Shrinking Class D amplifier moves to 0.4-mm pitch

Santa Clara, Calif. — Aimed at portable applications, National Semiconductor Corp. rolled out what it claims is the smallest 2.5-watt, Class D audio amplifier in the world.

In addition, the company introduced a high-output power stereo Class D amplifier. Both Boomer devices, which are filterless, 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, said Mike Polacek, vice president of National Semiconductor's Audio group.

The LM4673 mono Class D amplifier comes in a tiny, 0.4 mm-pitch micro SMD package (1.4 x 1.4 mm), and the stereo LM4674 Class D amplifier also provides board-space efficiency for stereo applications in a 2 x 2 mm, 0-5-mm pitch package, Polacek said.

The LM4673 is a fully differential, single-supply, 2.5-watt Class D switching audio amplifier. Consuming less power than any similar Class D audio amplifier available today, the LM4673 has a typical quiescent current of 2.1 milliamps at a 3.6 volts, Polacek said.

The LM4674 is a 2.4-W stereo Class D audio power amplifier. The chip also has a typical quiescent current of 4 mA at a 3.6 V supply for power savings.

National's LM4673 and LM4674 audio amplifiers employ a low-noise, filterless pulse-width modulation (PWM) architecture that eliminates output filters, Polacek said. The flexible power supply requirements of these parts allow operation from 2.4 V to 5.5 V.

Both of these Class D amplifiers have high efficiency with speaker loads compared to typical Class AB amplifiers, Polacek said. Each product features low-power shutdown mode, and advanced pop and click circuitry that eliminates output transients. 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 5-V supply, the LM4673 is capable of driving a 4-ohm speaker load at a continuous average output of 2.5 W with less than one percent total harmonic distortion plus noise (THD+N). In addition, the LM4673 offers a power supply rejection radio (PSRR) of 78 dB at 217 Hz, a common-mode rejection ratio (CMRR) of 70 dB, and a signal-to-noise ratio (SNR) of 97 dB. It also has a typical fast turn-on time of 17 microseconds.

The LM4674 delivers 2.4 W 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.

See related block diagram

The LM4673 is priced at 90 cents in a lead-free, 9-bump 0.4-mm pitch micro SMD package. The LM4674 is offered in a standard or lead-free, 16-bump micro SMD package and is priced at $1.50. Both are available now and pricing is in 1,000-piece quantities. Click here for the LM4673 data sheet. Click here for the LM4674 data sheet.

National Semiconductor , 1-800-272-9959,

National's Boomer family of audio amplifiers for portable applications range from 10 mW to 3 W. “Although National has hundreds of Boomer amplifiers, we currently offer eight Class D Boomer amplifiers,” said Gary Adrig, director of marketing, National's Audio Group. “It's a smaller subset of the family. The others are mostly Class AB amplifiers,” he said.

Previously, the smallest micro SMD package (aka flip-chip package) available for analog semiconductors was the .5-mm pitch, Adrig said. “That's been the industry standard up till now. National's LM4673 is the first one in the industry that has gotten down to the 0.4-mm pitch. One-tenth of a millimeter doesn't sound like much. But, if you take spacing between each bump into account, you save .2 mm on just three bumps. We are talking about nine bumps, which is a significant space reduction for the final product,” he said.

National introduced the first 0.5-mm pitch micro SMD package in 1999 and the rest of the industry followed suit, according to Adrig. He expects the 0.4-mm pitch to become the next industry standard.

The LM4673 is a one-channel mono amplifier, with specs that will amaze most people, Adrig said. “It's the smallest 2.5-W Class D amplifier with high PSRR and the lowest quiescent current in the industry, which ultimately translates to longer battery life,” he said.

At 3.6 V, this amplifier runs 2.1 mA typically, compared to other Class D amps that are in the 4 mA to 5-mA range at 3.6 V, Adrig said. National's year-old predecessor Class D amplifier, the LM4671, provides typical power consumption of 3.8 mA at 3.6 V. A key distinction between the LM4671 and newer devices is the maximum quiescent current limit, which is 6.2 mA for the LM4671, and only 2.9 mA for the LM4673. “The low quiescent current of the LM4673 significantly reduces the drain on the power budget of portable devices when compared with existing Class D audio amplifiers,” Adrig said.

Click here for more details on National Semiconductor’s LM4671 at eeProductCenter.

PSRR is 78 dB for the LM4673, compared to 68 dB for the LM4671. “This may not be considered very high for some high performance amps — but for a portable Class D amp, it's very high,” Adrig said.

PSRR for typical single-ended Class AB amplifiers for portable equipment runs in the mid- to high 60's. and differential Class AB amplifiers for portable equipment have PSRR around 82 dB to 85 dB PSRR, Adrig said. “With 78dB PSRR, the Class D LM4673 has PSRR approaching that of fully differential Class AB amps,” he said.

The main difference between the LM4673 and LM4674 amplifiers is that the LM4673 is a single-channel amplifier, while the LM4674 is a two-channel amplifier. Both are considered high-output power amplifiers.

“These kinds of amplifiers can operate up to 5.5 V. For mono Class-D amps operating in this voltage range, anything over 1 W is considered high power,” said Genevieve Vansteeg, product marketing manager for National's Audio Group. At 5 V, National's older LM4671 delivered 1.2 W into an 8-ohm load. The new device, with the exact same circumstances (5 V and 8-ohm load) is delivering 1.24 V.

“Even though we have reduced the size of the package, and the amount of current (2.6 mA under these conditions), we have boosted the amount of output power,” Vansteeg said. “The output power is so significant, that you will be sure to hear your Boomer -equipped mobile phone ringing in noisy environments such as crowded restaurants,” she added.

Once upon a time, all Class D amps required filters. “Now, they are designed in such a way that Class D amplifiers for portable devices don't require output filters — although very high power Class D amps (10 W to 20 W and up) still need them,” Vansteeg said.

“We optimized the architecture for low-power Class D amplifiers, allowing us to eliminate the large inductor and capacitor from each output trace, which saves four components for each channel,” Vansteeg said. “However, if the amplifier is some distance from the speaker, you may still need a small amount of inductance in the form of a ferrite bead,” she added.

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