TDA7562B Datasheet, PDF (17/30 Pages) STMicroelectronics – 4 x 46 W multifunction quad power amplifier with built-in diagnostics features

TDA7562B
Diagnostics functional description
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:
I_out > 500 mApk = normal status
I_out < 250 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 500 mApk in normal conditions and lower than 250 mApk 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 > 0 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 500 mA 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
,O AD\Z\/HM

,OWCURRENTDETECTIONAREA

/PENLOAD

$OFTHE$"XBYRES
)OUTPEAK M!
)OUTPEAK M!

)"$ 
(IGHCURRENTDETECTIONAREA

.ORMALLOAD
$OFTHE$"XBYTES





6OUT 0EAK
'!0'03
Doc ID 17984 Rev 2
17/30