Efficient 3D Stereo Class D Audio Subsystem (Maxim)

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Maxim Integrated Products introduces the MAX9775/MAX9776 high-efficiency Class D audio subsystem for 3D stereo boost. This family of devices is specifically designed for cellular phones or other applications where sound quality is high and the speakers are located very close together.

As stereo speakers become smaller and smaller, stereo sounds get worse. In handheld devices, the effect is almost mono, since the speakers are very close.

With the MAX9775's built-in Maxim 3D stereo enhancement, you can simulate stereo sound with speakers very close together. The waveform interface can be used to eliminate the left channel output of the listener's right ear and vice versa. This 3D lifting technology gives the speaker a clear separation with four times or more resolution in physical isolation!

The MAX9775 includes five output amplifiers: a mono, DirectDrive, 80mW receiver amplifier; a stereo, DirectDrive, 80mW headphone amplifier; and a high-efficiency, Class D stereo 1.5W audio power amplifier. The MAX9776 features four output amplifiers: a mono, 80mW receiver amplifier; a stereo, DirectDrive 80mW headphone amplifier; and a stereo Class D amplifier. The MAX9775/MAX9776 are designed with proprietary technology such as spread-spectrum modulation, active emission limiting (AEL), Class D filterless op amps, and DirectDrive. The advantages of each technology will be described in turn below.

The MAX9775/MAX9776 Class D audio systems include the company's proprietary spread-spectrum modulation and third-generation filterless active-radiation limiting (AEL) technology. This proprietary architecture reduces EMI emissions and ensures that the system meets the requirements of international communication standards.

The efficiency of a Class D amplifier depends on the operating state of the output stage transistors. In a Class D amplifier, the output transistor acts as a freewheeling switch and consumes almost negligible power. The power consumed by the Class D output stage is almost equal to the sum of the I2R loss and the quiescent current on the MOSFET's on-resistance.

The patented spread spectrum modulation technique flattens out the wideband spectral components, thus improving EMI radiation performance. At this point, a large amount of spectral energy will no longer be concentrated at the multiplier of the switching frequency, but will be spread throughout the bandwidth as the frequency increases. Above the megahertz frequency, the wideband spectrum is almost white noise for EMI. Proprietary technology ensures that the cycle-by-cycle switching cycle does not degrade audio reproduction or efficiency. The MAX9775/MAX9776 Class D speaker amplifiers have efficiencies greater than 70%, while the AEL circuit ensures EMI below the standard.

The device's stereo headphone amplifier and receiver amplifier use the company's patented DirectDrive architecture, which uses a charge pump to generate an internal negative voltage. This results in a ground-referenced output that nearly doubles the dynamic range with a single supply. This architecture eliminates the need for large-size DC blocking capacitors for output. The device's charge pump requires only two small ceramic capacitors, eliminating the need for two large-capacitance (typically 220μF) tantalum capacitors. DirectDrive technology also has complete noise suppression. The entire design saves board space, reduces cost, and improves the frequency response of the headphone driver.

The MAX9775/MAX9776 feature a user-defined input architecture with built-in input mixers and multiplexers. The three flexible input ports can be configured as single-ended stereo inputs or differential mono inputs. All input signals are mixed into three unique signals before being transmitted to the amplifier. This unique technology of input and output gives designers a flexible choice for a variety of end applications.

In addition, the MAX9775/MAX9776 include volume control, internal gain, and an I2C-compatible interface for software control.

Samples are available for the MAX9775 and MAX9776. The device is available in a standard 32-pin, 5mm x 5mm TQFN package or a 36-bump, 3mm x 3mm (RoHS compatible) UCSP package. The MAX9775 starts at $1.45, while the MAX9776 starts at $1.25 (from 10,000, FOB USA). An evaluation board is available to speed up the design process.

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