Boomer audio subsystems add RF suppression

Santa Clara, Calif.&#8212National Semiconductor's two new Boomer audio subsystems, the LM4946 and LM4947, integrate radio-frequency (RF) suppression technology to improve noise performance in cell phone and other portable applications.

The LM4946 and LM4947 contain a mono amplifier, stereo headphone amplifier that can couple directly to the headset (i.e., no coupling/blocking output capacitor, OCL mode) and programmable 3D sound enhancement. RF suppression circuitry prevents the outputs from picking up noise and injecting it back into the amplifier signal path. The subsystems' mono amplifier also provides a differential input to improve the power supply rejection ratio (PSRR), common-mode rejection rate (CMRR) and noise performance.

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The four-input LM4946 delivers 540 mW to a mono, 8-ohm bridge-tied-load (BTL) with 1 percent total harmonic distortion plus noise (THD+N). Its built-in headphone amplifier provides 35 mW per channel of continuous average power into stereo 32 single-ended (SE) loads with 1% THD+N, or OCL configuration with identical specs as the SE from a 3.3-volt power supply. The stereo headphone amplifier requires just seven external components in the OCL mode.

The six-input LM4947 delivers 500 mW with the same general specifications as the LM4946. Its mono amplifier is a fully differential, filterless, Class D audio amplifier that works off a 5-volt supply. The LM4947 consumes less power than any similar Class D audio amplifier available today, according to the company. Typical quiescent current is 2.1 mA with a 3.6 volt supply to drive ringtones with maximum efficiency.

Its built-in stereo headphone amplifier delivers 30 mW per channel. Its four-pin stereo input configuration allows system designers to directly connect one pair of differential stereo inputs from a differential output source, such as an MP3 decoder, or to directly connect to single-ended stereo input sources such as melodic ringtone integrated circuits or FM radio modules.

Both chips offer a 32-step digital volume control and eight output modes. The volume control, adjustable 3D sound enhancement and output modes are programmed through a two-wire I2 C compatible interface that allows flexibility in routing and mixing audio channels.

These amplifiers feature a shutdown mode, interrnal thermal shutdown protection mechanism, and advanced pop-and-click circuitry that eliminates output transients which would otherwise occur during power or shutdown cycles.'

For more information and to order samples click here and here. To learn about audio amplifiers for portable applications, visit National's library of online seminars (click here). The LM4946, available now in standard and lead-free 24-pin LLP packages, is priced at $2.95 each in 1k quantities. The LM4947, in a 25-bump micro SMD package, costs $3.25.

National Semiconductor , 1-800-272-9959,

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