Autonomous driving is one of the most far-reaching changes affecting today’s cars. A vehicle with autonomous driving features requires complex computing capabilities and networks which can handle large data transfers.
Typically, an advanced driver assistance system (ADAS) consists of a central unit and several radar sensors on the outside of the car. The sensors are usually connected via an automotive Ethernet network. If the system’s sensors include a camera, there will also be a high-speed video link.
To maintain reliable and safe operation, it is essential to protect an ADAS circuit against destructive ESD pulses, and the scale of protection offered by external ESD devices is far higher than that in the circuit’s main components. While an IC can typically tolerate ESD strikes up to 2 kV, which is sufficient only for handling and assembly purposes, external ESD protection can increase the system robustness up to 30 kV, the level which is usually required by car manufacturers.
Protecting automotive Ethernet networks
The OPEN Alliance has created detailed specifications for each of the individual components in the automotive Ethernet architecture. In these specifications, the ESD protection device has different electrical characteristics from those in most other automotive networks. These special characteristics reflect the topology of Ethernet, as well as providing for robust and reliable operation.
In particular, the OPEN Alliance specifies a new trigger voltage of higher than 100 V: this provides excellent protection not only against ESD but also EMI. Nexperia ESD protection products satisfy this requirement through the implementation of snap-back technology, which also produces a very low clamping voltage, the key to better ESD protection, shown in Figure 1.
Strict testing of Nexperia ESD devices according to the OPEN Alliance specifications ensures that every unit provides very reliable protection for the Ethernet transceiver as well as for the entire system-level circuitry. Low capacitance of as little as 1 pF also ensures that Nexperia ESD protection devices maintain high signal integrity.
Fig. 1: Block diagram of automotive Ethernet applications and dedicated ESD protection devices from Nexperia
Providing up to 15 kV of protection to automotive video links
For the video interfaces which connect an ADAS system’s cameras to a central processor, ESD protection is equally important. These video interfaces are often referred to by the umbrella term
SerDes links, but more specifically, automotive manufacturers use proprietary SerDes technologies such as GMSL, FPD Link, or APIX.
These interfaces provide high bandwidth of up to 12 Gbits/s, have a point-to-point topology, and operate mainly unidirectionally. If they operate bidirectionally, the bandwidth in one direction is substantially higher than in the other. The physical implementation of the serial data stream is differential when using low-voltage differential signaling (LVDS), or single-ended when using a single coaxial cable.
Because the topology of these video interfaces is different from that of Ethernet, the requirements for ESD devices are also somewhat different. For instance, an ESD protection device for video links will have a low voltage rating in the range of 5 V to 18 V, while offering ESD protection of up to 15 kV.
An ESD protection device may be placed in one of two locations: on the connector side, or after dc capacitors. On the connector side, voltages higher than 12 V can occur on the signal lines, and so ESD devices should have a voltage rating of around 18 V. If the ESD device is placed after the dc capacitors, then the normal signal line voltage is applicable: this calls for ESD protection devices with a voltage rating of around 5 V.
Fig. 2: Recommended Nexperia ESD protection devices for SerDes applications
In addition, as with automotive, Ethernet signal integrity is crucial because of the high data-transfer rates of higher than 1 Gbit/s. This requires ESD protection devices to offer very low capacitance of 0.5 pF or even lower. A compact and reliable package such as a DFN1006(BD)-2 is also recommended, as shown in Figure 2.
These requirements may be met by choosing products from the broad Nexperia range of ESD protection devices suitable for all modern mobile and automotive applications and interfaces.