Designers of portable electronic systems, such as handheld PCs, personal digital assistants, wireless telephones, portable music systems and others, will be able to reduce power consumption, system size and cost with a new Class-D audio power amplifier (APA) from Texas Instruments (TI). The device will extend the battery life and reduce the size and cost of power supplies in portable systems because it is three times more power-efficient than the older Class-AB APAs.

"The time has come for Class-D audio amplifiers," said Steve Goacher, new product development manager for TI's Advanced Analog Products. "With a fully integrated solution, designers can have a system that will be very competitive in the marketplace because it will produce quality sound with longer battery life. With these factors at work, we will start to see audio emerge into new segments, previously limited by available supply power."

The new device is the first in TI's new family of Class-D APAs and is designated as the TPA005D02. The TPA005D02 is a stereo Class-D amplifier capable of driving stereo speakers at 2 Watts (W) continuously with 5 W peaks. The device is the first in the industry to integrate the control circuitry and stereo output stage on a single monolithic die.

Until now, linear Class-AB audio amplifiers have been used in most portable systems, but advances in the sound quality of Class-D APAs and their power-efficiency advantages over Class-AB APAs is causing a shift toward Class-D devices. Class-D APAs feature a different, more efficient architecture than Class-AB. Unlike Class-AB devices, which typically waste more than 75 percent of their input power as heat, Class-D APAs dissipate as little as 10 percent of their input power. With power-efficiency in the 80-90 percent range or three times that of Class-AB amplifiers, Class-D APAs generate significantly less heat, which simplifies system assemblies and improves reliability. Because of their greater efficiency, Class-D APAs require smaller power supplies and eliminate heat sinks, significantly reducing overall system costs, size and weight.

The TPA005D02 device is well suited for battery operated applications because of its efficient utilization of supply current as well as its low-power shutdown mode. When not in use, the TPA005D02 can shift into a shutdown mode where it consumes just 400 micro-amps of power. With a 5V supply, the TPA005D02 will drive 4-ohm stereo speakers and has a very low total harmonic distortion level plus noise (THD+N) of just 0.5 percent across the entire audio frequency range.

Full Design Support

To help designers implement Class-D audio power amplifiers, TI has developed an evaluation module (SLOP223) fully compatible with its APA Plug 'n Play Platform, the only one of its kind in the industry. To help maximize sound quality, a comprehensive application note on low-pass filtering will be available later in the third quarter of 1998. Full documentation for the TPA005D02 includes a User's Guide with reference designs and schematics. Gerber files are available upon request. More information about TI's audio power amplifier products is available on the World Wide Web.

The first commercial Class-D audio amplifiers were in Analog Avenue's Editor's Choice three weeks ago and I wrote at the time that there would be further developments; this is the first. It is the first stereo Class-D commercial amplifier that I know of and is at a power level that is ideal for PC and portable uses.

All that TI says is accurate but bear in mind that you don't get all the savings in a Class-D amplifier simultaneously: Your design goals will reflect which emphasis you want to focus on using the dramatic improvements in dynamic efficiency. Class-D is a kind of Nirvana for the audio power engineer and is now possible because of the ability to drive the loads sufficiently while maintaining switching speeds. But there are disadvantages to the circuits compared with other audio amplifiers: they do take up more real estate with more pins; they do require more external components including the absolutely essential output filters (to recover the analog signals); and designers need to be aware that there is no ground on the speaker lines so those connections need to be thought of more carefully if they are available to the user. For those reasons the product will be used more in portable applications than anywhere else.

The THD+Noise figure and curve is impressive for a device like this up to the rated 2-W rms output (into 4 ohms.) The shutdown power consumption performance is extremely acceptable. TI will get a lot of design wins with the product, despite the limitations of implementation that I have noted. We will no doubt see the company looking for increases in power with fewer external components, lower pin counts and further functionality; we will also see copycat products, at least declared, sooner rather than later.

The TPA005D02 is in TI's 48-pin PowerPAD TSSOP (DCA) and will be shipping late in the third quarter at a price of $3.48 in 1000-piece lots.