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Class D audio subsystems meet Microsoft Vista compliance regime

Maxim has introduced the first Windows Vista-compliant, audio-subsystem ICs for notebook computers. The MAX9791 and MAX9792 combine a stereo 2W or mono 3W Class D speaker amplifier, a stereo headphone amplifier, and a 120mA low-dropout regulator (LDO) in a single chip.

Commented Kymberly Christman, an applications engineer in Maxim's MultiMedia Business Unit: “To obtain the Vista logo, certain audio performance criteria apply to both the speaker amplifier and the headphone amplifier. Additionally, the PCB layout and support component selection becomes key in realising a Vista-compliant system. These requirements apply to the entire signal path – not just the last amplifier in the chain.”

Unlike prior Microsoft operating systems, Vista's requirements are actually enforced. Notably, only the headphone amplifier performance is currently tested because the headphone amplifier is readily accessible via the headphone output jack. The speaker amplifier is more challenging to test as it is located inside the box. However, Christman believes that in-air testing could be a possibility in the future.

The MAX9791/92 cements the argument for using Class D architectures in space and power constrained applications, with these devices being said to consume 36% less board space and 37% less power than competitive solutions. The MAX9791/92 follow on from Maxim's MAX9789A Class AB speaker amplifier design, with the Class D designs resulting in longer battery life and less system heat than the previous solution. Thermal-design constraints are a key disadvantage of Class AB amplifiers in confined spaces, such as notebook PCs, or docking stations, where these are also applicable.

The headphone amplifiers employ Maxim's DirectDrive technology, which produces a ground-referenced output from a single supply. This technology eliminates the need for DC-blocking capacitors, improving bass response and minimising audio distortion.

Headphone performance is improved by incorporating ground-sense circuitry at the headphone jack. COM and SENSE pins in the MAX9791/2 can be used to sense the ground potential at the headphone output jack and correct for it at the headphone amplifier. Notes Christman: “Crosstalk is highly dependent on the amount of resistance in the headphone ground return. The further the headphone jack is from the amplifier, the more the resistance in the headphone ground return – resulting in more resistive crosstalk. Vista's crosstalk specification has been the most difficult criteria to meet because a lot of designers do not have the choice of where the headphone jack is placed – usually a marketing decision.”

The MAX9791 has a stereo 2W Class D speaker amplifier, whilst the MAX9792 combines a mono 3W speaker amplifier, offering higher output power levels for mono notebook applications. The integrated LDO can be used to power the analog portion of the PC's audio codec, improving system power supply rejection ratio and reducing unwanted noises, buzzes and clicks. Both devices are fully specified over the “40 to +85 oC temperature range, and are available in a 28-pin 4 x 4mm TQFN package.

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