STA7360

Built-in protection systems

Doc ID 9814 Rev 4

21/28

5

Built-in protection systems

5.1

Short-circuit protection

The maximum current the device 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 currents of this magnitude (5 A peak).However, it becomes more complicated

if AC and DC short-circuit protection is also required. In particular, with a protection 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 example) the protection circuit may

intervene during normal operation. For this reason each amplifier has been equipped with a

protection circuit that intervenes when the output current exceeds 4 A.

Figure 33 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 signal, 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 removed, the flip-flop is reset and

restarts the circuit (Figure 35). 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

limited.

Figure 33.

Circuitry for short-circuit detection

5.2

Polarity inversion

High current (up to 10 A) can be handled by the device with no damage for a longer period

than the blow-out time of a quick 2 A fuse (normally connected in series with the supply).

This features is added to avoid destruction, if during fitting to the car, a mistake on the

connection of the supply is made.

Obsolete

Product(s)

- Obsolete

Product(s)