In like fashion to switchmode (versus linear) power supplies, which previously made their grand contribution to ultraminiaturization in portable communications devices, the Class D "digital audio" power amplifier, using a topology similar to the DC/DC buck converter, is poised to do its part.
For chip makers of next-generation Class D amplifiers, (i.e, of several varieties, most often a PWM-driven stage), it's a no-compromise effort to provide superior fidelity and higher efficiency in a smaller package than traditional Class AB analog amplifiers. Indeed, today's high-range Class D amps boast higher power density and take up significantly less board space, provide the very high efficiency that's come to be expected from switched-mode devices, and cut down significantly on power supply noise while minimizing EMI.
A snapshot of the latest Class D devices (which can include some with analog inputs, others with digital inputs) reveals a fair degree of innovation for meeting these main objectives. The major thrust extends towards ultra-high power devices with greater fidelity and lower noise floor. In the high-power arena, TI's (Dallas) TAS5261 is the industry's highest power single-chip Class D amplifier stage, delivering more than 300 watts into a 4-ohm speaker (or 220 watts into 6 ohms, 125 watts in 8 ohms) for AV and DVD receivers and home theater systems. This performance represents a doubling of output capability compared to the nearest Class D competitor. A complementary product, the TAS5162, can deliver 200 watts per channel at 6 ohms and 125 watts at 8 ohms. In addition to their high-current capability (up to 17 amps for the TAS5261), the amps include two-stage current-limiting circuitry to deal with speaker resonance conditions and impedance drop versus frequency. Cycle-by-cycle short-circuit monitoring circuitry protects against speaker damage.
TI earlier released its TAS5414/24 devices, providing up to 45 watts, their first to focus on automobile applications. Suited for LCD TVs and home theatres, Microsemi's (Irvine, Calif.) LX1725 Class D amp delivers 15 watts per channel, or a single-channel output of 30 watts. It works with both split and single power supplies. THD is 0.1 percent at 1 kHz, and SNR is 85 dB. In addition, Toshiba's (San Jose, Calif.) TB2924FG audio amp delivers up to 21 watts per channel (8 ohms) in a heatsink-free design for consumer electronics.
At the lower powers suited to handheld device applications, Texas Instruments' TPA203xD1 , for portable consumer electronics, claims an A-weighted noise floor of 27 microvolts rms, the lowest in the industry. TI says these fixed-gain amps, available in 2, 3, and 4 V/V versions and providing up to 2.75 watts into a 4-ohm load working from a 5-volt supply, improve PSRR by 20 dB and CMRR by 12 dB over competing devices. STMicroelectronics' Class D amp for mobile phone and other portable applications, the TS4962 , is said to provide higher audio output while preserving and extending battery life. The amp, providing 3 watts into a 4-ohm load at 10 percent THD (or 1.75 watts into 8 ohms) when operating off a 5-volt supply, has an efficiency of 88 percent. Output power is 2.3 watts into 4 ohms at 1 percent THD. Signal to noise ratio is typically 85 dB. Its fully differential design is said to reduce RF rectification effects and eliminate the need for a bypass capacitor.
A trio of previously unannounced Class-D amps from Maxim (Sunnyvale), their MAX9741, 9746, and 9773, offer a comprehensive solution set for consumer applications. At the higher power end, the filter-free MAX9741 delivers 12 watts continuous into an 8-ohm load. The MAX9746 is a filterless 1.2-watt audio amp (8-ohm load) designed specifically for systems using a 1.8-ohm logic interface. The filterless MAX9773 delivers 1.8 watts into a 4-ohm load for cell/multimedia phones, handheld gaming consoles, MP3 players, and notebooks.
Also, two recent arrivals from Analog Devices (Norwood, Mass.) touting very low EMI and a filterless topology, the SSM2301 (mono) and SSM2304 (stereo) chips deliver 1.4 watts into an 8-ohm load, and 2 watts into a 4-ohm load, respectively. "While most Class D amplifiers use some variation of PWM, ours use a sigma-delta pulse density modulation (PDM) to reduce the amplitude of spectral components at high frequencies, thus significantly minimizing EMI emissions," said Steve Sockolov, product line director, Precision Signal Processing. The devices, in a 3-by-3 mm LFCSP, tout less than 1 percent total harmonic distortion plus noise (THD + N) when driving peak loads from a 5-volt supply. SNR is better than 98 dB.
National Semiconductor (Santa Clara, Calif.) says its LM4947, a six-input Class D amplifier delivering 500 mW, consumes less power than any competing amplifier. Its built-in stereo headphone amp delivers 30 mW per channel. The chip has a 32-step digital volume control, programmed through an I2C interface.
Other chips include Wolfson's (San Diego) WM8956 stereo DAC with Class D amplifier stage for games and portable devices. It touts 0.03 percent total harmonic distortion at 1 watt per channel into 8 ohms, and an operating efficiency of 87 percent. SNR is 98 dB. A second hybrid device, the company's WM8960 integrates a codec, stereo a/d converters, d/a converters, PLL, headphone driver, and 1-watt Class D speaker driver.
The major fidelity improvements extend to JamTech's (Austin, Tex.) JM2020, initially announced in January and formalized in October, which touts a step advance using its new sub-ranging pulse width modulator technology. The device, delivering 8 watts per channel from a 10-13.2 volt input source, touts 16-bit audio quality with 100 dB true dynamic range, 100 dB SNR, and an overall efficiency of 80 percent.
The sub-ranging technology, initially developed for data converters, divides the input data into the sum of coarse and fine signal components, which are converted separately and recombined at the output. As a result, the output signal driving the output stage has significantly higher precision for even the smallest audio inputs. The company points to a water bucket analogy using a large diameter hose (most significant bits) and a small diameter hose (least significant bits) each connected to a time-controlled PWM. The large hose fills the bucket quickly, and the small hose fine-tunes the level very precisely.
More reference designs
Several design kits have come aboard to acquaint designers with the advantages afforded by Class D amplifiers. Cirrus Logic's (Austin, Tex.) Class D reference design built around its newly released CS44130 quad amplifier, and the CS44600 digital amplifier controller provides up to 60 watts for digital TVs and home theater receivers. It's a heatsink-free design that's designed to showcase the company's complete solution for audio conversion and digital audio amplification. The CS44130 boasts a system level dynamic range of greater than 105 dB. Its outputs can be configured as four half-bridge channels, two full-bridge channels or a parallel one-bridge channel. Similarly, International Rectifier's (El Segundo, Calif.) IRAUDAMP3 reference design is a six-channel, 120-watt half-bridge Class D audio power amplifier. The heatsink-free design features the IRS20124S high-voltage analog IC and IRF6645 DirectFET power MOSFETs.
In addition, Freescale Semiconductor is sampling its Symphony Class D digital amplifier solution, comprising its FSA95601 digital amp controller chip and the MC33851 output power stage chip. Production is expected in 1Q 2007.