Diagnostics functional description TDA7575B
26/32 Doc ID 14103 Rev 3
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 > 500 mApk = normal status
– Iout < 250 mApk = open tweeter
● Low current threshold IB2 (D7) = 1
– Iout > 250 mApk = normal status
– Iout < 125 mApk = 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 250 mApk with IB2(D7)=0 (lower than 125 mApk 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
AC diagnostic function IB2<D2>) up to the I
2
C reading of the results (measuring period). To
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
are recommended for their negligible acoustic impact and also to maximize the impedance
module's ratio between with tweeter-on and tweeter-off.
