Suppliers of audio amplifier ICs are rushing to market a new generation of Class D amplifiers that the vendors expect will replace the linear Class A/B amplifiers currently used in cell phones, personal digital assistants, MP3 players and other handheld, battery-powered consumer products.
These portable systems would benefit from using Class D amps to achieve better power efficiency, longer battery life and lower parts counts, said Eric Droge, audio amplifier product-marketing manager at Texas Instruments Inc. (Dallas). Very high-end audio systems use Class A amplifiers, which offer the best fidelity but generate the most heat. Class B amplifiers, by comparison, offer lower fidelity and less heat. Class A/B is a compromise between A and B: Its fidelity is better than B but not as good as A, and A/B amplifiers generate more heat than Class B amps do.
"MP3 players and PDAs now use Class A/B amplifiers, but they are very inefficient, and more power is required to overcome their inefficiency," Droge said. "The extra power is generated as heat, which is a problem because of the small form factors of battery-operated products. Class D amplifiers generate two-and-a-half times less heat, which means two-and-a-half times more battery life, especially for audio applications." Class D amplifiers are used now in products from boom boxes to midrange hi-fi products.
Customers loved the low heat and longer battery life of currently available Class D amplifiers but there have been problems, according to Droge. "Traditionally, Class D required bulky filtering-a large inductor and capacitor." TI has optimized the output modulation in its newest Class D amplifier series, the TPA2000xD. "It doesn't require a bulky filtering network," Droge said.
Like other Class D amplifiers, TI's entry is based on pulse-width modulation (PWM) oversampling. Droge said PWM samples the input audio signal at 12 times the audio band, then recreates the signal at the speaker. "When the output transistors are on, there is very low resistance, and power is delivered more efficiently to the speaker," Droge said. "When the output transistors are off, no power is consumed or delivered, so there is no loss." TI's so-called "filterless" Class D amplifiers are said to reduce board space and system costs by about 40 percent while maintaining the high efficiency associated with Class D amplifiers.
The first device in the series, TPA2000D2, is a 5-volt stereo audio amplifier with an output power of 2-watts RMS (root mean square) or 4-watts peak into 4-ohm speakers. Droge said that besides operating without a filter, the part reduces the external resistor count by providing four internal gain settings. "Low-noise characteristics improve fidelity, and shutdown control helps prolong battery charge life by limiting total supply current to less than 1 microamp," he added. The TPA2000D2 is packaged in a 24-pin thin shrunk small-outline package (TSSOP) and priced at $2.50 in 1,000-piece quantities.
The advantages of Class D amplifiers in low-power applications are "smaller, cooler packages, and batteries that last twice as long, according to Skip Taylor, vice president of PWM technology at Cirrus Logic Inc. (Austin, Texas).
"Also, since a Class D amplifier is a digital device, more highly integrated solutions can be created," Taylor said. "When a pulse-width modulation amplifier is paired with an analog modulator, the cost of silicon is high. Our approach is totally digital. We're integrating more functions on a chip, including a digital signal processor (DSP), so we can process the signal digitally, on smaller silicon.
"Linear technology is mature and cheap," Taylor continued, "but you can't continue to shrink analog silicon and stay cost-effective. In analog technology, an A/D is needed, but we can connect directly to a DSP."
Taylor said Cirrus is readying the CS44L10 Class D amplifier for introduction this summer. "Everybody has a 'secret sauce,' especially for low-power applications, but in reality, every Class D amplifier has the same capabilities, including the same output stages. And no one's approach is less prone to needing filtering," he said. "In low-power applications, speakers can be used as part of the output filter topology, but that doesn't remove or eliminate the need for an output filter to get rid of noise from the RF switching signal. Whatever can't be filtered translates to heat, and speakers have to be able to deal with heat that hasn't been filtered out.
"For any substantial power level output, the best approach is low-pass filtering between the amplifier and the speaker so the speaker doesn't have to do all the filtering itself," Taylor continued. "That's TI's approach and ours. You can get by with much smaller filtering networks, but we're not doing away with filters."
Bob Samson, U.S. product marketing manager for automotive and audio products at STMicroelectronics (Lexington, Mass.), estimated the power efficiency of Class A/B audio amplifiers as 50 percent with maximum dissipation, which means that half the power required to generate the desired output is wasted, in the form of excess heat. Class D amps, on the other hand, are 90 percent efficient. Samson concurred that Class D amps have, until recently, required bulky, costly inductance/capacitance output filters to eliminate RF carrier noise and electromagnetic interference (EMI). "The size and cost of the complete Class D solution made it unwieldy in products with small form factors," he said.
