TDA7801PD datasheet(26/48 Pages) STMICROELECTRONICS

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Functional description

TDA7801

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DocID022674 Rev 4

channel(s) concerned are shut down. The diagnostic performs restart cycles every

1 ms until a fault condition is present. The amplifier restarts in play only once the

overload is removed.

DIagnostic mode, (D6-IB0='1'). It is enabled via I2C bus and self activates if an output

overload (so as to cause the intervention of the short-circuit protection) occurs to the

speakers outputs. Once activated, the diagnostics procedure develops as below:

–

The diagnostic performs 1 restart cycle in order to avoid momentary re-circulation

spikes which could give erroneous diagnostic results. If normal situation (no

overloads) is detected the channel returns active.

–

Instead, if after 1 restart cycle an overload is detected then the diagnostic circuit

performs a plateau cycle.

–

After the plateau cycle, the fault audio channel generates restart cycles every

1 ms until a fault condition is present. The relevant data acquired during plateau

cycle are stored and can be read by the microprocessor. A new plateau cycle can

be activated by an I2C reading only if the fault condition persists. This is to ensure

continuous diagnostics throughout the car-radio operating time.

The diagnostic performs another plateau cycle after each I2C bus registers read if the

diagnostic bit is activated, (IB0-d7="1"). If I2C read is performed during the diagnostic

permanent plateau cycle then, once the short is removed, the amplifier waits for another I2C

read before starting to play again, in order to communicate the diagnostic plateau cycle

results.

6.4

AC diagnostic

It is targeted at detecting accidental disconnection of tweeters in 2-way speaker and, more

in general, presence of capacitive (AC) coupled loads. This diagnostic is based on the

notion that the overall speaker's impedance (woofer + parallel tweeter) will tend to increase

towards high frequencies if the tweeter gets disconnected, because the remaining speaker

(woofer) would be out of its operating range (high impedance).

To determine the load impedance, a sinewave tone at a suitable (F > 10 kHz or even

ultrasonic) frequency should be fed to the output pins. Depending on the test's result the AC

diagnostic is able to determine if the tweeter is connected or not. The tweeter is not

connected if for four consecutive sine wave the current threshold on the load is not res-

passed.

AC diagnostic is managed by I2C commands. The AC diagnostic is enabled by bit D7 of

test are stored in the data register DB4 (bits D7:D4) which are set to logic level '1' if a

tweeter is detected on the correspondent channel.

Note, the results on D7:D4 are valid only if the bit D3 is zero; in order to avoid this condition

the pulse generated has to respect the condition Vout_peak < Vbattery-4V.

6.5

Output DC offset detection

Any DC output offset exceeding ± VO are signalled out. This inconvenient might occur as a

consequence of improper DC input signal. The offset detection is performed at the end of

the digital chain by a low pass digital filter. The offset detection is permanent, and also works

in play mode. The results of the DC offset detection are obtained from the right pin for each

channel (see I2C table in Section 9); the bits are continuously refreshed.

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