Today's performance vehicle owes a substantial debt to electronics for its performance, improved fuel economy and lower emissions, increased comfort and convenience, and improved safety. With each new vehicle system or electronic feature, increased connectivity has resulted. The emerging areas of telematics and driver information systems integrate a broad set of technologies to enhance vehicle safety, communications, convenience and entertainment. This includes dynamic navigation, wireless connectivity/Internet access, natural-language speech processing, car audio, virtual dashboards, collision-avoidance systems and more.
These electronic systems let vehicles interact with onboard systems, other cars, drivers and extended communications networks as well as support a multitude of in-vehicle applications. These systems also require an automotive-specific approach for their power.
A Car Has Several Buses
The pioneering vehicle system for complex electronics was the engine control system. With the expansion to include transmission control and advanced onboard diagnostics, this system is now called the powertrain system. Vehicle buses for power train must provide high speed as well as cost effective data transmission capability.
As shown in Table 1, the SAE initially defined three classes of vehicle networks for low-, medium- and high-speed data communications. Controller area network (CAN) and SAE's J-1850 have been dominant but aren't the only bus architectures used in these classes. SAE J-1850 encompasses variations that were initially developed by all three U.S. automakers.
CAN is a serial, asynchronous, multi-master communications protocol for connecting electronic control modules in automotive and industrial applications. CAN was designed for automotive applications requiring high levels of data integrity and data rates of up to 1 Mbps.
|Class A||Low||1 to 10 kbps||50 ms max|
|Class B||Medium||1 to 100 kbps||20 ms max|
|Class C||High||10 kbps to 1 Mbps||5 ms max|
|Medium-access control||single master||multimaster|
|Typical bus speed||2.4 to 19.6 kbps||62.5 to 250 kbps|
|Multicast message routing||6-bit identifier||11/29-bit identifier|
|Typical number of nodes||2 to 10||4 to 20|
|Bit/byte coding||NRZ with bit stuffing||NRZ 8N1 UART|
|Data byte per frame||2, 4, 8 byte||0 to 8 byte|
|Transmission time for 4 data bytes||3.5 ms at 20 kbps||0.8 ms at 125 kbps|
|Error detection data||8-bit checksum||15-bit CRC|
|Physical layer||single wire, 12 V||twisted pair, 5 V|
|Quartz/ceramic resonator required||no||yes|
CAN has low- and high-speed implementations, which communicate using a differential voltage on a pair of wires and are commonly referred to as a high-speed and a low-speed physical layer. The low-speed architecture can change to a single-wire operating architecture (referenced off ground) when one of the two wires is faulted through a short or open. Because of the nature of the circuitry required to perform this function, this architecture is expensive to implement at bus speeds above 125 kbps. As a result, 125 kbps is the dividing line between high-speed and low-speed CAN. Fig. 1 shows the connection of the CAN backbone to other vehicle networks.
For some vehicle applications, such as motor controls in the body electronics systems, an even lower-cost bus can be used. The local interconnect network (LIN) is a UART-based, single-master, multiple-slave networking architecture originally developed for automotive sensor and actuator networking applications. LIN provides a low-cost networking option for connecting motors, switches, sensors and lamps in the vehicle. The LIN master node connects the LIN network with higher-level networks, such as CAN, extending the benefits of networking all the way to the individual sensors and actuators (Table 2).
When fully implemented, drive-by-wire or X-by-wire systems will require high levels of power. The term “by-wire” denotes a control system that replaces traditional mechanical or hydraulic linkages with electronic connections between control units that drive electromechanical actuators. Drive-by-wire includes throttle-by-wire, brake-by-wire, shift-by-wire and steer-by-wire. Steer-by wire, also can mean front and rear steering, especially for automatic parking systems.
To implement these types of power-hungry control systems a fault-tolerant, time-triggered architecture is required. Today, three different protocols are vying for leadership in this area. One protocol, FlexRay, is designed for optical fiber for a 10-Mbps data transmission rate but also can run on copper. Another type, Time Triggered Protocol (TTP), uses copper for only 2 Mbps, but developers at the University of Vienna are exploring the use of fiber. With a third protocol, TT-CAN, both “time-triggered” and “event-triggered” applications are supported (Table 3).
The Automotive Multimedia Interface Collaboration, or AMI-C, was established to standardize requirements for mobile information and entertainment systems multimedia. With a focus on telematics and multimedia technologies, AMI-C is actively addressing the use of Bluetooth, IEEE-1394 and Media Oriented Systems Transport (MOST)-based in-vehicle networks. Bluetooth brings wireless connectivity to the vehicle environment, IEEE-1394 already connects several multimedia products in the consumer environment, and MOST is an optical media-based protocol developed specifically for automotive streaming video and audio data.
