Advanced power management device for LIN bus controlled door zone applications

　　New cars offer more and more comfort and convenience features to the driver and passengers: These features are further enriched by electronic functions of roof and door zone controls, HVAC systems, wiper and seat controls, lighting control systems, etc.

　　In automotive comfort electronics, the door zone module (DZM) has become a widely recognized solution: a typical module controls power windows, exterior mirrors (including heating components and motors for xy position adjustment and folding), door locks, footrest/exterior lighting, and turn indicators. Figure 1 shows the typical system architecture of a standard high-end front door module seen in a variety of cars on the road today.

　　Inside these modules, power management functions and a transceiver are also included. The latter is generally needed to "wake up" the DZM module and identify the key in the ignition lock as well as the usual diagnostic communications. Depending on cost and technical requirements, the transceiver can be CAN or LIN . With the increasing requirements for speed, control and diagnostic complexity of the front door, and based on high speed or body requirements, the CAN bus is chosen in most cases. The rear door electronics clearly tend to LIN because the number of functions in the rear door is significantly reduced. Ultimately, using LIN can reduce module costs, which is one of the main driving factors in body applications (as shown in Figure 2).

　　The door area is very compact and the space constraints are very demanding: the door is subject to strong vibrations, the space is small, and it is not convenient for factory workers to operate. In addition, the wiring harnesses that can be deployed to the door are limited because the door is separated from the body. Despite these restrictions, the number of functions is constantly increasing; if the mirrors are taken into account, these constraints are even more stringent. To reduce costs, the implementation of specific functions in the door module requires a certain degree of flexibility and can be tailored to special user specifications. For example, the car manufacturer may specify one or two engine locks, with or without folding mirrors. Other variations depend on the vehicle division and the distribution of loads between the front and rear doors.

　　ST is a leader in the development of products for this special application, offering an impressive range of devices that currently characterize these special automotive products. ST's system-on-chip technology enables complex automotive solutions with a high level of integration, such as the complete door module device L9950, which provides most of the functions required for a high-end front door. The L9950 actuator driver can be used to control the adjustment, folding and unfolding of the mirrors, while the advanced car lock system can be used to drive the door locks and engine locks. There are also five internal high-side drivers that can be used to control different loads, such as lights, LEDs and mirror defrosters. Complex diagnostic algorithms allow monitoring of digital and analog load status by reading the folder, car lock motor and defroster information and responding appropriately. The status of all loads can be accessed through the standard SPI and output through a programmable current monitor.

Figure 1: Typical system architecture diagram of a standard high-end front door module seen in a variety of cars on the road today.

Figure 2: Typical network topology for door area applications

　　ST's full range of products also includes different product models (L9951, L9953 and L9954), all of which can accept SPI drive and have different output configurations. These devices are fully compatible in pins and software, suitable for backdoors or mid-range frontdoors, all use PowerSSO-36 packaging, and have the following advantages:

1. The transmission drive series can cover various front door models;
2. Flexible series approach;
3. Hardware (HW) compatibility/pin to pin;
4. Software (SW) compatibility;
5. Devices can be changed using the same software;
6. All physical pins/outputs can be controlled by the same SPI bits;
7. Each diagnostic information from the same physical pin/output can be indicated by the same status bit;
8. The same PCB can be used for different models of car doors;
9. Can be defined on the customer's production line with corresponding drivers;
10. The user side only needs to develop, verify and maintain one PCB.

　　Special attention needs to be paid to optimizing the module's current absorption characteristics, especially when the car is not started (ignition key is off). Even though today's advanced regulators provide very low quiescent currents, and the latest BCD devices are optimized to consume very little battery power, the current absorption of the entire module does not meet the recent overall power consumption requirements of car manufacturers. To solve this tricky problem, dedicated, advanced current absorption control needs to be implemented on the board. This is why ST has developed the new L9952: it is a completely new device designed specifically to provide power management for car door modules.

