Class-D digital amplifier made for flat-panel TVs

Dallas — This digital-input Class-D audio power amplifier from Texas Instruments Inc. is said to enable the development of flat-panel displays (FPDs) with sound quality that matches the display's high-quality image, while minimizing cost and component count.

Suited for 27-inch and larger FPDs, the digital amplifier operates more efficiently than traditional linear amplifiers, minimizing heat and eliminating the space required for an external heat sink, said Eric Droge, TI's marketing manager for audio power amps. It also minimizes overall power consumption, which helps reduce the transformer and regulator size and cost, he said.

The amplifier operates from 8-V to 18-V supply voltages and has less than one percent (0.09 percent at 1 kHz) total harmonic distortion plus noise (THD+N) at half the rated output power level, and less than 10 percent THD+N at the maximum rated output level (see related image below).

“The amplifier's operating voltage range includes 12 V, a very common supply rail found in flat panel displays, and its capability to operate up to 18 V provides enough voltage swing on the outputs to drive 20 W at 10 percent THD+N or 10 W with only 0.09 percent THD+N,” Droge said.

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Harmonic Distortion Plus Noise vs. Output Power Chart

Some other features of the amplifier include three integrated gain settings, a power supply rejection ratio (PSRR) of -60 dB, thermal and short-circuit protection, and a power-saving shutdown mode.

The amplifier provides the key functions required by flat- panel manufacturers, including both a digital interface and power amplifier. The device's 250-kHz switching frequency oversamples the audio signal a minimum of 10 times to ensure high quality audio that is equal to or better than the linear amplifiers used in flat panels today, according to Droge. The amplifier's digital 24-bit Inter-IC Sound (I²S ) or 16-bit right-justified input connects directly to most audio processors and reduces layout concerns that can cause noise and affect signal integrity, he said.

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The TPA3200D1 is available today in volume in a 44-pin TSSOP package. Suggested resale pricing in 1,000-piece quantities is $2.95. Click here for the TPA3200D1 data sheet.

Texas Instruments , 1-800-477-8924,

Rarely do I get the opportunity to actually experience the products I'm writing about. I did, however, get a live demo of TI's latest Class-D audio power amp (APA).

Now, while I'm certainly no audiophile, I do enjoy listening to music. And, I was amazed with the clarity and quality of the music I heard generated from this amplifier, which is TI's first single-chip digital-input Class-D amp aimed at this market. I'm sure my husband, who is the surround sound expert at home, would be thrilled to have this sound emanating from our TV set.

For demo purposes, TI put the amplifier on a demonstration platform that converted the Sony/Philips Digital Interface format (SPDIF) to the I²S serial bus. This allowed them to use the output of a common DVD player.

Although the demo lacked volume control, its full scale output was a suitable listening level with the internal gain of the amplifier set at 12 dB — the lowest of the three gain settings.

The single IC takes a digital input signal and drives a pulse-width modulation (PWM) output signal to the speaker. Using a digital audio path instead of an analog input path eliminates the adverse affects of noise coupled on the input signal and the need for a digital-to-analog converter (D/A converter), Droge said.

The thin form factor of flat-panel displays necessitates an efficient audio solution because there is no room to dissipate the heat generated by the inherent inefficient operation of linear amplifiers (i.e. Class-AB amplifiers).

In contrast to a linear amplifier, a Class-D or digital amplifier is very efficient, Droge said. The high efficiency is the result of switching the output transistors of the amplifier on and off very quickly to produce the analog audio signal. During the 'on' period, the transistors conduct current with very little voltage drops, like a short circuit. During the 'off' period, the transistors are open circuits, through which no current flows. Very little power is dissipated as heat in either period because the voltage and current are not both a high value in the same period. Consequently, less heat is generated using Class-D or digital amps, which eliminates the need for heat sinks, Droge explained.

Previously, all amplifiers in the Class-D spectrum were linear-input devices. Since Class-AB amplifiers, which have ruled the market for many years, were also linear-input devices, analog-input Class-D amps were a natural succession.

Now, Class-D digital input devices are starting to emerge as applications continue to grow more complex and designers must find ways to improve the noise immunity of the audio signal, Droge said. “We are starting to see a shift from Class-AB or linear amps to Class-D digital amps especially in the home entertainment market,” he said.

Integrating three different gain settings within the amplifier that are common for various applications allows designers to increase or decrease the amplitude of the output signal — giving them three different options while utilizing fewer components, Droge said.

A key benefit of using the digital amplifier is that it is cost effective and provides the functions needed by flat-panel customers without excess features that drive up cost, Droge said.

Click here to learn more about how Class-D technology provides benefits as an audio power amplifier solution according to Droge’s Opinion column, entitled “Flat panels capitalize on cool, clear Class-D audio technology.”

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