Functional description TDA7375AV
14/20 Doc ID 6325 Rev 5
4.5.1 Rail-to-rail output voltage swing with no need of bootstrap capacitors
The output swing is limited only by the V
CEsat
of the output transistors, which is in the range
of 0.3 (R
sat
) each. Classical solutions adopting composite PNP-NPN for the upper output
stage have higher saturation loss on the top side of the waveform.
This unbalanced saturation causes a significant power reduction. The only way to recover
power consists of the addition of expensive bootstrap capacitors.
4.5.2 Absolute stability without any external compensation
Referring to the circuit of Figure 22 the gain V
Out
/V
In
is greater than unity, approximately
1+R2/R1. The DC output (V
CC
/2) is fixed by an auxiliary amplifier common to all the
channels.
By controlling the amount of this local feedback it is possible to force the loop gain (A*) to
less than unity at frequency for which the phase shift is 180°. This means that the output
buffer is intrinsically stable and not prone to oscillation.
Most remarkably, the above feature has been achieved in spite of the very low closed loop
gain of the amplifier. In contrast, with the classical PNP-NPN stage, the solution adopted for
reducing the gain at high frequencies makes use of external RC networks, namely the
Boucherot cells.
4.6 Built–in short circuit protection
Figure 22. The new output stage
Reliable and safe operation, in presence of all kinds of short circuit involving the outputs is
assured by BUILT-IN protectors. Additionally to the AC/DC short circuit to GND, to V
S
,
across the speaker, a SOFT SHORT condition is signalled out during the TURN-ON PHASE
so assuring correct operation for the device itself and for the loudspeaker.
This particular kind of protection acts in a way to avoid that the device is turned on (by
Standby) when a resistive path (less than 16 ohms) is present between the output and GND.
As the involved circuitry is normally disabled when a current higher than 5 mA is flowing into
the ST-BY pin, it is important, in order not to disable it, to have the external current source
driving the ST-BY pin limited to 5 mA.
This extra function becomes particularly attractive when, in the single ended configuration,
one capacitor is shared between two outputs (see Figure 23). Supposing that the output