"There's a trend toward minimizing filtering of Class D amplifiers by minimizing what the speaker sees coming from the amp," noted Tony Doy, senior product definer at Maxim Integrated Products (Sunnyvale, Calif.). "The higher the switching frequency, the smaller the inductor that's needed for filtering, but there's a trade-off in efficiency. These are design considerations for the customer, but some filtering is definitely needed."
Maxim is marketing the MAX4295 (mono) and MAX4297 (stereo), single-channel Class D audio amplifiers that operate from a single 2.7-V to 5.5-V supply and can deliver up to 2 watts per channel of continuous power to a 4-ohm load with an efficiency of better than 87 percent. They are being used for speaker/headphone drivers in portable multimedia, and in general-purpose audio applications that require either high efficiency or low heat dissipation. They feature total harmonic distortion plus noise (THD+N) of less than 0.4 percent, and quiescent current is 2.8 milliamps for the MAX4295 and 4.6 milliamps for the MAX4297. The parts also feature "clickless/popless" power-up and shutdown modes. Their active-low shutdown input disables the device and limits the supply current to less than 1.5 microamps.
Other features include an over-current limit of 1 amp, thermal overload protection, undervoltage lockout and a programmable switching frequency (125 kHz, 250 kHz, 500 kHz or 1 MHz) that allows users to optimize the size and cost of the output filter for specific applications. The MAX4295 is packaged in a 16-pin QSOP or a 16-pin small-outline (SO) package and the MAX4297 in a 24-pin SSOP or 24-pin SO packages. The stereo device is priced from $1.45 in 1,000-piece quantities.
National Semiconductor Corp. (Santa Clara, Calif.) markets a Class D amp, the LM4663, that can output 2 watts per channel of RMS power into a 4-ohm load with 83 percent efficiency (69 percent for typical music signals) and THD+N of less than 0.2 percent. It's packaged in a 24-lead TSSOP and priced at $3.30 in 1,000-piece quantities. But Ken Boyce, marketing manager for amplifiers, is less sanguine than others about the prospects for Class D devices in portable applications because "there are size constraints on inductors." He estimated that when board space and ancillary costs are taken into account, a Class D part could cost two-and-one-half times as much as an A/B amplifier.
"We're selling A/B amplifiers into portable applications," Boyce said. "Most designers know how to deal with them." Class D amplifiers, according to Boyce, are most efficient at higher power levels, which explains their popularity for midrange home entertainment equipment. The strongest selling point for Class D amps in portable applications is longer battery life, but a "filterless" Class D might change the equation.
Also lukewarm to Class D amplifiers is Bob Adams, manager of Analog Devices Inc.'s digital audio engineering group (Wilmington, Mass.). "If there's any use for them at all, it's at low power-below 1-watt," he said. "Above that, it's not worth going to Class D for the headaches you'll encounter."
Adams said the efficiency of filterless Class D designs is nearer 75 percent than 90 percent, "so the difference between a linear amplifier and a filterless Class D is questionable. The filterless designs count on the leakage inductance of the speaker as a low pass filter, and they deliver quite a bit of power to the speaker above the audio range.
"Since the leakage inductance of every speaker is different, you have to be careful about the choice of a speaker," Boyce continued. "Class D amplifiers have been around for a long time, and generation after generation has approached them with naivete, then encountered issues. It's something that a vendor and customer should go into with their eyes open."
STMicroelectronics was one of the first vendors to introduce a Class D amplifier, and the company will manufacture the LM1720 amplifier recently introduced by Microsemi Corp. (Santa Ana, Calif.). Under its own flag, it markets the TDA748x family, which includes the TDA7480, a 10-W amplifier in a 20-lead DIP package with a special copper head spreading leadframe, the TDA7481, an 18-W amplifier in a Multiwatt 15 package and the TDA7482, a 25-W amplifier, also in a Multiwatt 15 package. Samson said the devices could replace conventional linear amplifiers, either by providing equivalent power in less space or more power in the same space; however, there's plenty of life left in traditional (A/B) linear amps. His company and others are developing higher-power, more integrated solutions. "The trend toward higher-efficiency amplifiers is one that's taking off more slowly than we had predicted," Samson said.
From ST's perspective, the TV market is the hottest for Class D. "Television manufacturers want higher power for better sound quality, but they're working in confined space, without much opportunity to get the heat out. Class D hasn't taken off in battery-powered equipment because of its higher voltage requirements," Samson said. ST is working on a device that will combine a Class D output stage on the same chip with a decoder. The as-yet-unnamed part could surface later this year.
