Power Electronics

Expanded Amplifier Family Includes Class AB

National Semiconductor’s Boomer Class AB amplifiers are housed in a 0.4 mm-pitch SMD micro package. The LM4941 1.25-W audio amplifier and LM4985 stereo headphone amplifier provide low quiescent current and high output power. The 0.4 mm-pitch micro SMD package utilizes manufacturing processes for making silicon, bumping and back-grinding technologies.

The LM4941 Class AB, 1.2-W audio amplifier integrates National’s unique RF suppression circuitry. The 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-mm x 1.2-mm fully differential amp rejects the common-mode noise found in most systems and offers a low quiescent current of 1.7 mA at a 5-V supply.

With an I2C 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. The amp has a low quiescent current of 3 mA dual channel in OCL mode and 135 mW per channel of continuous average power into a 16-Ω load from a 5-V power supply.

The LM4985 may be used as either a capacitive-coupled or output capacitor-less (OCL) device for design flexibility. The LM4941 delivers 1.25 W of continuous power to an 8-Ω load with 0.04% total harmonic distortion plus noise (THD+N) from a 5-V power supply. It has a typical power supply rejection ratio (PSRR) of 95 dB at 217 Hz and a signal-to-noise ratio (SNR) of 108 dB at 1 kHz. The amp also provides a mono speaker output.

At 1% THD from a 3.6-V power supply, the LM4985 delivers 68 mW per channel of continuous average power into a 16-Ω load or 38 mW per channel into a 32-Ω load when operating in OCL mode. Its I2C compatible volume control allows 18 db to -76 dB 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 77 dB at 217 Hz in OCL mode.

Both chips work without output coupling capacitors or bootstrap capacitors and feature an internal thermal shutdown protection mechanism. Their pop & click circuitry eliminates noises which would otherwise occur during turn-on and turn-off transitions.
Available now, the LM4985 is priced at $1.46 and the LM4941 is priced at $0.62, both in 1000-unit quantities.

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