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Cap-free headphone amps improve fidelity, save space

Santa Clara, Calif.&#8212National Semiconductor's LM4920, LM4981, and LM4982 ground-reference stereo headphone amplifiers in the company's Boomer series do away with the output blocking/coupling capacitor usually required to connect to portable music players, mobile phones, CD players, personal digital assistants, media players, and notebook computers.

“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 the 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’ circuitry uses a low-noise, inverting charge pump to generate a negative supply voltage. This technique allows the outputs to be referenced at ground instead of a nominal DC voltage as with traditional headphone amplifiers, eliminating the output-coupling capacitors typically required to drive single-ended loads. This approach reduces the component count, system cost and board space, while improving the headphone's low-frequency response.

The LM4920 is a basic fixed-gain headphone amplifier in a 2-by-2 mm micro SMD package, which has a footprint that's one-third previous-generation LLP packages. The chip also includes low logic-level capability to interface easily with low-voltage digital integrated circuits. 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 3-volt power supply. It has a typical power supply rejection ratio (PSRR) of 70 db at 217 Hz and features internal thermal shutdown protection. Its low-power-consumption shutdown mode is individually selectable for either channel by driving the shutdown channel pins with a logic low command. The LM4920 also offers fixed logic levels, allowing seamless use with low-voltage basebands and microprocessors.

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 I2 C compatible volume adjustment with headphone sense to detect whether a mono or stereo headphone plug has been inserted into the output jack. The LM4981 delivers 83 mW of continuous power into a 16-ohm load with less than 1 percent THD+N while operating from a 3-volt power supply. It operates from a single 2 to 4.2-volt 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 3-volt power supply. It has a typical PSRR of 66 dB at 1 kHz. Its I2 C 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. Click here, here, and here for more information, samples, and evaluation boards. Available now, the LM4920 is offered in a 14-bump micro SMD package and is priced at 89 cents. The LM4981 is offered in a 16-pin LLP package and is priced at $1.15, while the LM4982 is available in a 16-bump micro SMD package and is priced at $1.50. All prices are for 1,000-unit quantities.

National’s WEBENCH on-line design tool supports a large selection of audio amplifiers.'To explore these solutions, click here. For on-line seminars, click here.

National Semiconductor , 1-408-721-5000, www.national.com

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