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National Semiconductor Introduces New Family of Output Capacitor-less Headphone Amplifiers

New Ground-Referenced Amps Well-Suited for Portable Applications

May 22, 2006

May 22, 2006 – National Semiconductor today introduced a family of three Boomer® ground-referenced stereo headphone amplifiers that eliminate external blocking capacitors for portable music players, mobile phones, CD players, personal digital assistants, media players, notebook computers and other portable applications.

The LM4920 is an output capacitor-less (OCL), fixed-gain headphone amplifier in a 2 mm by 2 mm micro SMD package, a 67 percent reduction in footprint size from previous-generation LLP® packages. The chip also includes low logic-level capability to interface easily with low-voltage digital integrated circuits.

Building on National’s LM4920 core, the LM4981 adds an up/down volume control that eliminates the need for complex software controls. The LM4982 offers a digitally controlled I2C compatible volume adjustment with headphone sense, a technique that detects whether a mono or stereo headphone plug has been inserted into the output jack.

“Small, consumer portable music devices are one of today’s fastest growing market segments, and the consumers who purchase these products expect high-quality audio headphones,” said Mike Polacek, vice president of National Semiconductor’s Audio group. “National’s new stereo headphone amplifiers provide superior sound and features in small packages, allowing system designers to shrink their systems without compromising audio fidelity.”

The headphone amplifiers’ circuit technology uses a low-noise, inverting charge pump to generate a negative supply voltage. This allows the outputs to be referenced at ground instead of a nominal DC voltage like traditional headphone amplifiers, eliminating the output-coupling capacitors typically required to drive single-ended loads. Using this circuit technology reduces the component count, system cost and board space, while improving the lower frequency response of the headphone outputs.

Key Features and Benefits
The LM4920 delivers 80 mW of continuous average power into a 16-Ohm, single-ended load or 40 mW per channel into a 16-Ohm load, with less than 1 percent total harmonic distortion plus noise (THD+N) from a 3V power supply. The typical gain of 1.5V is fixed internally, eliminating the need for gain-setting external resistors. It has a typical power supply rejection ratio (PSRR) of 70 db at 217 Hz and features an internal thermal shutdown protection mechanism. Its low-power-consumption shutdown mode is individually selectable for either channel by driving the shutdown channel pins with logic low. The LM4920 also offers fixed logic levels, allowing seamless use with low-voltage basebands and microprocessors.

The LM4981 delivers 83 mW of continuous power into a 16-Ohm load with less than 1 percent THD+N while operating from a 3V power supply. It operates from a single 2V - 4.2V supply and features up-down volume control that sets the gain of the amplifier between -33 dB to +12 dB in 16 discrete steps using a two-wire interface. A selectable (active high/low) low power shutdown mode provides flexible shutdown control.  It has a typical PSRR of 67 db at 217 Hz.

The LM4982 is a variable-gain amplifier capable of delivering 80 mW of continuous average power into a 16-Ohm, single-ended load or 40 mW per channel into a 16-Ohm load, with less than 1 percent THD+N from a 3V power supply. It has a typical PSRR of 66 dB at 1 kHz. Its I2C compatible volume control allows 18 db to -76 db gain settings.

All the chips contain advanced pop and click circuitry, which eliminates noises that would otherwise occur during turn-on and turn-off transitions.

Pricing and Availability
Available now, the LM4920 is offered in 14-bump micro SMD package and priced at 89 cents. The LM4981 is offered in a 16-pin LLP® package and priced at $1.15, while the LM4982 is available in a 16-bump micro SMD package and priced at $1.50.  All prices are for 1,000-unit quantities. More information, easy-to-order samples and evaluation boards are available at http://www.national.com/pf/LM/LM4920.html, http://www.national.com/pf/LM/LM4981.html and http://www.national.com/pf/LM/LM4982.html.  National’s WEBENCH® online design tool supports a large selection of audio amplifiers.  To explore these solutions, visit http://webench.national.com.  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 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 amplifiers, mono and stereo headphone amplifiers, and 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 audio.national.com.

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, communication 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, California, National reported sales of $1.91 billion for fiscal 2005, which ended May 29, 2005.  Additional company and product information is available at www.national.com.