Figure 3: L9952 power management device with embedded LIN transceiver

This device (Figure 3) features two low-dropout regulators with advanced contact monitoring and additional peripheral functions, and has the following key features:

1. 5V low dropout regulator, powering the microprocessor (μC)/250mA;
2. 5V low-dropout voltage regulator, powering peripherals/100mA;
3. Very low standby current; a. 7μA in VBATT standby b. 45μA in V1 standby c. 75μA in cycle wake-up
4. Car window watchdog;
5. Fault protection output;
6. Wake-up logic with cyclic contact monitoring;
7. Compatible with LIN2.0 and SAEJ2602 physical layer;
8. 24-bit SPI interface for mode control and diagnosis;
9. Output drive relays for window opening and LEDs (PWM enabled);
10. All outputs are short-circuit and over-temperature protected;
11. Two operational amplifiers for monitoring GND-compatible currents;
12. High temperature warning and overheating shutdown;
13. Undervoltage and overvoltage shutdown.

　　As mentioned above, the L9952 includes power management functions for the entire door area module. The purpose is to meet the required quiescent current in different sleep modes, which is required or specified by the car manufacturer. The typical value is 100μA per module, but the requirements are getting higher and higher, and even lower values ​​are needed in more system functions, such as cyclic reading of contact status.

　　The concept of the power management function of the L9952 can be described by the fact that the μC can be operated in stop or pause mode or even switched off by switching the 5V power supply. The contact status readout (for example in the lock or the handle) is required to be performed by the power management device itself. The device can therefore operate in a cyclic mode or in a configuration for static readout of the contact status, depending on the standards and target quiescent current of the car manufacturer. Before entering sleep mode, the corresponding settings are programmed via SPI and the L9952 can then work independently. The time base for cyclic contact power supply and readout is automatically generated inside the power management device by utilizing an integrated RC oscillator.

　　In addition, the filtering strategy (debounce) for the wake-up input can be implemented using an internal time base without the use of an additional microcontroller. This is important to avoid any wake-up events due to EMC noise, which is omnipresent in the harsh automotive environment. If a wake-up event occurs, the system will automatically start.

　　As mentioned above and shown in Figure 2, there is a clear trend that LIN nodes will become part of the front door as well as the back door modules. On one hand, due to cost issues, more and more back doors are controlled via the LIN bus, with the front door as the master node and the back door set as a slave node. There is another trend; the keypad used to control the different functions in the door is implemented as an electromechanical LIN node. Therefore, the physical layer needs to be embedded in the L9952. The physical layer itself complies with the LIN2.0 standard and is compatible with the SAEJ2602 standard.

　　Compliance with standards and EMC performance have been certified by independent bodies, and the LIN physical layer was initially released by car manufacturers such as Volkswagen, Audi and DaimlerChrysler. The LIN bus can be configured in master mode, for the example of the front door, where it acts as a master node for the back door, or in slave mode. In master mode, the required pull-up resistor can be provided by the switchable LINUP (LIN pull-up) output, so that the pull-up resistor can be disconnected in the event of a short circuit to ground on the LIN bus, thus avoiding an increase in quiescent current.

　　In addition to the power management functions and the LIN physical layer, some other functions are embedded in the L9952, which are also the characteristics of the door area application. The purpose is to reduce costs by using high integration and a single control module optimized for typical loads, rather than using standard devices. The use of this application-specific standard product (ASSP) approach can significantly reduce the PCB area, thus bringing price advantages. It should be pointed out that innovation is not only achieved at the silicon level by integrating different functions on a single silicon chip, but also in the development and industrialization of new, high-performance, small-size and advanced power packaging technologies such as PowerSSO-36. Compared with traditional plastic packages, the introduction of this exposed pad package helps save PCB space while providing superior thermal performance.

　　In future developments, CAN also tends to be integrated into the upcoming new power management devices to obtain a flexible and successful approach like the transmission driver. This is the natural continuation of ST's ASSP approach in automotive body applications. Therefore, the common goal of the industry is to optimize system costs and continuously reduce the total module quiescent current, which is the most challenging problem encountered not only in door area applications.