TDA7718B Datasheet, PDF (24/40 Pages) STMicroelectronics – 3 band car audio processor

Description of the audioprocessor
TDA7718B
4.10
DC offset detector
Using the DC offset detection circuit (Figure 18) an offset voltage difference between the
audio power amplifier and the APR's Front and Rear outputs can be detected, preventing
serious damage to the loudspeakers. The circuit compares whether the signal crosses the
zero level inside the audio power at the same time as in the speaker cell. The output of the
zero-window-comparator of the power amplifier must be connected with the WinIn-input of
the APR. The WinIn-input has an 50k internal pull-up resistor connected to 3.3 V. It is
recommended to drive this pin with open-collector outputs only.
To compensate for errors at low frequencies the WinTC-pin are implemented, with external
capacitors introducing the same delay  = 7.5 k * C_ext as the AC-coupling between the
APR and the power amplifier introduces. For the zero window comparators, the time
constant for spike rejection as well as the threshold are programmable.
For electrical characteristics see Chapter 3 on page 9.
A low-active DC-offset error signal appears at the DCErr output if the next conditions are
both true:
a) Front and rear outputs are inside zero crossing windows.
b) The Input voltage VWinIn is logic low whenever at least one output of the power
amplifier is outside the zero crossing windows.
After power-on, the external attached capacitor is rapidly charged (fast-charge) to overcome
a false indication.
Figure 18. DC offset detection circuit (simplified)
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