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National Semiconductor’s New Boomer Audio Subsystems with RF Suppression Help Shield High-Frequency Interference


LM4947’s Class D Mono Speaker Drives High-Powered Ringtones With Maximum Efficiency

June 19, 2006 – Two new Boomer® audio subsystems from National Semiconductor Corporation (NYSE:NSM) integrate radio-frequency (RF) suppression technology to improve noise performance in portable applications. National’s LM4946 and LM4947 audio subsystems contain a mono amplifier, output capacitor-less (OCL) stereo headphone amplifiers and National’s programmable 3D sound enhancement.

The subsystem’s RF suppression circuitry prevents the outputs from acting as antennas that can pick up noise and inject it back into the amplifier signal path. This makes the chips well- suited for cell phones and other portable applications with potential noise sources at various RF frequencies. 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.
The LM4947’s mono amplifier is a fully differential, single-supply, filter-less, 2.5W Class D audio amplifier. Consuming less power than any similar Class D audio amplifier available today, it has a typical quiescent current of 2.1 mA at a 3.6V supply to drive ringtones with maximum efficiency.

Key Technical Features and Benefits
National’s 4 mm by 4 mm LM4946 delivers 540 mW to a mono, 8-Ohm bridge-tied-load (BTL) with 1 percent total harmonic distortion plus noise (THD+N). The built-in stereo headphone amplifier provides 35 mW per channel of power into the stereo, 32-Ohm single-ended load or OCL configuration with 1 percent THD+N from a 3.3V power supply. The stereo headphone amplifier does not require output coupling capacitors in OCL mode.

The LM4946 has four inputs, one pair for a fully differential mono input and another pair for single-ended stereo inputs. It requires only seven external components in OCL mode, which eliminates the large DC blocking capacitors on the headphone outputs.

The LM4947 delivers 500 mW of power with the same general specifications as the LM4946. Its built-in stereo headphone amplifier provides 30 mW per channel of power. The LM4947 contains six inputs, including two inputs for the fully differential, mono Class D amplifier and four stereo inputs that may be configured as one pair of fully differential stereo inputs or two pairs of single-ended stereo inputs. This 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 intput sources, such as melodic ringtone integrated circuits or FM radio modules.

Both subsystems offer a 32-step digital volume control and eight distinct output modes. The volume control, adjustable 3D sound enhancement and output modes are programmed through a two-wire I2C compatible interface that allows flexibility in routing and mixing audio channels.

These amplifiers feature a low-power consumption shutdown mode and an internal thermal shutdown protection mechanism, as well as advanced pop-and-click circuitry that eliminates output transients which would otherwise occur during power or shutdown cycles.

Pricing and Availability
The LM4946 is available now in standard and lead-free 24-pin LLP® packages and priced at $2.95. The LM4947 in a 25-bump micro SMD package costs $3.25. All prices are in 1,000-unit quantities. More information and easy-to-order samples are available at and To learn about audio amplifiers for portable applications, visit National’s library of online seminars.

Note to editors: To view a high-resolution downloadable photo of these products, visit National’s photo gallery.

About National’s Audio Portfolio
National Semiconductor works with every major handset maker in the world and is the leading provider of audio amplifier systems for the cell phone market. The company’s line of Boomer audio amplifiers enhances and enriches the entire range of sound – from voice, ring sounds and music – that can be amplified in a mobile phone. National manufactures Class AB and Class D mono and stereo speaker and headphone amplifiers, as well as audio subsystems for desktop and notebook computers, multimedia displays and portable electronic devices. The company also offers its higher-power Overture® amplifiers for consumer stereo and home theater systems. For more information on National’s audio products, visit

About National Semiconductor
National Semiconductor, the industry’s premier analog company, creates high-value analog devices and subsystems. National’s leading-edge products include power management circuits, display drivers, audio and operational amplifiers, interface products and data conversion solutions. National’s key analog markets include wireless handsets, displays and a variety of broad electronics markets, including medical, automotive, industrial, and test and measurement applications. Headquartered in Santa Clara, Calif., National reported sales of $2.16 billion for fiscal 2006, which ended May 28, 2006. Additional company and product information is available at

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