On bridge (or bridge equivalent) devices if there is no short circuit to GND or to Vs, Isource goes into

saturation mode (for Vout > 3V), and in the load flows Isink. As the turn-on diagnostic thresholds are

fixed, it is possible to calculate the ranges of loudspeaker resistance in which short circuit, normal op-

eration and open load are detected.

S.C. across Load

x

Open Load

0

Ω

1.1

Ω

11

Ω

infinite

x

Normal Operation

0.27

Ω

27

Ω

D96AU500

S.C. across Load

x

Open Load

0

Ω

2

Ω

20

Ω

infinite

x

Normal Operation

0.5

Ω

50

Ω

The exact values of the above mentioned resistive ranges may vary a little, depending on the power

amplifier used. These values for the various possible ST power amplifiers will be communicated later.

When single-ended devices are used and the application circuit is as shown in fig. 5,6, it is necessary

to use:

over 1V;

In this case, the loudspeaker resistance ranges in which short circuit, normal operation and open load

S.C. across Load

x

Open Load

0

Ω

1.9

Ω

19

Ω

infinite

x

Normal Operation

0.47

Ω

47

Ω

D96AU501

S.C. across Load

x

Open Load

0

Ω

1.1

Ω

11

Ω

infinite

x

Normal Operation

0.27

Ω

27

Ω

The exact values of the above mentioned resistive ranges may vary a little, depending on the power

amplifier used. These values will be communicated later.

Turn-on diagnostic - CH1, CH2, CH3, CH4, CH5 - Short to GND and Vs.

To detect if there is short circuit to GND or Vs, the subsonic current pulse is exploited. The information

related to the status of the outputs are measured and memorized at the top of the current pulse (tm in

fig.1). If no faults are present, the pins connected to the audio outputs (CH1,..CH5) will reach about 3V.

If one or more outputs are shorted to GND, these voltages become lower than 3V.

If one or more outputs are shorted to Vs, the output voltage increases over 3V.

The fault status can be know by sensing the output voltages. The reason way voltage threshold has

been preferred instead of a current threshold to declare short circuit resistor ranges is two fold:

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

quently 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.

Rcs = 3.3k

Ω

Rcs = 1.8k

Ω

Rcs = 4.7k

Ω

Rcs = 2.7k

Ω

(*) Vthr is the threshold described in the table on page 3/14 - 4/14 (for example Pgnd-min, Pvs - max, Pnop - min, Pnop - max etc..)

TDA7476

5/15