Car buyers who may, just a few years ago, have been struck by the novelty of seeing a single USB port in a new vehicle are now increasingly likely to ask how many ports their latest purchase will have.
However, the electrical environment in a car differs significantly from that in a non-automotive scenario, in terms of Electro-Static Discharge (ESD) hazards and the large transients likely to be present. In addition, the 12V battery voltage represents a hazard to USB data lines. The vehicle's USB ports require effective protection against such hazards, in order to maintain signal integrity and ensure long-term reliability, which equates to an increased level of perceived quality, and fewer product returns or manufacturer recalls.
Connectivity is key
Leading car brands are increasingly looking to innovations in electronics to help create new features that win sales and build customer loyalty. Hand in hand with this trend, cars are becoming an integral part of today’s digital consumer lifestyles. Today’s car buyers want to bring not only their smartphones but also tablets, MP3 players, USB/SD storage and other devices into the vehicle, and connect these to the vehicle infotainment system to enjoy their content and access services while on the move.
To meet these demands, and also to support enhanced functionality such as telematics, car-to-car communication and ADAS (Advanced Driver Assistance Systems), the control unit for this functionality is evolving from a simple car radio to become a sophisticated computing platform. Powerful microcontrollers are emerging to fulfill these next-generation applications for the head unit. Such devices comprise sophisticated multicore processing, off-chip memory interfaces, and rich connectivity often including CAN, MOST, Ethernet, video camera inputs, and interfaces for wireless connections, as well as USB2.0 and/or USB3.0 ports. High-end microcontrollers may also provide the ability to output to multiple HD displays, while support for HDMI connections is expected to become widespread in the future.
Today, USB connectivity is vital to give vehicle users the flexibility to connect their own devices and storage and so access content and services as required. Increasingly, multiple USB ports are needed to allow front and backseat passengers, as well as the driver, to connect their own devices for various purposes, which may be different for the driver and passengers.
USB in the car
To provide satisfactory in-car USB connectivity, appropriate consideration must be given to regulating the USB power-supply voltage and current and protecting all of the USB data and power connections from various electrical hazards.
The USB power supply connection (Vbus) can be powered from either the HOST controller or from an external source (device). The Vbus voltage ranges from 4.75V to 5.25V, while the current ranges from 500 mA up to 2.1A, and more, for tablets and newer mobile devices. This can vary depending on the continuous and fast-charging requirements of individual equipment.
Among the electrical hazards likely to be encountered are ESD and transient events which may occur during manufacturing and assembly, or may be caused by vehicle occupants or other electrical circuits in the vehicle. The major documents discussing ESD and transient events as far as road vehicles are concerned are ISO 10605 (Road vehicles -- Test methods for electrical disturbances from electrostatic discharge) which describes ESD test methods and ISO 7637 (Road vehicles -- Electrical disturbances from conduction and coupling) for effects caused by other electronics in the vehicle. IS0 10605 is based on IEC 61000-4-2 Industry Standard, which specifies the various levels of ESD signal characteristics, but also includes additional vehicle-specific requirements. Further, OEM specific test requirements are usually also imposed.
In addition, all data-signal pairs in USB2.0 Full Speed/High Speed or multi-channel USB3.0 SuperSpeed interfaces, as well as Vbus and ground lines, require protection from short-to-battery (16 VDC) and short-to-ground faults.
A suitable protection solution must satisfy well-known constraints, such as low capacitive loading of the signal lines to minimise signal attenuation, and also respond quickly to surges and transients with low clamping voltage. In addition, small package sizes help to minimise demand for board-space while providing the ability to route the trace signals with minimal bending to maintain signal integrity.
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