TDA7350 datasheet(14/22 Pages) STMICROELECTRONICS

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BUILT-IN PROTECTION SYSTEMS

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 (5A peak).

However, it becomes more complicated if AC and

DC short circuit protection is also required.In par-

ticular,with a protection circuit which limits the

output current following the SOA curve of the out-

put transistors it is possible that in some condi-

tions (highly reactive loads, for example) the pro-

tection 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 34 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 (fig. 38). In case of AC short circuit or load

shorted in Bridge configuration, the device is con-

tinuously switched in ON/OFF conditions and the

current is limited.

Figure 33: Amplifier Block Diagram

Figure 32: A Classical Output Stage

Figure 34: Circuitry for Short Circuit Detection

TDA7350A

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