Combine Low Cost, High EMI Immunity, And Ultra-Low Emissions In The LIN PHY

The standardization of in-vehicle network communications has been a major step forward in the democratization of automotive electronics and plug-and-play option management across platforms. The global automotive industry’s acceptance of the Local Interconnect Network (LIN) standard has fostered the rapid growth of this network topology. In fact, Strategy Analytics, a marketing and research firm, predicts that up to 700 million LIN nodes will be delivered in the automotive market by 2014.

At the same time, the automotive industry is driving the need for more reliable, stronger, and competitive solutions through the development of specifications providing greater protection against harsh stresses caused by both electromagnetic interference (EMI) and electrostatic discharge (ESD). These solutions must also reduce the impact of the communication IC on the frequency spectrum through ultra-low emission, especially at low frequencies, while maintaining a very low quiescent current.

Body electronics was the first application for LIN technology. In these systems, cost-driven plug-and-play solutions support distributed architectures with load management located close to the application loads, such as window lifts, mirror drivers, flap controls, and more. Today, LIN technology is penetrating the powertrain arena in applications such as alternator regulators and battery management. As a result, LIN is becoming a key hardware connection topology to manage diagnostics and state control within automotive energy management strategies and programs.

The evolution of both LIN and SAE J2602 (SAE’s version of the LIN specification) over the past decade includes the progression from LIN 1.3 to LIN 2.2 to allow engineers to design to cost goals while increasing the flexibility of the physical-layer (PHY) signal management. The flexibility is accomplished using high-level definition compared to parametric signal definition. Thus, the evolution of this specification is becoming increasingly mature, attaining ISO specification on the LIN side.

Numerous specifications had to be considered during the process of defining the critical stresses to be supported by a LIN physical layer (Fig. 1). The trend is to self-protect the LIN PHY against system-level stresses without the use of external components. An example is the recently introduced Freescale MC33662, the latest automotive qualified and certified LIN PHY. It uses SMARTMOS8MV mixed-signal and power technology to provide excellent electromagnetic compatibility (EMC) and ESD performance.

ACHIEVING ULTRA-LOW EMISSION

Since the shape of the signal transition can significantly impact the device’s emission spectrum, Freescale developed innovations to improve the shape of the signal emitted by the PHY. These include LIN slope management and LIN corner shape management.

While LIN 1.3 was more rigid on the signal specification and limits, including transition and slope, the LIN 2.x specification allows more signal flexibility to achieve the same and even better transitions from dominant to recessive states and vice versa. Transition time and slope are key to managing emissions at low frequencies. As a result, the MC33662 provides a rounded shape on both the rising and falling edges that significantly reduces the emitted noise in the low-frequency spectrum range (Fig. 2). The emission level is 40 dBµV at 150 kHz, which is about –10 dB compared to other PHYs. The result is minimal external signal disturbance.

2. The new PHY IC uses a proprietary rounded shape on both rising and falling signal edges that significantly reduces emitted noise at lower frequencies.

The second innovation, improved management of the LIN transition’s corner shape, directly influences the energy emitted at high frequencies. To reduce this emission, Freescale enhanced current management to control the transition using a linear variation of current, which minimized the di/dt that generates high-frequency noise.

Measurements made both with and without 68 pF on the LIN terminal show that the MC33662 delivers an emission level lower than 18 dBµV from 700 kHz to 1 GHz. That’s about –20 dBµV below the average for previous solutions, including the previous Freescale MC33661 solution (Fig. 3). The result is ultra-low emissions in the low-frequency regime, which is particularly important since the component’s primary purpose is communication.

3. In conducted-emission measurements using the 150-Ω method, the MC33662 (orange plot) exhibited emissions –20 dBµV below the average of other devices on the market, even without an external 68-pF external capacitor.

DESIGN FOR IMMUNITY

As with every network inside an electrical or electronic environment, the LIN behaves as an antenna, absorbing electromagnetic noise caused by load switching, including motors and inductive switches. Characteristics of this noise are related to the switching frequency and energy.

Each electronic control unit (ECU) within the network is connected through a PHY that converts digital information into analog waves for transmission. With proper electromagnetic immunity, the integrity of the signal during a LIN communication won’t be disturbed when electromagnetic noise stresses the network. The automotive industry currently uses two methods of noise generation to simulate and validate the robustness of PHYs versus the noise absorbed by the network: direct power injection (DPI) and bulk current injection (Fig. 4).

4. The auto industry uses two techniques to generate noise to simulate real-world conditions and test the robustness of PHYs. The direct power injection and bulk current injection methods each have their own strengths.

To correlate the component simulations with actual behavior during EMI stress, transistor models have been improved to account for parasitic components like diodes, capacitors, and resistors. Simulation accuracy has been improved by using more accurate models for the external components, such as bond wire models, external capacitors, and external inductors, in addition to using an accurate model of the injection path and test equipment. This results in a complete design-for-EMC flow:

• Very accurate design and layout guidelines
• An extensive simulation plan on the block level and top cell level
• Process and temperature variations accounted for during EMC simulation to ensure a certain margin versus the specification

These improvements provide a LIN signal integrity margin of more than 4 dBm at the minimum level versus the OEM specification when injected on the LIN without any external capacitor.

The MC33662 ensures total signal integrity throughout the EMI frequency range (150 kHz to 1 GHz). Testing was performed by injecting 4 W on the LIN pin using the DPI methodology without a capacitor (Fig. 3, again) and with three 68-pF capacitors (Fig. 5), or on the VSUP (supply) pin, with and without an LIN capacitor. The measurements were performed at both Freescale and external certification laboratories.

