STA550

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FUNCTIONAL DESCRIPTION

The circuit contains all the blocks to build a stereo amplifier. Each single channel is based on the Output Bridge

Power Amplifier, and its protection circuit. Moreover, the compression function and a signal rectifier are added

to complete the circuit.

The operation modes are driven by The Turn-on/off sequence block. In fact the IC can be set in three states by

the Stby/mute pin:

In the Standby mode all the circuits involved in the signal path are in off condition, instead

in Mute mode the circuits are biased but the Speakers Outputs are forced to ground potential.

These voltages can be get by the external RC network connected to Stby/Mute pin.

The same block is used to force quickly the I.C. In standby mode or in mute mode when the I.C. dangerous

condition has been detected. The RC network in these cases is used to delay the Normal operation restore.

The protection of the I.C. are implemented by the Over Temperature, Unbalance Ground, Output Short circuit,

Under voltage, and output transistor Power sensing as shown in the following table:

Table 1. Protection Implementation

See the POWER PROTECTION paragraph for the details

Compression

An other important function implemented, to avoid high power dissipation and clipping distortion, is the Com-

pression of the signal input. In fact the preamplifier stage performs a voltage gain equal to 5, fixed by Ri and Rr

external resistor, but in case of high input signal or low power supply voltage, its gain could be reduced of 26dB.

This function is obtained with a feedback type compressor that , in practice, reduces the impedance of the ex-

ternal feedback network. The behavior of compression it's internally fixed but depends from the Audio input volt-

age signal level, and from the Threshold voltage applied to the Threshold pin. The attack and release time are

programmable by the external RC network connected to the Att_Rel pins.

The constraints of the circuit in the typical application are the following:

Vthreshold range

= -5 to 0

Vin peak max

= 8V

Vout peak max

= 10V

Fault Type

Condition

Protection strategy

Action time

Release time

Chip Over

temperature

Tj > 130 °C

Mute

Fast

Slow Related to

Turn_on sequence

Chip Over

temperature

Tj > 150 °C

Standby

Fast

Slow, Related to

Turn_on sequence

Unbalancing

Ground

|Vgnd| > ((CD+) -

(CD-))/2 + 5V

Standby

Fast

Slow, Related to

Turn_on sequence

Short circuit

Iout > 6A

Standby

Fast

Slow, related to

Turn_on sequence

Under Voltage

|Vs+| + |Vs-|< 20V

Standby

Fast

Slow, related to

Turn_on sequence

Extra power

dissipation

at output transistor

Pd tr. >25W

Reducing DIGITAL

CONVERTER output

voltage.

Related to the

DIGITAL

CONVERTER

Related to the

DIGITAL

CONVERTER

Maximum power

dissipation

at output transistor

Pd tr. > 48W

Standby

Fast

Slow, related to

Turn_on sequence