5 General structure

STA543SA

16/23

CD00061065

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.

5.6

Absolute Stability Without Any External Compensation.

Referring to the circuit of

R1. The DC output (VCC/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*

β) 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.

Figure 9.

The new output stage

5.7

BUILT–IN Shortcircuit Protection

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 VS, across

the speaker, a SOFT SHORT condition is signalled out during the TURN-ON PHASE so

assuring correct operation for the device it self and for the loudspeaker.

This particular kind of protection acts in such a way to avoid the device is turned on (by ST-BY)

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 5mA is flowing into the ST-BY

pin, it is important, in order not to disable it, to have the external current source driving the

STBY pin limited to 5mA.