TPA2018D1 Datasheet, PDF (17/36 Pages) Texas Instruments – 3-W Mono Class-D Audio Amplifier with SmartGain AGC/DRC

TPA2018D1
www.ti.com ................................................................................................................................................................................................ SLOS592 – AUGUST 2009
Large Limiter
Small
Range
Compression
Region
Rotation
Point @
higher gain
Rotation
Point @
lower gain
Small Limiter
Range
Large
Compression
1:1
Region
V_IN - dBV
Figure 33. Output Signal vs Input Signal State Diagram Showing the Effects of the Limiter Range to the
Compression Region
Noise Gate Threshold: The noise gate threshold prevents the AGC from changing the gain when there is no
audio at the input of the amplifier. The noise gate threshold stops gain changes until the input signal is above the
noise gate threshold. Select the noise gate threshold to be above the noise but below the minimum audio at the
input of the amplifier signal. A filter is needed between delta-sigma CODEC/DAC and TPA2018D1 for
effectiveness of the noise gate function. The filter eliminates the out-of-band noise from delta-sigma modulation
and keeps the CODEC/DAC output noise lower than the noise gate threshold.
Noise Gate Threshold
No
Audio
Gain does not change
in this region
time
time
Figure 34. Time Diagram Showing the Relationship Between Input Signal Amplitude, Noise Gate
Threshold and Gain Versus Time
Maximum Gain: This variable limits the number of gain steps in the AGC. This feature is useful in order to
accomplish a more advanced output signal vs. input signal transfer characteristic.
For example, to prevent the gain from going above a certain value, reduce the maximum gain.
However, this variable will affect the limiter range and the compression region. If the maximum gain is
decreased, the limiter range and/or compression region is reduced. Figure 35 illustrates the effects.
Copyright © 2009, Texas Instruments Incorporated
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