Optimization of desired functional performance at an acceptable and reasonable cost is a worthwhile objective for any new product. In fact, this objective is at the heart of value which can be defined as the relationship between benefit received and expectations. A product design can be envisioned in many different forms to achieve this value goal.

The Fairchild Dual Integrated Solenoid Driver, FDMS2380, is an example of a product that delivers exceptional value as an intelligent low side driver for solenoids and other inductive loads. In addition to exact parametric specifications, the FDMS2380 was developed to perform in the harsh automotive physical and electrical environment. Partitioning of power and signal processing functions as well as packaging were considered as critical to achieve optimal reliability and performance in powertrain applications. Through the use of state-of-the-art power silicon, high performance BiCMOS control functions and the latest power packaging approach, the FDMS2380 provides a new higher power, lower dissipation complex Functional Power solution to address today's automotive design challenges.

Designed for the Automotive Environment

Physical Environment

The challenges of designing for the automotive environment are well documented. Power semiconductors used in automotive powertrain controls must endure harsh physical environments. Today's automotive manufacturers often need to have fully tested systems, such as engines and transmission. This requires that electronic control units be mounted on or near the deliverable tested system and the thermal environment for these power semiconductors can reach ambient temperatures of 150oC or more (figure 1).

Engine, Transmission are run at up to 200°C

Combustion chambers up to 500°C

Exhaust systems up to 800°C

Wheel systems up to 300°C

Sensors see respective environments

Figure 1. Ambient Temperatures

For a semiconductor, Tj max is the critical factor since blocking capability gate threshold voltages as well as other vital characteristics are all bounded by this parameter. Exceeding Tj max is the cause of most failures. Couple this with the fact that in many automotive applications the power device must operate in energy absorption modes rarely experienced in other power designs, and it becomes clear that an understanding of thermal limits of power semiconductors and consideration of thermal management details is absolutely necessary for insuring that designs will continue to provide the reliability required by the automotive market.

The FDMS2380 utilizes a multiple die approach to optimize the thermal management of the product. The power die are selected and packaged to achieve reliable performance in the thermal environment. Additional thermal protection is internally provided by a temperature sensing circuit. When junction temperatures exceed the protection limit, thermal shutdown occurs; the output excitation circuit is automatically switched off and the recirculation and demagnetization circuit is forced into recirculation mode to discharge load coil energy (figure 2).

Figure 2. FDMS2380 Block Diagram - For higher resolution, click here

Electrical Environment

The electrical environment for automotive applications is also quite different from that of most power systems. Unlike other transient environments where external influences have the greatest impact, the transient environment of the automobile is one of the best understood. The most severe transients result from either a load dump condition or a jump start over-voltage condition. Other transients may also result from relays and solenoids switching on and off, and from fuses opening.

The circuit designer must ensure reliable circuit operation in this severe transient environment. The transients on the automobile power supply range from the severe, high-energy transients generated by the alternator/regulator system, to the low-level "noise" generated by the ignition system and various accessories. Figure 3 shows some of the voltage transients that must be considered in automotive systems design.

Figure 3. Automotive Electrical Transients Figure

The FDMS2380 has many built-in features such as over-current and over-voltage detection circuits and has been designed to operate from 6V to 26V. During fault conditions, the output is turned off and the recirculation path is turned on to dissipate inductive energy.

The Functional Power Solution

To get the most out of power solutions, a multiple die approach to optimizing Functional Power technologies in an automotive environment is a benefit. While monolithic solutions are reaching their limits in the basic power and analog requirements primary to any power application, a multiple die approach offers several advantages, including:

• Use of the lowest power loss switch for a given area

• Use of the latest technology advances in power discrete technology

• Improved isolation between the power and sensitive analog blocks

• Improved modularity of the design by having one control die used with various power discrete devices.

• Ability to combine all types of discrete power technologies with high performance analog control blocks.

• Improved optimization of the switching function for on resistance or switching losses.

