TDA7572 Datasheet, PDF (38/64 Pages) STMicroelectronics – 200W mono bridge PWM amplifier with built-in step-up converter

Input stage and gain compressor
TDA7572
6.2.1
is clipping during that period. The THD estimate is calculated from the ratio between these
times. Clipping means are any of the following conditions occurred: maximum modulation
reached, output current limiting active, or voltage clipping at the AOUT pin. These are used
to estimate THD, which is then mapped to a desired number of discrete steps of gain
reduction. Attenuation is then changed at the next zero crossing of the signal at the Input
Stage block
The attack time sets the minimum time allowed between gain reductions. At low frequency
signals, where the time between zero crossings is greater than the attack time, the attack
rate is dictated by the signal frequency, rather than this setting. Similarly, the decay time sets
the minimum time allowed between gain increases, with the same caveats about rate
dictated by the signal frequency.
The major tuning control here is the distortion-to-attenuation lookup table. It will determine
how aggressively to operate and thus the relative amount of audible artifact. Decay time
adjustment can be varied for audible effect and to mange average power.
Following are reported the correspondence between I²C bus registers and coefficients for
Attack and decay time. The first table reports the one for compressor setting:
Setting in I²C bus mode
GainTable[1:0]: Selects the distortion versus gain step size table to be used, including the
ability to disable the gain compressor.
Table 28. Distortion versus gain step size
GainTable [1:0] Pseudo THD,% / T2/T1 ratio
Number of gain steps
00
Gain compressor disabled
0.02
1
01
3.0
2
0.02
1
10
3.0
2
5.0
3
0.02
1
3.0
2
11
5.0
3
15.0
4
RELEASE[1:0]: Sets the maximum release rate of the gain compressor according to the
table below:
Table 29. Sets the maximum release rate of the gain compressor
Release [1:0]
Clock counts
Nominal time at 400KHz clock
00
2¹³
01
2¹⁴
10
2¹⁵
11
2¹⁶
20.48ms
40.96ms
81.92ms
163.4ms
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