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Capacitor-less headphone amps target portable apps

Posted: 24 May 2006 ?? ?Print Version ?Bookmark and Share

Keywords:National Semiconductor? headphone? amplifier? LM4920? LM4981?

National Semiconductor Corp. has introduced a family of three Boomer ground-referenced stereo headphone amplifiersthe LM4920, LM4981and LM4982that aims to eliminate external blocking capacitors for portable music players, cellphones, CD players, PDAs, media players, notebook computers and other portable applications.

The LM4920 is a fixed-gain headphone amplifier that does not have an output capacitor. The product is housed in a 2-by-2mm micro SMD package that is said to be 67 percent less in footprint size than previous-generation LLP packages. The chip also includes low logic-level capability to interface with low-voltage digital ICs.

Building on National's LM4920 core, the LM4981 adds an up/down volume control aimed at eliminating the need for complex software controls. The LM4982 offers a digitally-controlled, I2C-compatible volume adjustment with headphone sense, a technique said to detect whether a mono or stereo headphone plug has been inserted into the output jack.

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.

The LM4920 delivers 80mW of continuous average power into a 16-ohm, single-ended load or 40mW 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.

It has a typical power supply rejection ratio (PSRR) of 70db at 217Hz 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 use with low-voltage basebands and microprocessors.

The LM4981 delivers 83mW 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 2-4.2V supply and features up-down volume control that sets the gain of the amplifier between -33dB to 12dB in 16 discrete steps using a two-wire interface. A selectable (active high/low) low power shutdown mode provides shutdown control. It has a typical PSRR of 67db at 217Hz.

National said the LM4982 is a variable-gain amplifier capable of delivering 80mW of continuous average power into a 16 ohm, single-ended load or 40mW 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 66dB at 1kHz. Its I2C compatible volume control allows 18db to -76db gain settings.

All the chips contain advanced pop and click circuitry said to eliminate noises that would otherwise occur during turn-on and turn-off transitions.

The LM4920 is available in a 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.




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Related Datasheets
Part Number Description Category
? LM4982 Ground-Referenced, Ultra Low Noise, 80mW Stereo Headphone Amplifier with IntelliSense and I2C Volume Control Passives - Filters, Oscillators and Resonators
? LM4920 Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW Stereo Headphone Amplifier Passives - Filters, Oscillators and Resonators

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