5. Measured with the direct power injection method, the MC33662 maintained total signal integrity of 34 dBm throughout the EMI test frequency range both with external capacitance (a) and without external capacitance (b).

IMPROVING ESD PERFORMANCE

The MC33662 has been specifically designed to withstand the most severe automotive ESD standards defined at both the IC and system level. It passed the ESD tests specified in AECQ100, such as the Human Body Model (HBM) up to 10 kV, the Machine Model (MM) up to 200 V, and the Charged Device Model (CDM) up to 750 V.

The device has been optimized to handle system-level stress according to the tests defined in ISO10605:2008, IEC61000-4-2:2008 standards, the Human Metal Model (HMM) standard practice, and the car OEM’s ESD requirements for LIN interface. All of these test procedures had to be considered during the development of the IC because the setup variations lead to different stresses.

The tests use an ESD gun to reproduce the impact of an ESD when a human being is handling an electronic system subassembly or touching the car structure. IEC61000-4-2 addresses situations where the electronic system is not referenced to ground. In this case, a 330-Ω/150-pF network is used.

The ISO10605 standard recently evolved from its 2001 release to the 2008 release by adding a 330-Ω resistor to existing 2-kΩ/150 pF/330 pF networks. This standard is dedicated to road vehicles, and it describes both electronic modules and vehicle tests.

The 2-kΩ resistor tends to simulate the resistance of a human when the discharge occurs through the human skin (i.e., a finger) while the module using 330 Ω simulates the discharge of a human body through a metallic part (i.e., a key). Two different capacitors are used to simulate the equivalent capacitance of a human placed either outside the vehicle (150 pF) or inside the vehicle (330 pF).

After the ESD stress, the unit’s functionality and electrical parameters are evaluated using dedicated automated test equipment. This measurement provides a complete IC validation. The test setup has a strong impact on the results, and most of the configurations have been considered to cover a wide range of system-level ESD requirements required by carmakers.

During the development of the chip, different R-C modules of the gun were considered with several ground connections and different biasing conditions. The circuit’s transient behavior was optimized for four gun combinations (150 pF or 330 pF with either 330 Ω or 2 kΩ) as defined in ISO 10605. Figure 6 shows the device’s current-discharge profile using the four gun combinations.

6. Testing of the LIN PHY included injection of ESD noise using four ESD gun variations that provided different current discharge stresses defined by AECQ100 and ISO/IEC standards.

The IC can resist severe system-level stress directly applied at the LIN pin with and without external protections. To achieve such high performance (up to 25 kV), the System Efficient ESD Design (SEED) approach was used. The ESD strategy and the design of the analog blocks were achieved by taking into account external devices to ensure a very high robustness versus ESD gun tests done at the applications level.

The MC33662 covers most of the ESD system standards required by automotive OEMs. The table summarizes ESD performance. The combination of high ESD and DPI performance is really a challenge of energy absorption, without compromising the LIN communication. The MC33662 achieves this performance without requiring external components and with an optimum die area.

The MC33662 standalone LIN PHY is pinout compatible with all eight-pin solutions in the market. The device comes in three versions, with dedicated part numbers, and no risk when switching from one baud rate to another: the L version for LIN 2.1 networks, the J version for J2602-2 networks, and the S version for those applications that require sharp signal symmetry.

Quiescent current is as low as 6 µA, and automatic level detection and setup (5.0 V or 3.3 V) provides digital flexibility. A fast baud rate accommodates software programming or any diagnostics that might be required during the life of the application. The IC also includes a Tx-permanent dominant timeout that prevents a complete network blocking situation during a failure condition.

DAVID LOPEZ is product line manager for the SBC and physical layer portfolio at Freescale Semiconductor. He received an engineering master level degree from the Higher National School of Physics of Marseille with a master’s in electronic physics and a master’s in business administration from Toulouse University, France.

KAMEL ABOUDA is a digital designer at Freescale Semiconductor. Since 2007 he has worked on improving designs for EMC flow. He received his graduate engineer diploma from ENIS Tunisia in the electronics and electric field and a post-master’s in transistor modeling at IXL, University of Bordeaux I. He has defended his PhD at IXL.

HAMADA AHMED is senior analog IC designer at Freescale Semiconductor with 11 years experience in analog, mixed-mode, and power circuits for automotive applications. He received a master’s in electronics from the National Institute of Applied Sciences, Toulouse, France.

PATRICE BESSE is member of the technical staff at Freescale Semiconductor. He is involved in the development of ESD solutions for analog products. He received his master’s in electronics and post-master’s in EMC from the Blaise Pascal University of Clermont-Ferrand, France. He received his PhD in electronics (ESD) from the University of Paul Sabatier, Toulouse.

LAURE PORTAL is a product engineer at Freescale Semiconductor. She is in charge of evaluating EMC performance and works with auto makers to certify products according to their EMC specifications. She is Freescale’s representative to the LIN Consortium. Portal received her engineering degree in electronics from the Polytech’Montpellier, France.

ALAIN SALLES is a member of the Analog and Mixed Power Division at Freescale Semiconductor, where he is in charge of ESD structures characterization for automotive applications. He received his engineering degree in electronics from the National Polytechnic Institute and his master’s and PhD in microelectronics from Paul Sabatier University, Toulouse.

THIERRY SICARD is a senior member of the technical staff at Freescale Semiconductor, with more than 23 years of international experience in analog, mixed-mode, power design, and electromagnetic modeling. He received his engineering master’s level degree from the Higher National School of Electronic and Radio Electricity in Bordeaux, France.