Also pursuing an integration strategy, Motorola Semiconductor Products sector (Austin, Texas) is alpha-testing class D "Symphony" amplifier technology, which is designed to be integrated with the company's 24-bit DSP-56300 line of digital signal processors and which incorporates patent-pending methods for pulse-width modulation linearization and noise shaping. Motorola said OEMs could replace class A or A/B amplifier stages with no degradation in audio performance. By using a two-sided PWM approach, the output sample rate is effectively doubled and signal-to-noise ratio increases by more than an order of magnitude. Digital-to-analog converters are eliminated because the signal processing is performed entirely in the digital domain.
Motorola said it plans to expand its line of stereo and multichannel amplifiers from 100-W up. Ken Obuszewski, Motorola's Audio Solutions operations manager, said Symphony amplifier chips operate independently from the back-end switching stage that determines amplification power.
Jean Van Eeghen, marketing manager for analog audio ICs at Philips Semiconductors (Mijmegen, the Netherlands), said his company expects to have Class D audio amplifiers available as early as the third quarter of this year. "Lower power consumption is driving us to Class D," he noted. "Also, the world is becoming increasingly digital, and if we stick with analog inputs, our customers need a converter. Since customers want to minimize their parts count, our goal is a digital source for the amplifier input. The downside of Class D amplifiers is more EMI radiation, because they switch at frequency. Our target is a filterless amplifier, or one that could be used with or without filters."
A different approach
TriPath Technology (Santa Clara, Calif.) is taking a different approach, according to Neal Carney, vice president of marketing. Its "Class T" switching amplifier uses a proprietary and patented architecture that includes feedback to correct for nonlinearity and distortion. "It provides the audio fidelity of class A and B amplifiers with the power efficiency of class D, in the 85 to 95 percent range," Carney said. THD+N varies with the power level, but for normal listening is about 0.02 percent. "Our amplifiers are designed to produce an undistorted signal at a comfortable listening level. For 100 watts of audio output, we generate about 10 watts of heat. An A/B amplifier at about 30 percent efficiency would dissipate more than 200 watts," Carney said.
TriPath's feedback scheme is designed to correct for the distortion that occurs when square waves are used to represent an analog signal. "We can vary the frequency by switching at appropriate times to avoid distortion, earlier or later, depending upon the distortion effect," Carney said. "The amplifier output goes through an LC or low pass filter that transforms the square wave back to an analog signal. We're switching the power FETs field-effect transistors on the output, between the high-voltage rails of the device. The output voltage and the ability of the FETs to withstand breakdown voltage could be plus/minus 35 volts into an 8-ohm speaker. We get power in the 60-W to 80-W range."
According to Carney, comparatively low sound quality has kept Class D amplifiers out of home-stereo and home-theater applications. "Class D is being used in lower-end products like boom boxes, where distortion isn't as important as higher frequency. Our applications are in the hi-fi portion of the market," Carney said. "We enable audio companies like Sony to shrink the size of their A/B receivers." Also taking a different approach to better power efficiency with less of a filtering requirement is Apogee Technology Inc. (Norwood, Mass.), which is marketing an amplifier chip set consisting of a DDX2000 processor and a DDX2060 output stage. The latter chip is capable of delivering 30-W RMS per channel into 8 0hms. The chip set is priced at $6.98 in 1,000-piece quantities. Michael Danielson, a corporate fellow at Apogee, explained that the company's direct digital amplification technology accepts PCM data and modulates it as a PWM signal. "Unlike the classic Class D design, which is constantly connected to a power supply, we connect in a positive or negative direction only as required to produce a signal. Whenever we're not supplying power to the load, we put a short across the load on the ground side so that the load is disconnected from the power supply," Danielson said. "We don't have the carrier switching on and off, so there's not as much energy and it's easier to filter the signal."
Meanwhile, linear amplifer makers aren't ready to concede portable markets. Microchip Technology Inc. (Chandler, Ariz.), for example, recently introduced the TC4864 linear audio amplifier in an MSOP that occupies about half the space of an SOIC-8. It's designed for use in devices with dc power sources as low as 2.7 V, including cellular and cordless phones, desktop and notebook PCs, radios, hearing aids and portable videogames and toys. Product applications manager Patrick Maresca said the amp simplifies overall system design, saves board space and low-
ers manufacturing costs. "It's suited for applications where minimizing audible noise and waveform distortion are critical," he said. The TC4864 is optimized for single-supply operation over an input range from 2.7 V to 5.5 V. It's priced at 87 cents in lots of 1,000.
Analog Devices Inc.
(800) ANALOGD (262-5643)
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Apogee Technology Inc.
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Cirrus Logic Inc.
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Maxim Integrated Products
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Microchip Technology Inc.
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National Semiconductor Corp.
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(800) 234 7381
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Texas Instruments Inc.
(800) 477-8924, ext. 4500
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