The Class-D Amplifier Is the Star of Acoustic Vehicle Alerting Systems, ST Suggests
ADAS and AVAS are known to add a number of speakers to an automotive audio system. Class-D amplifiers, however, may be a useful solution for cutting size, costs, heat, and power consumption.
Automotive manufacturers are now pushing the conventional audio layout limits by adding more speakers. These additional speakers, such as midrange and woofer, provide over six speakers per vehicle, making it necessary to place an external amplifier in the trunk space.
Power dissipation between Class A, B, and D amplifiers. Image used courtesy of Analog Devices
To avoid this, hardware designers must develop smaller audio amplifier solutions with low power and heat dissipation for advanced automotive audio systems.
Design Considerations for Amplifiers in Modern Automotive Audio
When engineers are determining the size of a new audio amplifier system, some crucial considerations include:
- Thermal performance
- Switching frequency
- Inductor size
- Package design
Each amplifier differs greatly in specifications, making each one fit for different design goals and applications.
Class-A amplifiers provide reliable sound quality but deliver a high amount of power dissipation because of a large DC bias current that flows through the transistors instead of to the speaker. These simple power amplifiers have a smooth midrange but lack dynamic, powerful sound.
Class-B amplifiers solve the issue of power dissipation but deliver poorer sound quality with crossover distortion. They are mainly used for experimental testing and research, and they are more efficient than Class-A solutions; however, the distortion is a significant disadvantage for consumer devices.
Class-AB amplifiers provide good linearity and functional sound, making them more appropriate for car radio systems than Class-A or Class-B amps. The drawbacks are Class-AB amplifiers tend to generate a large amount of internal heat and require a sizable heat sink, increasing the overall size of the audio amplifier system.
Finally, Class-D amplifiers have a clear sound quality and a higher switching frequency, making them more efficient than Class-AB setups. Some of the benefits of choosing a Class-D amplifier are low heat dissipation, a small footprint, extended battery life, and cost-effectiveness.
Class-AB's efficiency compared to Class-D. Superior results put Class-D ahead of traditional amplifiers that are widely used vehicles. Image used courtesy of Texas Instruments
With these details in mind, many designers turn to the Class-D amplifier as a candidate for automotive applications.
New Class-D Amplifier from ST
Cueing into the efficacy of Class-D in automotive audio applications, ST recently announced the HFDA801A, a 2 MHz pulse width modulation (PWM) Class-D amplifier with a quad-bridge configuration.
The HFDA801A from STMicroelectronics. Image used courtesy of STMicroelectronics
The amplifier has a 124-bit digital-to-analog converter (DAC) to ensure higher sound quality. It also includes a noiseless turn-on/off with a signal-to-noise ratio (SNR) of 120 dB range to help with unwanted distortion.
Block diagram of ST’s HFDA801A, configured with a low-pass filter, an 80-kHz frequency response, and wide bandwidth HD audio. Image used courtesy of STMicroelectronics
While a standard Class-D amplifier achieves linearity and minimal distortion, the trade-off is that it requires more components. However, ST says its embedded solution adds depth to traditional Class-D-based hardware by including a digital impedance meter and real-time load-current monitors. This amplifier solution can also draw its supply from the vehicle's battery without additional power converters.
Class-D Amplifiers for AVAS and ADAS
Acoustic vehicle alerting systems (AVAS) emit warning sounds to alert pedestrians of the presence of electric vehicles. These systems, forecasted to be of use in smart cities, can also open doors for more sophisticated sound algorithms in semi- and fully-autonomous vehicles.
Meanwhile, ADAS adds to the number of speakers needed within each vehicle—unless a well-equipped Class-D audio system is used.
Flow diagram depicting a robust AVAS system powered by the car battery. Image used courtesy of STMicroelectronics
A challenge to overcome in these systems is shoot-through current, which occurs during long periods of operation. Shoot-through current can be addressed with current-sensing output-transistor protection circuitry. This circuitry will set a safety threshold, and should the current surpass it, the device shuts off.
Even with low heat dissipation output by Class-D designs, overheating can still occur during long periods of operation. Temperature monitoring is necessary to avoid overheating—a feature built into ST's HFD801A.
What design challenges have you faced in automotive audio systems? Do you find these challenges shifting as more ADAS and AVAS appear in vehicles? Share your experiences in the comments below.