LM49352 datasheet(61/106 Pages) TI

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DECAY

HOLD

ATTACK

Audio

Input

ALC

Output

TARGET

LM49352

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SNAS467E – MARCH 2009 – REVISED MARCH 2013

signal amplitude instantaneously, but tracks decreases in audio input signal amplitude at programmable rate

(PEAK DECAY TIME). ATTACK RATE, DECAY RATE, HOLD TIME, and PEAK DECAY TIME are fully

adjustable which allows flexible operation of the ALC circuit. The ALC’s timers are based on the sample rate of

the DAC or ADC, so the closest corresponding sample rate must be programmed into the DAC SAMPLE setting

(for DAC ALC) or the ADC SAMPLE (for ADC ALC).

Figure 68. ALC Example

Limiter

The LM49352’s ALC features a limiter function. The purpose of the limiter is to limit the maximum level of the

audio signal to the specified ALC target level. When the limiter is enabled, the ALC will decrease the gain of the

volume control whenever the audio signal is higher than the specified target level. The programmed I2C gain

setting when the limiter is first enabled is the maximum gain setting that the ALC limiter will apply to the audio

signal. Gain increases beyond the original I2C gain setting are disabled. This is in contrast to ALC operation with

the limiter disabled, where the ALC may increase gain of audio signals below target level using gain settings

beyond the original I2C gain setting. Therefore, it is important to set the gain of the audio path to the desired

setting before enabling the ALC limiter function.

The limiter’s target level can be set just below the clipping level of the output amplifier or ADC in order to prevent

harsh distortions delivered to the loudspeaker or headphone on the receiving end. This method of ALC limiter

operation is also known as “no clip” mode. Operating the ALC limiter in “no clip” mode maximizes the dynamic

range of the audio amplifier or ADC while ensuring that the audio signal will never clip. Utilizing the ALC limiter in

“no clip” mode also protects the loudspeaker from damage due to harmful overdriven conditions.

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LM49352 数据表(PDF) 61 Page - Texas Instruments