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Manage growing complexity in automotive design

Posted: 16 Sep 2008     Print Version  Bookmark and Share

Keywords:automotive design  electric power windows 

- Ng Jit Keah David
Infineon Technologies Asia Pacific Pte Ltd

Electric power windows have become one of the standard features in cars today. Even in emerging markets where cost is of priority, it is one of the most important factors influencing the buyer's decision. Most car buyers see power window as a must-have comfort feature and it is thus, widely adopted as a basic feature by car OEMs.

While the current trend for doors is towards a modular and lightweight approach, another growing trend is towards the inclusion of side airbags in the car. Some changes to the door structure are also happening whereby the load specification of the door waist beam has been increased to achieve better crash management. Such increased reinforcement and added components of the airbag function greatly drive the need to achieve a smaller footprint for the door/window-lift electronic control unit (ECU). Higher semiconductor integration delivers the key to manage increasing complexity like anti-pinch functionality as well as the need for small size and lightweight.

The same window anti-pinch functionality also applies to sunroof modules which shares the same window-lift mechanism as the normal door window.

This article discusses the use of the embedded power (ePower) TLE7810G device from Infineon to address the window-lift application. In a single package, it incorporates an 8bit MCU compatible to the standard 8051 core with on-chip debug support (OCDS) and a system-basis-chip (SBC). The SBC is equipped with LIN2.0/ SAEJ2602 compliant LIN Transceiver, low-dropout voltage regulator (LDO) as well as two low-side switches (relay drivers) and supply for external hall sensors. An additional high-side driver for driving LEDs is also available.

This IC is realized as multichip-module (MCM) in a PDSO-28 package, and is designed to withstand the severe conditions of automotive environment.

The 8bit MCU with on-chip flash 16Kbyte memory is required to handle several control tasks, one of which is window anti-pinch functionality. This feature is mandatory in the United States for vehicles manufactured after October 2008, with several countries likely to follow soon. Several OEMs have already included this feature in their new models.

Window regulators are high-current devices and in most solutions in today's passenger cars, a relay method is still used to drive the window lift motors. To cater to such popular method, the TLE7810G embeds two relay drivers to control external relays for driving the window lift motor.

Integrated in the TLE7810G is also low drop-out (LDO) regulator to supply voltage to the Hall sensors. Hall sensors are typically employed in window anti-pinch system to detect both the speed/torque and position of the window.

The single package with high integration greatly reduces the PCB space and this means it can be fitted into a smaller (or cheaper) housing. The chip also draws very low quiescent current, an important criteria for today's window-lift/door modules.

Embedded in the chip is also a LIN transceiver that addresses the growing trend of decentralized architecture, whereby each window is controlled by a dedicated ECU. With a connection to the LIN bus, a decentralized modular approach can be realized, thereby greatly reducing the number of wiring harnesses and thus the overall body weight.

Furthermore, with such an integrated approach, one clear benefit is that savings can be achieved for the chip packaging versus the individual discrete devices of microcontroller, transceiver, relay drivers and voltage regulator. Other benefits that Tier 1 suppliers of the window-lift ECU may find in adopting a high integrated ePower solution over a discrete solution, are higher reliability due to fewer components, better inventory management and higher manufacturing assembly efficiency.

Currently TLE7810G also boasts a total of five wake-up inputs and that the LIN transceiver can withstand ESD up to 4kV.

Figure 1: Block diagram of TLE7810G is shown.

Below are key technical requirements to address for this design.

Anti-pinch and window position saving
Anti-pinch is the key safety requirement in window lift applications. Correct operation even under fault conditions in the car has to be guaranteed. In particular, the management of battery fault or ignition turn-off while power window is moving, is critical. In order to ensure correct operation of the anti-pinch function, when the system is powered up again, the system must always know the correct position of the window. Thus, it is required to store the last position in the short timeframe before the power is shutoff.

Generally the anti-pinch mechanism depends on the position of the window. For example, if the window is completely opened and moving up and if an anti-pinch case is detected, then the window simply stops. But if the open space is 250mm or less, the window has to stop and reverse for 100mm. To allow proper closing of the window, no anti-pinch will be detected if the open space is less than 4mm.

Due to energy in the mechanical system, it can take up to 50ms from turning off the window lift motor until it actually stops moving. During this time, the ePower device has to monitor the hall sensor and store the final position before the electronic is shut down. During this time, the ECU is powered out of buffer capacitor. The size of the capacitor depends on the time to bridge, on the current consumption and on the voltage requirements of the application, which is the difference between the voltage level where the system starts shutting down and the level of the low voltage reset. The TLE7810G has been optimized to have a very low undervoltage reset threshold of 3.3V, which allows for smaller and thus cheaper buffer capacitor to be used.

Low quiescent current required
The current consumption of the window regulator ECU has to be minimized typically in the region of a few hundreds of µA, even in active mode. This is because the driver may turn off the engine while still putting the ignition key at the battery-on position, and the window-lift and anti-pinch functionality still have to work, and ideally draining as little current as possible.

This can be achieved by periodic wake-up of the controller to run in active mode but with long period of low power consumption. This can be achieved in low power mode of TLE7810G, which is defined in the specification as communication device block (SBC) in stop with cyclic wake-up and MCU device block in power-down mode. When the MCU is in power-down mode, it consumes the lowest current. There is no activity from the core or peripheral. The MCU can be woken up (with or without Reset) via external interrupt. So the SBC generates this internal "wake-up" signal periodically to allow MCU to return to active mode to service its application. Total power consumption is in the region of 30µA + 80µA = 110µA. The key feature is the wake-up on LIN or monitor pins and the additional periodic wake-up allows active monitoring of additional signals or inputs.

Figure 2: A decentralized body control architecture is shown.

Looking ahead
Looking ahead, there is a trend to use external high current semiconductor switches (e.g. BTM7960B) instead of relays to drive the window lift motors. The benefits include lower noise, longer lifetime, smaller PCB size and enhanced feature such as soft-start with the use of PWM. Moreover, with a harsh environment of varying motor currents due to varying friction in the mechanics from temperature changes and aging of the rubber sealants, semiconductor solution offers better protection and better diagnostic management.

Also with anti-pinch software becoming more intelligent, robust and adaptive to the harsh environment of the cars, TLE7810G offers scalability with increasing flash sizes from 16Kbytes, 24Kbytes to 32Kbytes. It also supports LIN BSL (Boot Strap Loading) for downloading of program should the software be upgraded for increased anti-pinch robustness, via single wire LIN.

As of current, in APAC, particularly China, power window-lift for both passenger door modules and sunroof modules are gaining strong popularity. According to market data in China, there was a sudden surge in the sales of sunroof modules in recent years. This is mainly due to increasing numbers of private cars being sold to young people who considered the electric sunroof an appealing feature matching that of a modern lifestyle. In 2004, for safety reasons, the China government stopped the aftermarket sales of electric sunroofs. This factor has also led the OEM market to increase in China.

Embedded power offers a high-integrated approach and scalable family of devices, at the same time giving benefits of smaller PCB size, light-weight, offering higher reliability with lesser components, better inventory management and higher manufacturing assembly efficiency.

The single-chip approach also greatly simplifies design and improves the overall time to market.

About the author
Ng Jit Keah David
is a senior marketing engineer
of automotive body segment division under the automotive, industrial and multimarket business group at Infineon Technologies Asia Pacific Pte Ltd.

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