9/20

STA575

Figure 2. Compressor attenuation vs. input amplitude

ABSOLUTE VALUE BLOCK

The absolute value block rectifies the signal after the compression to extract the control voltage for the external

digital converter. The output voltage swing is internally limited, the gain is internally fixed to 14.

The input impedance of the rectifier is very high , to allow the appropriate filtering of the audio signal before the

rectification (between Out_pre and Trk pins).

OUTPUT BRIDGE

The Output bridge amplifier makes the single-ended to Differential conversion of the Audio signal using two

power amplifiers, one in non-inverting configuration with gain equal to 2 and the other in inverting configuration

with unity gain. To guarantee the high input impedance at the input pins, Pwr_Inp1 and Pwr_Inp2, the second

amplifier stages are driven by the output of the first stages respectively.

POWER PROTECTION

To protect the output transistors of the power bridge a power detector is implemented (fig 3).

The current flowing in the power bridge and trough the series resistor Rsense is measured reading the voltage

drop between CD+1 and CD+. In the same time the voltage drop on the relevant power (Vds) is internally mea-

sured. These two voltages are converted in current and multiplied: the resulting current , Ipd, is proportional to

the instantaneous dissipated power on the relevant output transistor. The current Ipd is compared with the ref-

erence current Ipda, if bigger (dissipated power > 32W) a current, Iprot, is supplied to the Protection pin. The

aim of the current Iprot is to reduce the reference voltage for the digital converter supplying the power stage of

the chip, and than to reduce the dissipated power. The response time of the system must be less than 200

µSec

to have an effective protection. As further protection, when Ipd reaches an higher threshold (when the dissipated

value is higher then 60W) the chip is shut down, forcing low the Stby/Mute pin, and the turn on sequence is

restarted.

1

-24

-18

-12

-6

0

2345

6

7

8

|Vinpk|

Attenuation(dB)

D01AU1265

|Vth=1|

|Vth=2.5|

|Vth=5|