Diagnostics functional description

TDA7575B

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7.3

Output DC offset detection

Any DC output offset exceeding +/- 2 V are signalled out. This inconvenient might occur as a

consequence of initially 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.

7.4

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, and it is enabled by setting (IB2-D2) = 1. Two different detection levels are

available:

–

HIGH CURRENT THRESHOLD IB2 (D7) = 0

Iout > 500mApk = NORMAL STATUS

Iout < 250mApk = OPEN TWEETER

–

LOW CURRENT THRESHOLD IB2 (D7) = 1

Iout > 250mApk = NORMAL STATUS

Iout < 125mApk = 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 with IB2(D7)=0 (higher than 250mApk with IB2(D7)=1) in normal

conditions and lower than 250mApk with IB2(D7)=0 (lower than 125mApk with IB2(D7)=1)

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

confirm presence of tweeter, it is necessary to find at least 3 current pulses over the above

threholds 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