Low-Power Class D Audio Subsystems Target Portable Devices
National Semiconductor has added two devices to its family of low-power Boomer Class D audio subsystems, the LM49352 mixed-signal audio subsystem and LM49151 analog subsytem, which target smart phones, full-featured phones, portable gaming devices, and portable GPS devices (see the figure).
The LM49352 integrates audio codec functionality with ground-referenced stereo headphone amplifiers, an earpiece driver, a low-EMI (electromagnetic interference) Class D loudspeaker, and an audio DSP in a 36-bump, 3.3- by 3.3-mm micro SMDxt package. The loudspeaker delivers 93% efficiency at 970 mW from a 4.2-V supply, while the headphone amplifier offers 18-mW MP3 playback to extend battery life.
Also, the subsystemís stereo digital-to-analog converter (DAC) provides a signal-to-noise ratio (SNR) of 103 dB with a 24-bit, 48-kHz signal. Its stereo analog-to-digital converter (ADC) provides a 97-dB SNR with a 24-bit, 48-kHz signal. The loudspeaker amplifier delivers up to 970 mW to an 8-Ω load with less than 1% harmonic distortion plus noise (THD+N) at 4.2 V.
The LM49352ís digital signal processing includes dedicated effects circuits for both the DACs and ADCs. These circuits include volume control, automatic level control (ALC) with compression and limiting, and five-band equalization (EQ) for quality audio processing. The ADC also is equipped with a flexible wind noise filter that can operate in voice or high fidelity recording mode. The LM49352ís dual bi-directional I2S interface and PCM audio bridge enable easy interface with peripherals such as Bluetooth modules, as well as an I2C-compatible interface for programming different gains and nodes.
The LM49151 integrates a Class AB earpiece driver, a mono enhanced emission suppression (E2S) Class D loudspeaker amplifier, stereo ground-referenced headphone amplifiers, and ALC in a 2.2- by 2.6-mm, 20-bump micro SMD package. It offers the lowest power in its class, according to the company, consuming 7.3 mA of quiescent current at 3.3 V for the loudspeaker and headphone. Its ALC gives handset manufacturers a variety of options to control audio levels and prevent speaker damage, National says.
Also, the LM49151 includes volume control and input mixer/multiplexer and speaker protection. The Class D amplifierís patented, ultra-low EMI pulse-width modulation (PWM) architecture significantly reduces RF emissions while preserving audio quality, National says. Its ALC prevents distortion as battery voltages drop by allowing small signals to be amplified at high gains, while preventing clipping of high-level signals. In addition, the LM49151ís clip-level select ALC produces high output power at a controlled distortion level, while the power-level select ALC provides speaker protection.
Available now, the LM49352 costs $7.55 each and the LM49151 costs $1.60 each, both in 1000-unit quantities.
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