A Functional Power device is first and foremost a power device. It normally has limited data processing responsibilities compared to other IC devices. Therefore, it is important to approach Functional Power from a power perspective. For a power device, the primary concerns are voltage blocking capability, current handling capability and thermal performance. Heat-related power losses in the device must be removed to keep the junction temperature below the point at which potential damage due to extremely localized heating can occur.

With Functional Power systems, there is a need to process both power and data. In some cases the data processing function is so complex that it is more cost-effective to use a silicon process optimized for signal processing for the smart functions of the device, and to use an entirely different silicon process optimized for power devices for the power functions of the device. To achieve this dual goal, the power and control functions could be placed in separate packages, but separate packages consume board area.

The need for smaller electronics requires integration of the separate, optimized silicon processes into a single, smaller package. These more compact devices must also provide the power handling, die interconnect, power and signal connections""and possibly die substrate isolation""along with physical support and environmental protection.

Fairchild Semiconductor's FDMS2380 is an example of this type of integration (figure 4). This device is a dual, intelligent low-side driver with built-in recirculation and demagnetization circuits designed specifically for driving inductive loads. Its inputs are CMOS compatible. The diagnostic output on the device provides an indication of open load and demagnetization mode. Built-in over-current, over-voltage and over-temperature circuits protect the device and, in case of over-current or over temperature, this product will automatically operate in freewheeling recirculation mode for inductive loads.

Figure 4. Multiple die in PQFN package optimizes power and control in the automotive environment.

With a multi-chip functional power technology, excellent electrical isolation between power and control silicon is provided. The thermal communication between power and control silicon is dampened. This improves product ruggedness and reliability particularly in harsh electrical environments.

With a multi-chip smart power technology it makes sense to use the most efficient power silicon for the smart power functions that the footprint (board space) will allow. The lowest RDS(ON) MOSFETs can be used as needed for the lowest power loss.

Future Flexibility

Most automotive designs are somewhat customized, yet automotive system designers are under constant pressure to reduce design cycle time. Traditional "smart power" monolithic technologies have complicated fabrication processes, which reduce the speed and flexibility of developing new devices. Since a monolithic development needs to make both power and signal processing on the same fabrication process, iterative designs can be costly and slow. With a multi-chip smart power technology the power and signal device development can occur in parallel. This enables faster development of new products optimized for a customer application. Often an IC process is optimized for driver functions only (not power); and while the IC portion may stay the same, the power handling requirements often vary. Use of the latest-generation power technologies can expand system life by migrating the power section to the latest power technologies.

The FDMS2380, while designed as a specific product for controlling low side, inductive loads in the automotive powertrain, provides the technical foundation for additional products that are similarly customized for other applications.

Performance

Lower RDS(ON) products are needed for less power loss. Low RDS(ON) means less voltage drop across the switch to measure at a given current. Any noise in the measurement method makes accurate measurement difficult. Reduction of noise and accurate sensing of load conditions are important considerations in automotive module design. Multi-die smart power devices allow for more accurate measurement of low voltage drops and low currents. With a multi-chip smart power technology excellent isolation between the power and the control silicon is provided. This isolation improves product ruggedness and reliability particularly in harsh electrical environments. Future products based on the FDMS2380 technology can take advantage of ongoing improvements in power consumption in MOSFET technology.

Conclusion

Semiconductors for automotive powertrain must operate in one of the harshest physical and electrical environments used in high volume production. Products are needed which provide less power loss, higher degrees of flexibility, and the ability to process both small signals and large amounts of power. The solutions need to be cost effective. Through the combined use of innovative packaging and different semiconductor processes designed for specific functions like power or signal processing, new solutions will allow for less expensive more efficient and reliable powertrain systems. The FDMS2380 from Fairchild Semiconductor is an excellent example of this design approach for controlling inductive loads. It also provides a foundation for future products to deliver optimal value to new automotive electronic systems.

Gary Wagner is Director of Body Electronics and Smart Switches at Fairchild Semiconductor. Mr. Wagner has 29 years of experience in the development of electronics and semiconductor products for the automotive industry. He is responsible for automotive functional power products at Fairchild.