TDA7476

6/19

s

Isink and Isource are depending on the external resistor Rcs. The minimum allowed value for Rcs is

1.65KOhm. The relationship among Isink, Isource and Rcs is the following:

Isink = (3.3/Rcs) · 11

Isource = 1.5 · Isink

s

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 (S.C.), normal

operation and open load are detected as following:

For example, here below are two cases, with Rcs = 3.3KOhm and Rcs = 1.8KOhm..

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

used.

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 volt-

age 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:

S.C. across Load

x

Open Load

0

Ω

infinite

x

Normal Load

D01AU1256

R1

L

sc,max

I

k

sin

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

L

sc ,max

36.3

------------------- R

cs

⋅

==

R2

L

no p,min

I

k

sin

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

L

nop ,min

36.3

---------------------- R

cs

⋅

==

R3

L

no p,max

I

k

sin

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

L

no p,max

36.3

----------------------- R

cs

⋅

==

R3

L

op ,max

I

k

sin

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

L

op min

,

36.3

-------------------- R

cs

⋅

==

S.C. across Load

x

Open Load

0

Ω

1.1

Ω

11

Ω

infinite

x

Normal Operation

0.24

Ω

27

Ω

D96AU500A

S.C. across Load

x

Open Load

0

Ω

2

Ω

20

Ω

infinite

x

Normal Operation

0.45

Ω

50

Ω