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New Class D Boomer Audio Amplifier from National Semiconductor Significantly Reduces EMI


Unique Spread Spectrum Architecture Lowers Noise While Maintaining Output

October 23, 2006 – A new mono, 2.65W, Class D Boomer® audio amplifier from National Semiconductor Corporation (NYSE:NSM) reduces electromagnetic interference (EMI) more than 11 dB below the United States Federal Communications Commission (FCC) limit to lower system noise in portable devices including cell phones, smart phones, music players, notebook computers, DVD players, gaming devices and other portable electronics.

The LM4675 is a single-supply, filter-less, high-power-output, Class D mono audio amplifier in space-saving LLP® or 0.5 mm-pitch micro SMD packages. This new class of power amplifier employs a unique spread spectrum technology and a low-noise, filter-less, pulse-width modulation (PWM) architecture that significantly lowers electromagnetic interference (EMI) and eliminates output filters. This combination reduces external component count, saves board space, lowers system cost and simplifies circuit design.

National optimized the design of its new Class D amplifiers with spread spectrum technology for high efficiency. With a 3.6V supply driving an 8-Ohm speaker, the amplifiers’ efficiency for a 100 mW power level will be greater than 80 percent, reaching 89 percent at 400 mW output power. The company plans to leverage this ultra-low EMI technology for future products, including a stereo version of the LM4675.

Key Technical Features and Benefits
National’s LM4675 consumes 1.6 mA of power at a 2.7V supply, enabling longer talk times in mobile phones and more listening or playing time for portable devices. The device operates on a wide power supply voltage range of 2.4V to 5.5V.
National designed the amplifier with added features such as differential inputs for reduce noise. The amp also includes the industry’s lowest-current shutdown mode, output short circuit and thermal overload protection, as well as internal, advanced pop-and-click circuitry that eliminates output transients which would otherwise occur during power or shutdown cycles. The gain of the LM4675 is externally configurable, allowing users to connect different multiple inputs while optimizing the gain for each source or system.

Operating on a single 5V supply, the LM4675 drives a 4-Ohm speaker load at 2.2W continuous output with less than 1 percent total harmonic distortion plus noise (THD+N). In addition to its industry-leading low quiescent current, the LM4675 offers a power supply rejection ratio (PSRR) of 82 dB at 217 Hz, a common-mode rejection ratio (CMRR) of 80 dB and a signal-to-noise ratio (SNR) of 82 dB. It also has a typical fast turn-on time of 17 microseconds.

Pricing and Availability
Available now, the LM4675 is priced at 98 cents in an 8-pin LLP® package and 95 cents in the 9-bump, 0.5 mm-pitch micro SMD package, all in 1,000-unit quantities. More information and easy-to-order samples are available at To learn about high-performance audio amplifiers for portable applications, visit National’s library of online seminars.

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

About National’s Audio Portfolio
National Semiconductor has been a leading provider of audio solutions for more than 30 years and offers a wide range of products in its Boomer®, Overture® and high-performance audio lines. National works with every major handset maker in the world and is the leading provider of analog audio solutions for the cell phone market. The company’s line of Boomer audio amplifiers and subsystems enhance and enrich the entire range of sound – from voice, ring sounds and music – that can be amplified in mobile phones and other portable equipment. National manufactures Class AB and Class D speaker amplifiers, headphone amplifiers and audio subsystems for many applications ranging from portable devices to high-power, high-fidelity audio systems. The company offers its higher-power Overture amplifiers for mid-performance consumer applications, and National’s new high-performance series brings dramatically improved sound quality to high-fidelity consumer and professional audio equipment. 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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