TDA7476 Datasheet, PDF (6/15 Pages) STMicroelectronics – CAR RADIO DIAGNOSTIC PROCESSOR

TDA7476
Turn-on diagnostic - AUX1, AUX2
To detect if there is a short circuit to GND or an open load involving to the AUX output of the car radio,
the voltage across a sensing resistor Rsens is detected.
These output voltages (for example for the active antenna and for the booster) are usually generated
by high side drivers, but also voltage regulators with Vout > 5V are admissible.
The detection ranges can be set by adjusting the sensing resistors Rsens1 or Rsens2.
For example, if Rsens = 5Ohm, the following detection table will be operative (I = Vthr/Rsens):
S.C. to GND x Normal Operation x Open Load
150mA 100mA
25mA 17mA
D96AU503
Permanent diagnostic - CH1, CH2, CH3, CH4, CH5 - Short to GND and Vs
During the CAR-RADIO normal operation, to detect a short circuit to GND (or to Vs), the output volt-
ages are sensed. If one or more outputs stay at any voltage below 0.8V or over Vs-0.8V for more than
2 sec. (typ), the warning pin is pulled down.
The µP can address the TDA7476 to know the status.
The subsonic current pulse is activated also in this case. The fault is correctly detected if it remains
until the memorization (tm in fig.1).
In this phase, faults regarding shorts across the load and open loads cannot be detected.
For single-ended devices as in fig. 5, 6 a short citcuit to ground is detected both when a short to
ground is really present and when the load is missing.
Permanent diagnostic - AUX1, AUX2 - Short circuit to GND and open load
The detection mode of the auxiliary inputs is equal to what is in place during turn-on, but the fault
must be lasting without interruption for more than 2 seconds.
The warning pin is pulled down only in case of short to GND. This is to avoid that this pin remains per-
manently to 0 level if one or both AUX outputs of the car radio are unused.
Timing
From the byte ”ADD1” to the byte ”ADD2” the µP must wait a period Twait1(see Software Specifica-
tion) that is depending on the timing capacitor CT according to the following table (Twait1> tm, max):
CT (µF)
3.3
4.7
10
22
47
100
tm max (ms)
45
65
130
290
620
1300
Note: any (positive) spread of the capacitor value must be added. The relation to be used to determine
Twait1 from the value of CT according to the following:
Twait1 > tm, max = (130 x CT/10µF)ms
After the byte ”ADD2” the power amplifier can be switched-on. In some cases, the µP has to wait until
the current pulse is finished (Twait2). This time (ts in fig.1) is given by:
Twait2 > ts, max = (140 x CT/10µF)ms
For bridge or bridge equivalent devices (figg.7,8), Cs = 10µF will be enough.
For the TDA7375, connected as in fig.5, Cs = 47µF if Cout = 1000µF, and Cs = 100µF if Cout = 2200µF.
If the circuit is as shown by fig. 6, the suggested values of Cs are as follows: Cs = 22µF if Cout =
1000µF, Cs = 47µF if Cout = 2200µF.
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