11/19

TDA7566

OUTPUT DC OFFSET DETECTION.

Any DC output offset exceeding ±2V are signalled out. This inconvenient might occur as a consequence of ini-

tially defective or aged and worn-out input capacitors feeding a DC component to the inputs, so putting the

speakers at risk of overheating.

This diagnostic has to be performed with low-level output AC signal (or Vin = 0).

The test is run with selectable time duration by microprocessor (from a "start" to a "stop" command):

START = Last reading operation or setting IB1 - D5 - (OFFSET enable) to 1

STOP = Actual reading operation

Excess offset is signalled out if persistent throughout the assigned testing time. This feature is disabled if any

overloads leading to activation of the short-circuit protection occurs in the process.

AC DIAGNOSTIC.

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

of capacitively (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 con-

ditions and lower than 250mApk should the parallel tweeter be missing. The test has to last for a minimum num-

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.

Fig. 19 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 19. Current detection: Load impedance magnitude |Z| Vs. output peak voltage of the sinus

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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