Application hints

TDA7385

12/17

Doc ID 8160 Rev 6

3.3

Standby and muting

Standby and muting facilities are both CMOS-compatible. If unused, a straight connection to

Vs of their respective pins would be admissible. Conventional low-power transistors can be

employed to drive muting and stand-by pins in absence of true CMOS ports or

microprocessors. R-C cells have always to be used in order to smooth down the transitions

for preventing any audible transient noises.

Since a DC current of about 10 µA normally flows out of pin 22, the maximum allowable

muting-series resistance (R2) is 70 k, which is sufficiently high to permit a muting capacitor

reasonably small (about 1µF).

If R2 is higher than recommended, the involved risk will be that the voltage at pin 22 may rise

to above the 1.5 V threshold voltage and the device will consequently fail to turn OFF when

the mute line is brought down.

About the stand-by, the time constant to be assigned in order to obtain a virtually pop-free

transition has to be slower than 2.5V/ms.

3.4

Diagnostics facility

The TDA7385 is equipped with a diagnostics circuitry able to detect the following events:

●

Clipping in the output stage

●

overheating (thermal shut-down proximity)

●

Output misconnections (OUT-GND and OUT-VS shorts)

Diagnostics information is available across an open collector output located at pin 25

(Figure 15) through a current sinking whenever at least one of the above events is

recognized.

Figure 15.

Diagnostics circuit

Among them, the Clipping Detector acts in a way to output a signal as soon as one or

more power transistors start being saturated.

As a result, the clipping-related signal at pin 25 takes the form of pulses, which are perfectly

synchronized with each single clipping event in the music program and reflect the same

duration time (Figure 16). Applications making use of this facility usually operate a

filtering/integration of the pulses train through passive R-C networks and realize a volume

(or tone bass) stepping down in association with microprocessor-driven audioprocessors.

The maximum load that pin 25 can sustain is 1 k

.

Due to its operating principles, the clipping detector has to be viewed mainly as a power-

dependent feature rather than frequency-dependent. This means that clipping state will be

immediately signaled out whenever a fixed power level is reached, regardless of the audio