Nikkei Electronics Asia -- October 2008
Insights
Door Zone Devices Improve Car Comfort, Convenience

In modern vehicle doors, the control of various loads and functions is realized increasingly by using smart power semiconductor devices. The quantity of the functions to be controlled has been constantly increasing during the last few years, and different loads have been integrated inside the electronics.

Selecting Topology

The control of door electronics is dominated by two topologies: a decentralized door zone module located inside the door, or respectively, a centralized approach. Which concept is better? There are so many unique variables involved, making it almost impossible to provide a definitive analysis. Each approach has its advantages and drawbacks in terms of cost, efficiency, and design suitability. Typically, the arguments for each topology center on several factors: the wiring harness - considering cable costs and weight, the reliability of connectors, the location and function of control elements and sensors, space requirements, and the infiltrations grade of the functions addressed in the door. 

In the end, the choice of topology is a cooperative effort between the OEM, the Tier 1 supplier, and the semiconductor company, made to optimize system cost. A mixed approach is sometimes the final choice.

Decentralized Approach

Wiring harness weight considerations directly impact fuel consumption as well as add costs due to raw material usage. The trend here is for the decentralized approach. Consequently, in automotive comfort electronics, door-zone modules have emerged as viable, cost effective and reliable solutions: a typical module is responsible for controlling power windows, exterior mirrors including heating elements, motors for mirror adjustment and folding, electro-chromic, and automatic dimming mirrors; memory seat selection, door locks, safety/foot-well lamps and side-turn indicators (blinkers).

The Fig shows the L9950XP (one of the devices belonging to STMicroelectronics' Door Zone Portfolio) block diagram. The device comes with six half-bridges (for mirror, folder, and lock driving), a mid power high-side driver for the defroster (heater), and four high-side drivers for bulbs or LEDs (puddle, blinker, courtesy, lamps of the door). Driving and diagnostics are handled by the SPI bus (only four pin), and direct input (for some outputs) is implemented. Besides standard diagnosis for short circuit, overload and open load detection, under- and over-voltage shutdown, temperature pre-warning and temperature shutdown information, in addition, a current sense output is implemented. This feature can be used to control the heater's power dissipation by applying a variable PWM signal, or it can be used to identify a missing load for paralleled loads, for example, using two bulbs or LEDs in parallel for the side mark indicator. Additionally, a charge pump output is provided to control an active reverse battery protection MOSFET. This insures a minimization of power dissipation in the reverse battery protection device to increase system efficiency, and in comparison to passive protection like a diode, it allows the system to work at lower voltage levels.

Other door zone variants, with fewer or more power outputs, are needed to address every kind of partitioning. In order to reduce costs, implementations of specific functions for door modules need to be flexible and tailored to meet customer specifications. Additional variants can depend on the vehicle partitioning and distribution of loads between the front and rear doors.

The drivers support all regular door-zone loads such as door-lock motors, mirror folders and leveling, defroster, and lighting functions from low current LEDs up to 10W incandescent bulbs.

The power-management system device enables applications to achieve a very low quiescent current of the overall ECU, which is one of the key requirements for the future. For this purpose a power management device family which covers different topologies and supports different physical layers like LIN and CAN has been developed.

Advantages of Door Approach

This kind of approach brings in a number of benefits. Control and diagnostics for all devices are managed via a serial peripheral interface (SPI). Actuator drivers are available in small power packages, offering superior thermal performance - ideal for compact and lightweight systems. The car-body door actuator family housed in the same packages is pin-to-pin compatible, therefore able to target different market segments. This provides a real family approach since project designers can consider one option or another, without any further change on the board.

Functions can be implemented for driving an electro-chromic mirror glass and supporting more and more LEDs, which are quickly replacing conventional incandescent bulbs.

Besides addressing new features, reducing the number of external passive components to minimize the overall system costs is also important. By implementing new control and diagnosis topologies, future devices will also achieve enhanced robustness, which is mandatory for the increasing EMC and EMI requirements prevalent in the harsh automotive environment.

Lighter Vehicles

Rising gasoline prices, as well as legislations mandating cuts in average CO2 emissions from new vehicles, are pushing automakers to look for ways to lighten their vehicles while never compromising safety, performance, or comfort. On top of the well known advantages of solid-state switches with respect to relays (higher degree of reliability, longer life, possibility to drive loads in PWM, built-in diagnostics, reduced electromagnetic interference, faster response times, and vibration and shock resistance), the L995x devices offer weight, space, and cost reduction. For this reason, markets historically far from "pure" solid-state switch adoption are showing interest in the door zone devices.

by Giovanni Torrisi, Manuel Gaertner,
STMicroelectronics

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