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National unveils 'smallest' Boomer Class AB amps

Posted: 17 Aug 2006 ?? ?Print Version ?Bookmark and Share

Keywords:National? LM4941? audio amplifier? LM4985? stereo headphone amplifier?

National's Boomer Class AB amps

National Semiconductor Corp. has introduced two Boomer Class AB amplifiers in a 0.4mm-pitch package, said to be the world's smallest micro SMD package. The LM4941 1.25W audio amplifier and LM4985 stereo headphone amplifier promise low quiescent current and high output power, extending battery life in portable electronic devices, MP3 players, mobile phones and other similar applications.

The company said its 0.4mm-pitch micro SMD package uses very little space on a PCB, facilitating easier layout in small spaces. This package uses advanced manufacturing processes for making silicon, bumping and back-grinding technologies.

The LM4941 Class AB 1.2W audio amplifier integrates the company's RF suppression circuitry, which promises up to a 20dB improvement over existing designs. This RF circuitry prevents the outputs from acting as antennas that can pick up noise and inject it back into the amplifier signal path. This 1.2-by-1.2mm fully differential amp rejects the common-mode noise found in most systems and offers a low quiescent current of 1.7mA at a 5V supply.

With an I?C compatible controlled, 32-step, non-linear volume control, the LM4985 low-noise stereo headphone amplifier reduces the need for software to increase the system's volume-control resolution, according to the company. To improve the customer's listening experience, National said the volume control curve is tailored for the human ear. The amp has a low quiescent current of 3mA, dual channel in OCL mode and 135mW per channel of continuous average power into a 16次 load from a 5V power supply.

The LM4985 may be used as either a capacitive-coupled or output capacitor-less (OCL) device for design flexibility, while also allowing companies to speed product qualification by creating additional platforms using this one amplifier.

Key features
The LM4941 delivers 1.25W of continuous power to an 8次 load with 0.04 percent total harmonic distortion plus noise (THD+N) from a 5V power supply. It has a typical power supply rejection ratio (PSRR) of 95dB at 217Hz and an SNR of 108dB at 1kHz. The amp also provides a mono speaker output.

At 1 percent THD from a 3.6V power supply, the LM4985 delivers 68mW per channel of continuous average power into a 16 次 load or 38mW per channel into a 32次 load when operating in OCL mode. Its I?C compatible volume control allows 18db to -76db gain settings. At very low volumes, where the human ear needs a large jump in volume to identify the change, the step sizes are large. At high volumes, where the human ear can discern much smaller level changes, step sizes are very small. It also has a PSRR of 77dB at 217Hz in OCL mode.

Both chips work without output coupling capacitors or bootstrap capacitors and feature an internal thermal shutdown protection mechanism. Their advanced pop-and-click circuitry eliminates noises which would otherwise occur during turn-on and turn-off transitions, the company explained.

Already available, the LM4985 is priced at $1.46 and the LM4941 is priced at 62 cents, both in 1,000-unit quantities.

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Related Datasheets
Part Number Description Category
? LM4985 Boomer ? Audio Power Amplifier Series Stereo 135mW Low Noise Headphone Amplifier with Selectable Capacitively Coupled or Output Capacitor-less (OCL) Output and Digitally Controlled (I2C) V Passives - Filters, Oscillators and Resonators
? LM4941 1.25 Watt Fully Differential Audio Power Amplifier With RF Suppression and Shutdown from the PowerWise? Family Passives - Filters, Oscillators and Resonators



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