How to upgrade automotive high-side drivers using a smart multiplexer

Intelligent control of interior and exterior lighting is becoming increasingly important in automotive electronic systems; more and more functions are integrated into compact body control modules, and this trend will inevitably bring corresponding technical challenges. Automotive lighting systems have increasingly higher requirements for electronic components, and intelligent multiplexers can solve PWM channels, diagnostic functions and system reliability issues.

The body control module (BCM) is an electronic control unit that performs various functions, including control and diagnostics, as well as failure management of interior and exterior lighting or submodule power distribution. The BCM also handles door lock actuators, wipers, anti-theft functions and access control systems. In addition, the BCM can also perform gateway functions and serve as a bridge to connect different communication networks in the car. Today's BCMs are still evolving. Let's explore the development trends of the control and diagnostic functions of the BCM related to automotive lighting.

The development trend of modern body control module

• PWM control of high-side drivers: Solid-state switches (also called high-side drivers) are replacing electromechanical relays to save cost and weight and enhance diagnostics. The requirements of these high-side drivers are becoming increasingly stringent in target applications. In fact, in order to correct transient events caused by battery voltage changes and keep the brightness of LED lamps and ordinary incandescent lamps stable, the system must control these drivers using pulse width modulation (PWM) signals. This control function also prevents incandescent filaments from melting quickly or LED lamps from overheating when the car battery voltage is high.

• High-side driver diagnostics: The high-side driver provides a digital status or current sense signal that is proportional to the load current. Based on this information, the system must identify and report different failure conditions in a reliable way: overload, short circuit to ground or battery, overtemperature, open circuit in the on state, open circuit in the off state.

• Versatility of platform solutions: The body control module not only has to control several devices for a given vehicle model, but also has to meet the special requirements of different markets (Asia, Europe, America). In addition, the same body control module is shared between different vehicle models. The main reason behind the aforementioned development trend is to reuse modules, reduce development costs, and improve product quality.

These trends have made designers realize that developing a cutting-edge body control module will face many challenges. Below we discuss some of the main challenges.

New Challenges

A wide range of high-side drivers with PWM control and synchronous diagnostics

Traditional cars were originally equipped with three body control modules, located in the front, rear and exterior of the car, while modern cars have integrated the functions of these three modules into two or even one module. The upper bridge arm driver of the body control module of a passenger car usually has more than 40 channels, while the upper bridge arm driver of a truck usually has more than 80 channels. The phase shift control implemented between the PWM channels helps to prevent the huge voltage drop caused by the simultaneous inrush current of the bulbs and improve the electromagnetic compatibility of the system. In addition, the output current of each activated load needs to be monitored and PWM controlled regularly; false failure detection must be avoided, such as overload detection caused by the high inrush current of the cold incandescent filament. The diagnostic function should also filter out spurious failure conditions, such as failure conditions caused by ISO pulses. The PWM signal generator with phase shift function combined with the synchronous diagnostic function of a large number of channels may lead to an increased load on the microcontroller.

Scalability of high-side drivers

To comply with the functional safety requirements of ISO26262 (ASIL B), the supply voltage of the high-side driver is usually divided into 2-4 voltage rails. Sometimes, a multi-channel high-side driver cannot control more than one safety load (low beam, brake light, clearance light...). In addition to these limitations, the optimization of the power stage size according to the load characteristics and the variation within the automotive platform may lead to a driver area consisting of single-channel and dual-channel high-side drivers. In summary, we need a scalable high-side driver solution with very low internal integration to perform synchronous diagnostic and PWM control functions.

Excellent short-circuit robustness (also known as robustness) to filter out intermittent load conditions

In line with the zero-defect principle of the automotive industry, the safety, fault tolerance and robustness of electronic components and systems against abnormal load conditions are becoming increasingly important. In particular, a short circuit to ground of the output of a high-side driver must not lead to a burnout of the failed output. Despite built-in protection mechanisms, standard high-side drivers can still burn out under short-circuit conditions caused by thermomechanical stress and electromigration phenomena. The longer the filtering time, the higher the risk of a high-side driver breakdown. Today, when diagnostic functions are directly handled by the microcontroller, the sampling time for current sensing or digital states of the high-side driver reaches 10 ms to 100 ms. In addition, a filtering function must be implemented to avoid erroneous failure reports caused by transient disturbances before the failure condition is verified.

Using a smart multiplexer to reduce microcontroller load and improve system robustness

The L99PD08, also known as AMICO (Advanced Multiplexer and Integrated Coprocessor), is an intelligent multiplexer capable of controlling and diagnosing eight channels of high-side drivers (Figure 1). The high-side drivers can be any of the STMicroelectronics VIPOWER M0-5, M0-5E and M0-5T series, regardless of the Rdson value, diagnostic type and number of channels of each device. The L99PD08 is the interface between the microcontroller and the high-side drivers. The multiplexer communicates with the microcontroller via an SPI bus interface, reducing the number of microcontroller pins required for 16-channel high-side drivers from 32 to 11 (Figure 2).

The clock of the 8-bit resolution PWM signal comes from two external clock sources. To prevent the huge voltage drop caused by the simultaneous bulb surge current and improve the system electromagnetic compatibility, the L99PD08 introduces a phase shift control function in each PWM channel. The L99PD08 monitors the 8 channels of the upper bridge arm driver at the same time and samples each diagnostic signal every 32µs. During the on-state or off-state process, the real-time diagnostic function always considers whether the sampling is completed. When a PWM signal is applied, the final synchronization process can avoid wrong diagnosis.

Figure 1: Schematic diagram of the structure of L99PD08[page]

Figure 2: Application diagram

When a short circuit occurs in the high-side driver, the reaction time of the L99PD08 is increased from 64 to 100µs to filter out potential hazards caused by fast ISO transient events. The L99PD08's shutdown lock protection function can significantly improve the robustness of the high-side driver in the event of a short circuit without the need for intervention or intervention by the microcontroller. The performance evaluation of the L99PD08 under the test conditions specified in the AECQ100-012 standard shows that during the long pulse test at -40°C (300ms reaction time before shutdown, see Figure 3), the L99PD08's robustness improvement factor is 300 times that of a direct diagnostic architecture of the high-side driver by the microcontroller.

Figure 3: Improving device robustness under short-circuit conditions through automatic shutdown

in conclusion

L99PD08 helps developers meet the increasingly stringent requirements of body control modules, built-in PWM signal generator and synchronous diagnostic functions, short reaction time under short-circuit conditions, these features help to reduce the load on the microcontroller and improve the robustness of the upper bridge arm driver. AMICO also improves the performance of the upper bridge arm driver of the M0-5, M0-5E and M0-5T series through the SPI interface, reduces the number of microcontroller pins required for 12V and 24V systems, and reduces the complexity of the software, which is very beneficial to the future development of automotive software, because with the introduction of the AUTOSAR standard in the automotive industry, the complexity of automotive software is an important issue that the automotive industry will soon face.