take control. The master is referred to as the "upstream" device or host/hub and the slave as the "downstream" device. The interface consists of four wires: D+, D-, Vbus, and Ground. The two data lines are for bi-directional data transfer and use differential drive techniques. Vbus and Ground can be used to distribute power from the upstream host to the downstream device.

One of the reasons for the successful adoption of this interface is the very thorough specification and certification process. For a more complete description please refer to the materials available at the USB Website, including the complete specification.

The auto environment
But the inclusion of this bus architecture in vehicles poses some issues that weren't considered or included in the industry specification. Because the original applications were intended primarily for connecting digital devices together that operate at low voltage, the maximum allowed voltages on the data and power lines are less than the levels that automotive manufacturers require on wiring that is external to electronic modules. The OEM's all have their own specific requirements but many refer to the international standard ISO 7637.

The methods of protection used for the interface connections differ due to their respective characteristics. The protection of Vbus is easier so let's consider it first.

There are a few key limits in the USB spec that determines the circuitry that must be used. The first is the maximum regulated voltage on Vbus of 5.25V. The second is the amount of current that must be supplied and the greatest amount of voltage drop allowed. A simple zener diode or TVS clamp can be used in systems where there are only transient voltages possible above the 5.25V limit. But this approach cannot be used on a vehicle to meet the ISO 7637 specification since the DC level of 13.8V from a low impedance source would quickly overheat the clamp. Instead, a series limiting circuit has to be used.

Figure 1 (above) shows an approach to Vbus protection using an integrated circuit containing a voltage reference and comparator. The downstream Vbus voltage is monitored and compared to the reference. If the voltage exceeds the threshold of 1.27V for the comparator the output switches and drives the Vgs voltage to the P-Channel MOSFET to zero. Because this NCV33161 voltage monitor has an open collector output, a separate PNP transistor is used to turn off the MOSFET quickly to prevent current from passing through to the upstream Vbus regulation device. Below is the circuit's response to an overvoltage event.

Figure 2 (immediately above) shows this same approach but with a part from ON Semiconductor that includes the MOSFET for a greater level of integration and fewer parts for a smaller footprint on a PCB. Below is the response of this single chip device. Because the MOSFET has the usual parasitic body-drain diode, the current resulting from short circuits on Vbus on the downstream side are not blocked.

Compensating for ESD and other short duration and high source impedance events on Vbus can be handled by the bulk capacitor, C₁. The TVS zener diode, D₄, can be added for additional overvoltage protection, if necessary. Any transient energy that gets through the switch is filtered by C₂. The USB specification calls for the value of this capacitor to be at least 120 μF with low ESR (equivalent series resistance).

Another possible fault condition is a short circuit to the vehicle ground or an overcurrent condition inside the device that is connected to the USB port. There are many devices available that provide current limiting and fault reporting to the controller as required in the specification so they aren't presented here. If one of these components isn't being used for the Vbus supply then the short circuit situation has to be accommodated.