TDA7405 datasheet(11/56 Pages) STMICROELECTRONICS

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TDA7405

2.1.3 Mono-differential Input 1 (MD1)

The MD1-input is designed as a basic differential stage with 56k

Ω input-impedance. This input is configurable

as a single-ended stereo input (see pin-out).

2.1.4 Mono-differential Input 2 (MD2)

The MD2-input has the same topology as MD1, but without the possibility to configure it to single ended.

2.1.5 Single-ended stereo Input (SE1), single-ended mono input (AM) and FM-MPX input

All single ended inputs offer an input impedance of 100kW. The AM-pin can be connected by software to the

input of the stereodecoder in order to use the AM-Noiseblanker and AM-High-Cut feature.

2.2 AutoZero

The AutoZero allows a reduction of the number of pins as well as external components by canceling any offset

generated by or before the In-Gain-stage (Please notice that externally generated offsets, e.g. generated

through the leakage current of the coupling capacitors, are not canceled).

The auto-zeroing is started every time the DATA-BYTE 0 is selected and needs max. 0.3ms for the alignment.

To avoid audible clicks the Audioprocessor is muted before the loudness stage during this time. The AutoZero-

feature is only present in the main signal-path.

2.2.1 AutoZero for Stereodecoder-Selection

A special procedure is recommended for selecting the stereodecoder at the main input-selector to guarantee

an optimum offset-cancellation:

(Step 0: SoftMute or Mute the signal-path)

Step 1: Temporary deselect the stereodecoder at all input-selectors

Step 2: Configure the stereodecoder via IIC-Bus

Step 3: Wait 1ms

Step 4: Select the stereodecoder at the main input-selector first

The root cause of this procedure is, that after muting the stereodecoder (Step 1), the internal stereodecoder

filters have to settle in order to perform a proper offset-cancellation.

2.2.2 AutoZero-Remain

In some cases, for example if the µP is executing a refresh cycle of the IIC-Bus-programming, it is not useful

to start a new AutoZero-action because no new source is selected and an undesired mute would appear at the

outputs. For such applications the A631 could be switched in the AutoZero-Remain-Mode (Bit 6 of the subad-

dress-byte). If this bit is set to high, the DATABYTE 0 could be loaded without invoking the AutoZero and the

old adjustment-value remains.

2.3 Pause Detector / MUX-Output

The pin number 40(Pause/MUX) is configurable for two different functions:

1. During Pause-Detector OFF this pin is used as a mono-output of the main input-selector. This signal is

often used to drive a level-/equalizer-display on the carradio front-panel.

2. During Pause-Detector ON the pin is used to define the time-constant of the detector by an external ca-

pacitor.

The pause-detector is driven by the internal stereodecoder-outputs in order to use pauses in the FM-signal for

alternate-frequency-jumps. If the signal-level of both stereodecoder channels is outside the programmed volt-

age-window, the external capacitor is abruptly discharged. Inside the pause-condition the capacitor is slowly re-

charged by a constant current of 25µA. The pause information is also available via IIC-Bus (see IIC-Bus

programming).

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