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.

Figure 1: Examples for centralized and decentralized approaches

The control of door electronics is dominated by one of the two topologies: a decentralized door zone module located inside the door, or respectively, a central approach (Figure 1). Which concept is best? There are so many unique variables involved that it makes it almost impossible to provide a definitive analysis. Each approach has its advantages and drawbacks in terms of costs, 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 last, but not least, 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.

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).

Figure 2: L9950 comes with six half-bridges, a mid-power high-side driver for the defroster, and four high-side drivers for bulbs or LEDs.

Figure 2 shows the L9950XP (one of the devices under 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 a SPI bus (only four pin), and direct input (for some outputs) is implemented.

Figure 3: ST has designed actuator drivers to address all automotive door-zone applications for the multiplicity of door-electronics variants today.

Besides standard diagnosis for short circuits, overload and open load detection, under- and overvoltage 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. A charge pump output is also 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 a passive protection like a diode, it allows the system to work at lower voltage levels.

Other door zone variants, 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. For example, the customer may specify a single or double motor lock, with or without folding mirrors. Additional variants can depend on the vehicle partitioning and distribution of loads between the front and rear doors. STMicroelectronics has designed a complete group of actuator drivers (Figure 3) to address all automotive door-zone applications for the multiplicity of door-electronics variants today.

Figure 4: PowerSO-36, PowerSSO-36 Zthj-a

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 STMicroelectronics is working on a power management device family approach to cover different topologies and support different PHYs like LIN and CAN. With the L9952GXP LIN companion chip, the first family member offers the market high performance in terms of the current consumption in standby mode. In its lowest quiescent mode, the device is said to set the market benchmark with a typical current of 7?A.

Figure 5: A L9950XP counterpart can be and the equivalent discrete solution to provide same functionality as L9950XP output types, load current and level of diagnostics.

Control and diagnostics for all devices are managed via a serial peripheral interface (SPI). ST's 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. The STMicroelectronics door approach offers the following advantages:

As stated earlier, specific door configurations can be addressed by specific drivers. For example, L9950/L9953 devices are the right solutions for high/mid-end cars (L9950 is used to drive lock and power lock configuration; L9953 is used to drive only a single lock). The L9951 is tailored for rear doors (no mirror and/or folder control), and the L9954 perfectly fits doors that do not require lock control (needed in cases where door locks are, in a main BCM, centrally driven).

New devices will be available soon to complement the described portfolio, offering the opportunity to drive additional loads (or those requiring specific driving) within the door car. Here functions will 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, STMicroelectronics also focuses on reducing the number of external passive components to minimize the overall system costs. 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. STMicroelectronics' dominant position in this specific market is proven by its impressive billing figures: within 2009, after six years of mass production, 100 million parts will be sold.

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 锟絧ure' solid state switch adoption are showing interest in the door zone devices. Figure 5 shows what a L9950XP counterpart can be (showing the equivalent discrete solution to provide same functionality as L9950XP? Output Types, Load Current and level of Diagnostics). The relays/discrete solution includes 2 Lock Bridge Relays, 3 Small Bridge Relays, and six Sense FETs (in series to folder bridges for actively controlling the current, to drive the bulbs/LEDs and the heater).

The comparison shows the following results:

Figure 6: 'Top views' of different boards based on two different approaches.

In Figure 6, it is shown how two different boards, based on different approaches, look: the space reduction is visually evident, and the connection wiring, which is not visible, is optimized.

- Giovanni Torrisi, Manuel Gaertner
STMicroelectronics

Notes:
1. Refers only to components weight, including PCB and material molding.
2. Routing projected for 4 layers PCB.
3. For typical design made by relays, I/Os (digital) and A/D inputs are required to control the loads and provide feedback signals to the micro. Conventional design uses shunt resistors to control the load, and each load has a dedicated A/D current feedback. For the L9950 design, micro I/O usage is 4 for the SPI interface, 1 A/D input, and 1 PWM control.
4. Cost delta from L9950. It's important to highlight how the cost is not only related to the BOM but takes into consideration different items (including PCB, housing delta, wiring). Total cost is not including that of a low pin count micro, in case of L9950 usage, that can be used to save further on uC and PCB size.