TDA7476_01 Datasheet, PDF (6/19 Pages) STMicroelectronics – CAR RADIO DIAGNOSTIC PROCESSOR

TDA7476
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:
S.C. across Load x Normal Load x
Open Load
0Ω
R₁
R₂
R₃
R₄
infinite
D01AU1256
R1
=
-^L---^s---^c--^,--^m----^a---^x-
I _{sin k}
=
-^L---^s---^c---^,-^m----^a---^x-
36.3
⋅
R_cs
R2
=
-^L---ⁿ---^o---^p---^,--^m----ⁱ-ⁿ-
I _{sin k}
=
L_{----n---o---p---,--m----i-n-}
36.3
⋅
R_cs
R3
=
L_{----n---o---p---,--m----a--x-}
I _{sin k}
=
-^L---ⁿ---^o---^p---^,--^m----^a--^x-
36.3
⋅
R_cs
R3
=
L_{----o---p---,-m----a---x-}
I _{sin k}
=
-^L---^o---^p---^,--^m-----ⁱ-ⁿ-
36.3
⋅
R_cs
where L_sc, L_nop, L_op as specified in the ELCTRICAL CHARACTERISTICS.
For example, here below are two cases, with Rcs = 3.3KOhm and Rcs = 1.8KOhm..
S.C. across Load x Normal Operation x
Open Load
0Ω
0.45Ω 2Ω
20Ω 50Ω
infinite
S.C. across Load x Normal Operation x
Open Load
0Ω
0.24Ω 1.1Ω
11Ω 27Ω
infinite
D96AU500A
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:
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