SOURCE: Maxim Integrated Products

February 21, 2008 12:05 ET

Simplify Design With Maxim's Flexible, High-Efficiency Class D Audio Subsystems

SUNNYVALE, CA--(Marketwire - February 21, 2008) - Maxim Integrated Products (PINKSHEETS: MXIM) introduces the MAX9775/MAX9776 audio subsystems comprised of DirectDrive headphone amplifiers that need no capacitors and highly efficient Class D speaker amps.

Synopsis for Investor:
--  The MAX9775/MAX9776 are intended for cell phones or any application
    where audio quality is paramount and speakers must be positioned close
    together.
--  Both products are designed around several of the Company's proprietary
    technologies, specifically spread-spectrum modulation, Active Emissions
    Limiting (AEL), Class D filterless amps, and DirectDrive technology.
--  The AEL proprietary architecture reduces EMI radiated emissions and
    ensures acceptance by international communications standards.
--  The patented DirectDrive technology allows a ground-referenced output,
    almost doubling the dynamic range while operating from a single supply.
    This architecture removes the bulky DC-blocking capacitors required on
    the output, thus conserving both board space and saving cost.
--  These products employ Maxim's 3D stereo enhancement technique, which
    yields an "apparent" separation between the speakers that is a factor
    of four or greater than the actual physical separation.

Flexible Inputs and Outputs

The MAX9775/MAX9776 incorporate a user-defined, input architecture that features an input mixer and multiplexer (See Figure 1). Three flexible inputs can be configured as singled-ended stereo inputs or differential mono inputs. All input signals are summed into three unique signals before being routed to the amplifiers. This unique approach for the inputs and outputs gives the designer flexible options for each end application.

Unprecedented Integration of Amplifiers

The MAX9775 integrates 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 (See Figure 2). The MAX9776 has four output amplifiers: the mono, 80mW receiver amplifier; the stereo, DirectDrive 80mW headphone amplifier; and a mono Class D amplifier. The MAX9775/MAX9776 are both designed around several of the Company's proprietary technologies, specifically spread-spectrum modulation, Active Emissions Limiting (AEL), Class D filterless amps, and DirectDrive technology. The benefits of each of these design approaches will be discussed in turn.

Class D Speaker Efficiency and EMI

The MAX9775/MAX9776 Class D audio systems feature the Company's spread-spectrum modulation and third-generation filterless Active Emissions Limiting (AEL) technology. This proprietary architecture reduces EMI radiated emissions and ensures acceptance by international communications standards.

The efficiency of a Class D amplifier is attributed to the operation of the output stage transistors. In a Class D amplifier, the output transistors act as current-steering switches and consume negligible additional power. Any power loss associated with the Class D output stage is mostly due to the I2R loss of the MOSFET on-resistance and quiescent current overhead.

Patented spread-spectrum modulation* flattens the wideband spectral components, thus improving EMI emissions. Instead of a large amount of spectral energy present at multiples of the switching frequency, the energy is now spread over a bandwidth that increases with frequency. Above a few megahertz, the wideband spectrum looks like white noise for EMI purposes. Proprietary techniques ensure that the cycle-to-cycle variation of the switching period does not degrade audio reproduction or efficiency. The MAX9775/MAX9776 Class D speaker amplifiers exhibit greater than 70% efficiencies, while AEL circuitry keeps the EMI levels well below acceptable standards.

DirectDrive Headphone Amplifiers Need No Capacitors, Improve Audio Performance, and Save Valuable Space

The devices' stereo headphone amplifier and receiver amplifier both feature the Company's patented DirectDrive architecture**, which uses a charge pump to produce an internal negative supply voltage. This allows a ground-referenced output, almost doubling the dynamic range while operating from a single supply. This architecture removes the bulky DC-blocking capacitors required on the output. Instead of two large (typically 220 microfarad) tantalum capacitors, the charge pump requires just two small ceramic capacitors (See Figure 3). DirectDrive technology also provides comprehensive click-and-pop suppression. The overall design conserves board space, reduces cost, and improves the frequency response of the headphone driver.

3D Audio Enhancement

As stereo speaker applications become more compact, the quality of stereophonic sound is jeopardized. Close together in a handheld device, speakers almost sound mono.

With Maxim's 3D stereo enhancement in the MAX9775, it is now possible to emulate stereo sound in situations where the speakers must be positioned close together. Wave interference can be used to cancel the left channel in the vicinity of the listener's right ear, and vice versa (See Figure 4). This 3D enhancement technique yields an "apparent" separation between the speakers that is a factor of four or greater than the actual physical separation.

Samples and Further Information

Additional MAX9775/MAX9776 features include volume control, internal gain, and an I2C-compatible interface for software control.

The MAX9775 and MAX9776 are sampling now. Both are available in a standard 32-pin, 5mm x 5mm TQFN package or a 36-bump, 3mm x 3mm (RoHS-compliant) UCSP™. The MAX9775 is priced at $1.45, and the MAX9776 is $1.25 (10,000-up, FOB USA). Evaluation kits are available to speed designs. For more information on these products, please visit: http://www.maxim-ic.com/3D-Audio

UCSP is a trademark of Maxim Integrated Products, Inc.

*U.S. Patent #6,847,257

**U.S. Patent #7,061,327

Figure 1. http://www.maxim-ic.com/MAX9775-Figure1

Figure 2. http://www.maxim-ic.com/MAX9775-Figure2

Figure 3. http://www.maxim-ic.com/MAX9775-Figure3

Figure 4. http://www.maxim-ic.com/MAX9775-Figure4

Contact Information

  • Editors' Contact:
    Drew Ehrlich
    Public Relations
    408-737-7600

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