Diagnostics functional description

TDA7566

16/29

Doc ID 9801 Rev 6

4.4

AC diagnostic

It is targeted at detecting accidental disconnection of tweeters in 2-way speaker and, more

in general, presence of capacitive (AC) coupled loads.

This diagnostic is based on the notion that the overall speaker's impedance (woofer +

parallel tweeter) will tend to increase towards high frequencies if the tweeter gets

disconnected, because the remaining speaker (woofer) would be out of its operating range

(high impedance). The diagnostic decision is made according to peak output current

thresholds, as follows:

Iout > 500mApk = normal status

Iout < 250mApk = open tweeter

To correctly implement this feature, it is necessary to briefly provide a signal tone (with the

amplifier in "play") whose frequency and magnitude are such to determine an output current

higher than 500mApk in normal conditions and lower than 250mApk should the parallel

tweeter be missing. The test has to last for a minimum number of 3 sine cycles starting from

the activation of the AC diagnostic function IB2<D2>) up to the I2C reading of the results

(measuring period). To confirm presence of tweeter, it is necessary to find at least 3 current

pulses over 500mA over all the measuring period, else an "open tweeter" message will be

issued.

The frequency / magnitude setting of the test tone depends on the impedance

characteristics of each specific speaker being used, with or without the tweeter connected

(to be calculated case by case). High-frequency tones (> 10 KHz) or even ultrasonic signals

are recommended for their negligible acoustic impact and also to maximize the impedance

module's ratio between with tweeter-on and tweeter-off.

Figure 24 shows the Load Impedance as a function of the peak output voltage and the

relevant diagnostic fields.

This feature is disabled if any overloads leading to activation of the short-circuit protection

occurs in the process.

Figure 24.

Current detection: load impedance magnitude |Z| vs. output peak voltage

of the sinus

1

2

34

5

6

78

1

2

3

5

10

20

30

50

Vout (Peak)

Load |z| (Ohm)

Iout (peak) <250mA

Iout (peak) >500mA

Low current detection area

(Open load)

D5 = 1 of the DBx byres

High current detection area

(Normal load)

D5 = 0 of the DBx bytes