With all of the network protocols that have been discussed (except LIN), more powerful microprocessors are required to implement full-system capability.
Special Requirements for Power
Requirements for multiple supply voltages and for accuracy over a wide temperature range frequently dictate the use of special power-supply circuits in automotive applications. Two specific examples are the physical-layer power requirements and the power-up sequencing requirements for the multiple supply voltages in advanced microprocessors.
A simple physical-layer device isn't necessarily sufficient for some automotive applications. For example, all automotive modules require a regulated power supply. Sometimes a local switch or sensor might need to wake up the module from sleep state to active running state. That switch or sensor might be running at vehicle battery levels. To simplify the power design for physical-layer applications in vehicles, power-supply ICs, such as those in Motorola's System Basis Chip (SBC) family, have been developed.
One example, the MC33389 SBC, combines the CAN physical layers for automotive CAN connectivity with voltage regulation, independent watchdog timer and local wake-up circuitry to allow greater flexibility with fewer components. Combining these functions into one package requires an advanced mixed-signal semiconductor process, such as Motorola's SMARTMOS™ process. The integrated design reduces assembly costs, increases reliability and increases design flexibility.
|Protocol||Data Rate||Media (Design for)||Deterministic||Fault Tolerant|
|Time-Trigger Protocol||2 Mbps||Copper||Yes||Yes|
The MC33389 combines a low-speed fault-tolerant CAN transceiver, with dual low drop voltage regulators that have 100-mA and 200-mA current capabilities. The IC also has current limitation and overtemperature detection with pre-warning and a 5-V output voltage for the V1 regulator that has a monitor and reset function.
The IC provides three operational modes (normal, standby and sleep mode) separated from the CAN interface operating modes and addresses low-speed 125-kBaud fault-tolerant CAN Interface. The chip is designed for dc operating voltages up to 27 V and can withstand a 40-V maximum transient voltage. In addition, the IC can operate within the -40°C to 125°C operating temperature of automotive underhood applications.
The high-speed logic core in advanced VLSI logic devices is designed for a lower voltage (3.3 V and 2.6 V) while the power for input/output (I/O) interfaces is typically 5 V. Microprocessors (MPUs) requiring two supplies typically require a prescribed sequence for power-up. Using a power-supply IC designed for this purpose and targeted at specific MPUs can ensure the correct sequence.
Time and voltage specifications for the power for advanced MPUs can impact both the power-up and power-down. If design specifications are exceeded, excessive voltage stress or latch-up condition can occur between the device's I/O port and the core because of current flowing in isolation structures within the chip. The damage may be immediate or latent but, in either case, the reliability may be affected. Even for MPUs designed to tolerate extended operation with one of the supplies below its proper operating voltage, power sequencing provides an added degree of security for increased reliability.
Powertrain Power-Supply IC
For powertrain applications, power-supply ICs, such as Motorola's MC33997 and MC33998, offer a complete power-supply system solution for the electronic control unit (ECU). These chips are designed to withstand the harsh automotive environment (-40°C to +125°C), while operating from a 6-V to 26.5-V input source with transients up to 40 V. The MC33997 and MC33998 supply the voltages required for the core and I/O in Motorola MPUs with power sequencing, output-voltage monitoring, supervisory functions and special protection features required for the powertrain application. These devices also support standby mode, power external sensors and provide other features.
Fig. 3 shows a simplified application diagram for a dual-voltage MPU. The MC33997 circuit converts the battery voltage down to 5 V via a step-down switching regulator. A linear regulator in the MC33997 provides a 3.3-V output using an external pass transistor. Also, this chip offers an additional 3.3-V output for standby use along with two internally protected low-RDS(on) 5-V outputs for sensor use.
The MC33998 is a similar device but provides 2.6-V outputs instead of 3.3 V for MPUs that need the lower voltage. The switching regulator in both chips is a high-frequency (750-kHz) buck converter with an integrated high-side p-channel power MOSFET.
Different voltage requirements exist for applications in multimedia and telematics. As a result, custom ICs that address other voltages are being developed for the newest vehicle requirements.
The increased connectivity in vehicles will provide customer-desired improvements that help manufacturers differentiate their vehicles. This connectivity requires special considerations for power. Power-supply ICs and functions that require special power considerations simplify the design of end products, and provide the durability to survive in the harsh automotive environment.
For more information on this article, CIRCLE 343 on Reader Service Card