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STA7360

Figure 29. A Classical Output Stage

Figure 30. Amplifier Block Diagram

BUILT-IN PROTECTION SYSTEMS

Short Circuit ProtectionThe maximum current the de-

vice can deliver can be calculated by considering the

voltage that may be present at the terminals of a car

radio amplifier and the minimum load impedance.

Apart from consideration concerning the area of the

power transistors it is not difficult to achieve peak cur-

rents of this magnitude (5A peak).However, it be-

comes more complicated if AC and DC short circuit

protection is also required.In particular, with a protec-

tion circuit which limits the output current following

the SOA curve of the output transistors it is possible

that in some conditions (highly reactive loads, for ex-

ample) the protection circuit may intervene during

normal operation. For this reason each amplifier has

been equipped with a protection circuit that inter-

venes when the output current exceeds 4A

Fig 16 shows the protection circuit for an NPN power

transistor (a symmetrical circuit applies to PNP). The

VBE of the power is monitored and gives out a sig-

nal,available through a cascode.

This cascode is used to avoid the intervention of the

short circuit protection when the saturation is below a

given limit.

The signal sets a flip-flop which forces the amplifier

outputs into a high impedance state.

In case of DC short circuit when the short circuit is re-

moved the flip-flop is reset and restarts the circuit (fig.

36). In case of AC short circuit or load shorted in

Bridge configuration, the device is continuously

switched in ON/OFF conditions and the current is lim-

ited.

Figure 31. Circuitry for Short Circuit Detection