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TDA7476

1) The amplifier can drain current in the resistive path of the short circuit, hence this current and consequently

the short circuit resistor cannot be determined with a sufficient level of accuracy.

2) The voltage difference between the car radio ground (reference) and the position of the chassis of the car

where the loudspeaker line is connected (due to an accidental short circuit) can be up to some hundreds of

mV. This does not permit a correct measure of the short circuit resistor.

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 volt-

age 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 admis-

sible. The detection ranges can be set by adjusting the sensing resistors Rsens1 or Rsens2 as following:

For example, if Rsens = 5 Ohm, the following detection table will be operative :

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 voltages are

sensed. If one or more outputs stay at any voltage below 0.8V or over Vs-0.6V 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.

S.C. to GND

x

Open Load

x

Normal Operation

D01AU1258

I

4

A

OL,max

R

sens

----------------------

=

I

3

A

nop ,min

R

s ens

----------------------

=

I

2

A

n op,max

R

sens

------------------------

=

I

1

A

gn d,min

R

sen s

----------------------

=

Open Load

x

S.C. to GND

100mA

25mA

x

Normal Operation

170mA

17mA

D96AU503