TDA7350A

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4.9 BUILT-IN PROTECTION SYSTEMS

4.9.1 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 cur-

rents of this magnitude (5A 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 4A.

Fig 37 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. 41). 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 37. Circuitry for Short Circuit Detection

4.9.2 Load Dump Voltage Surge

The TDA7350A has a circuit which enables it to withstand a voltage pulse train on pin 9, of the type shown

in fig. 39.

If the supply voltage peaks to more than 40V, then an LC filter must be inserted between the supply and

pin 9, in order to assure that the pulses at pin 9 will be held within the limits shown.

A suggested LC network is shown in fig. 38. With this network, a train of pulses with amplitude up to 120V

and width of 2ms can be applied at point A. This type of protection is ON when the supply voltage (pulse

or DC) exceeds 18V. For this reason the maximum operating supply voltage is 18V.

Figure 38.