WM9090 Datasheet, PDF (29/81 Pages) Wolfson Microelectronics plc – Ultra Low Power Audio Subsystem

Production Data
WM9090
AGC POWER LIMITING
The second mechanism used by the AGC to monitor signal conditions is the power limit function.
The speaker output voltage is measured, and the corresponding power output is determined. The
power limiting function can be disabled using the AGC_PWR_ENA bit. It is enabled by default.
The power output threshold at which the AGC will apply attenuation is set using the AGC_PWR_THR
register. Power levels in the range 300mW and 1050mW can be selected. Note that these are RMS
power levels, assuming an 8Ω speaker.
The power output threshold is also controlled by the AGC_PWR_AVG register. When
AGC_PWR_AVG = 1, then the AGC responds to the RMS power level as quoted above. When
AGC_PWR_AVG = 0, then the AGC responds to the instantaneous voltage at the speaker output.
Selecting the RMS power level is recommended, as this represents the average signal level.
When the AGC applies signal attenuation triggered by the power limit threshold, the signal gain is
reduced at a rate that is set by the AGC_PWR_ATK register. When the power limit threshold is no
longer met (due to the signal level reduction), then the AGC increases the signal gain at a rate set by
the AGC_PWR_DCY register.
Note that, when the anti-clip and power limiting thresholds are both triggered concurrently, then the
signal gain is reduced at the rate set by the AGC_CLIP_ATK register and is increased at the rate set
by AGC_PWR_DCY. These fields are defined in Table 13 and Table 14 respectively.
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PD, November 2010, Rev 4.1
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