STA540

Practical information

Doc ID 13907 Rev 6

19/25

6.7

Built–in protection

6.7.1

Diagnostic facilities (pin 10)

The STA540 is equipped with diagnostic circuitry that is able to detect the following events:

●

clipping of the output signal,

●

thermal shutdown,

●

output fault:

–

short circuit to GND,

–

short circuit to VS,

–

soft short circuit at turn-on.

The event is signalled when the open collector output of pin 10 begins to sink current.

6.7.2

Short-circuit protection

Reliable and safe operation in the presence of all kinds of output short circuit is assured by

the built-in protection. As well as the AC/DC short circuit to GND and to VS, and across the

speaker, there is a soft short-circuit condition, which is signalled on pin 10 (DIAGNOSTICD)

during the turn-on phase, to verify output circuit integrity in order to ensure correct amplifier

operation.

This particular kind of protection acts in such a way as to prevent the device being turned on

(via pin ST-BY) when a resistive path (that is a DC path) less than 16

Ω exists between the

output and GND. This would avoid loud speaker damage should, for example, the output

coupling capacitor develop an internal short circuit.

As mentioned previously, it is important to limit the external current driving pin ST-BY

to 5 mA. The reason is that the associated circuitry is normally disabled with currents

greater than 5 mA.

The soft short-circuit protection is particularly attractive when, in the single-ended

configuration, one capacitor is shared between two outputs (see Figure 23).

Figure 23.

Shared capacitor in single-ended configuration