World's Smallest Class D Audio Amplifiers

To save even more space in portable applications, National Semiconductor Corporation introduced a Boomer mono Class D audio amplifier in the world's smallest micro SMD package. In addition, the company introduced a high-power stereo Class D amplifier. Both provide high-quality output power with a minimal number of external components for cell phones, smart phones and portable audio devices such as DVD players and gaming devices.

The LM4673 is a fully differential, single-supply, filter-less, 2.5W Class D switching audio amplifier in a 1.4 mm by 1.4 mm, 0.4 mm-pitch micro SMD package. This package, which uses very little space on the printed circuit board, facilitates easier layout in small spaces. For example, the chip can be placed very close to speakers, virtually eliminating electromagnetic interference. Consuming less power than any similar Class D audio amplifier available today, the LM4673 has a typical quiescent current of 2.1 mA at a 3.6V supply, enabling longer talk times in mobile phones and more listening time for portable audio devices.

The LM4674 is a filterless, 2.4W stereo Class D audio power amplifier available in a 2 mm by 2 mm, 0.5 mm-pitch package. The chip also has a typical quiescent current of 4 mA at a 3.6V supply for power savings.

National's LM4673 and LM4674 audio amplifiers employ a low-noise, filter-less pulse-width modulation architecture that eliminates output filters, reducing external component count, saving space on the board layout, lowering system cost and simplifying the circuit design. The flexible power supply requirements of these parts allow operation from 2.4V to 5.5V.

Both of these Class D amplifiers have high efficiency with speaker loads compared to typical Class AB amplifiers. Each product features a low-power shutdown mode and internal, advanced "pop and click" circuitry that eliminates output transients which would otherwise occur during power or shutdown cycles.
The gain of the LM4673 is externally configurable to allow the user to connect different multiple inputs while optimizing the gain for each source.

Operating on a single 5V supply, the LM4673 is capable of driving a 4-Ohm speaker load at a continuous average output of 2.5W with less than one percent total harmonic distortion plus noise (THD+N). In addition to its low quiescent current, the LM4673 offers a power supply rejection radio of 78 dB at 217 Hz, a common-mode rejection ratio of 70 dB and a signal-to-noise ratio of 97 dB. It also has a typical fast turn-on time of 17 microseconds.

The LM4674 delivers 2.4W per channel of continuous output power to a 4-Ohm load with 10 percent THD+N. The chip features independent left and right shutdown controls to maximize power savings in mixed mono and stereo applications. To give designers flexibility, the LM4674 has four gain settings of 6 dB, 12 dB, 18 dB and 24 dB. The chip also has a PSRR of 75 db at 217 Hz.

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