Class D amplifiers for automotive generate less heat

Dallas — Texas Instruments Inc. (TI) developed its first digital amplifiers for the automotive market, which are based on its PurePath Class D amps for the home.

Specifically designed for automotive audio applications like head units and external amplifiers, the TAS5414 is a single-ended input amplifier and the TAS5424 has differential input.

TI's devices are said to be ultra-efficient four-channel digital audio Class D amplifiers. “With the TAS54x4 amplifiers, TI delivers 90 percent power efficiency at normal radio listening levels for automotive audio systems. Comparatively, two TAS54x4 amplifiers can provide eight channels of audio, while generating less heat than a four-channel Class AB-based system, enabling a whole new class of cost-effective eight-channel audio systems that are lighter, smaller and more power efficient than existing systems,” said Jeff Akgul, TI's digital audio business manager. In addition, TI's new digital amplifiers also can be used with 2-ohm speakers to offer twice the output power of an AB amplifier into 4-ohm speakers, while generating less heat, he said.

With these automotive digital amplifiers, TI overcomes electromagnetic interference (EMI) at the source through innovative architecture and process advancements, such as an enhanced digital pulse-width modulation (PWM) topology, optimized gate drive technology and patented AM interference avoidance, Akgul said. These advancements make the TAS54x4 TI's first Class D amplifier to meet demanding automotive requirements, eliminating the need for expensive shielding and other EMI countermeasures, now making it possible for developers to bring the power advantages of digital amplification to automotive applications, he said.

By reducing heat dissipation, the TAS54x4 needs only a thin heat sink to spread heat compared to the large heat sinks and fans Class AB-based systems require, Akgul said. Radio head units with more functionality or more output channels can now be designed, eliminating the need for expensive external amplifiers and wire harnesses, he said.

Additionally, on-chip diagnostics simplify radio installation by testing all speaker connections, including tweeters, or short circuits created on the assembly line.

The TAS5414 and TAS5424 are designed to meet the defective parts per million (dppm) requirements of automotive OEMs. In addition, the amplifiers are TS16949 certified, which is the highest standard for automotive quality and reliability, according to Akgul. For an additional charge, TI also offers monitored burn-in during production.

See related block diagram

Currently sampling, the TAS5414 and TAS5424 are expected to be available for volume production in late 2006 and are scheduled to be AEC Q100 qualified at that time.

Pricing for the TAS5414IDKD in a 36-pin PSOP3 package is $9.75 in 1,000s. Pricing for the TAS5424IDKD, offered in a 44-pin PSOP3. is $10.50 in same quantities. Click here for additional information. Data sheets are not available yet.

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Digital amplifiers have taken off in the last five years as evidenced by TI's success with its PurePath digital audio amplifier products for the home, said Ryan Reynolds, TI's business development manager for the digital audio car group. “Our customers have been asking for PurePath quality digital amplifier for the automotive space as well, so that's what we gave them,” he said. Until now, there hasn't been a digital amplifier that was made specifically for automotive audio applications, according to Reynolds.

Making a Class D digital amplifier for automotive applications was no small feat since there are some major requirement differences, Reynolds said.

Automotive EMC requirements are much more stringent, for instance. Furthermore, car manufacturers have their own high quality standard specs (AEC-Q100) that have to be met. And lastly, auto applications demand more integration since there is obviously a lot less space to play with in a car, he said.

“Not only are we dealing with a tighter space, but we need to integrate diagnostics as well. Analog-based audio amplifiers have integrated diagnostic capabilities, but there aren't any digital amplifiers with diagnostic capabilities,” Reynolds said.

Digital amplifiers make a lot of sense in the automotive space because they are 90 percent more efficient using a 4-ohm load compared to Class AB amplifiers, Reynolds said. “They generate a lot less heat so they don't require fans and large heat sinks,” he said. With a power output of 22 watts (4 x 22 W), at 1 percent total harmonic distortion plus noise, dissipation is 7.8 W for the digital amplifier compared to 44 W for a Class AB amplifier, Reynolds said.

Digital amplifiers typically generate so much EMC that they can't be used in automotive applications because they can interfere with other systems in the car such as brakes or airbags.

TI has two methods for dealing with AM/FM tuner interference that can impact digital switching frequency. First, TI has a patented tuner frequency avoidance technology that it uses in its Class D amps for the home. This method involves an internal clock oscillator designed with three internally selectable switching frequencies, an I2C command to select a switching frequency that doesn't interfere with the tuner frequency, and additional TAS5414 devices and switched mode power supply (SMPS) that can be synchronized so no audible noise is created.

In addition, TI's patent pending clock dithering method includes an internal clock oscillator that can be dithered with an external, user defined frequency modulation signal that eliminates the chance of the tuner picking up noise from the amplifier. “Essentially, it slowly changes the frequency so it doesn't pick up interference,” Reynolds said. This feature also reduces measured radiated emissions by 10 dB to 20 dB in the AM/FM band without audio performance degradation, he said.

TI's goal is to get as close to possible to zero dppm. In the home, less than 100 dppm is acceptable, according to Reynolds. Since the parts aren't actually in mass production yet, TI doesn't have an actual dppm number for the new parts. However, if they come close to their home counterparts, TI's dppm for its PurePath digital amplifiers is a mere 6, Reynolds said.

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