ST7538Q datasheet(27/44 Pages) STMICROELECTRONICS

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ST7538Q

Functional description

27/44

5.8.1

Automatic Level Control (ALC)

The Automatic Level Control Block (ALC) is a variable gain amplifier (with 32 non linear

discrete steps) controlled by two analog feed backs acting at the same time. The ALC gain

range is 0dB to 30 dB and the gain change is clocked at 5KHz. Each step increases or

reduces the voltage of 1dB (Typ).

Two are the control loops acting to define the ALC gain:

●

The Voltage control loop acts to keep the Peak-to-Peak Voltage constant on Vsense.

The gain adjustment is related to the result of a peak detection between the Voltage

waveform on Vsense and two internal Voltage references. It is possible to protect the

Voltage Control Loop against noise by freezing the output level (see Section 6.6:

Output voltage level freeze on page 38).

If Vsense < VCLTH - VCLHYST

The next gain level is increased by 1 step

If VCLTH - VCLHYST < Vsense < VCLTH + VCLHYST

No Gain Change

If Vsense > VCLTH + VLCHYST

The next gain level is decreased by 1 step

●

The Current control loop acts to limit the maximum Peak Output current inside

ATOP1 and ATOP2.

The current control loop acts through the voltage control loop decreasing the Output

Peak-to-Peak Amplitude to reduce the Current inside the Power Line Interface.

The current sensing is done by mirroring the current in the High side MOS of the Power

Amplifier (not dissipating current Sensing). The Output Current Limit (up to

400mApeak), is set by means of an external resistor (RCL) connected between CL and

PAVss. The resistor converts the current sensed into a voltage signal. The Peak current

sensing block works as the Output Voltage sensing Block:

If V(CL) < CCLTH - CCLHYST

Voltage Control Loop Acting

If CCLTH - CCLHYST < V(CL) < CCLTH + CCLHYST

No Gain Change

If V(CL) > CCLTH + CLCHYST

The next gain level is decreased by 1 step

Figure 14 shows the typical connection of Current an Voltage control loops.

Figure 14. Voltage and current feedback external interconnection example

ALC

VOLTAGE

LOOP

CURRENT

LOOP

Vsense

ATOP/ATO

VRPK

VCLHYST

VCLTH

1.865V (Typ)

CCLHYST

CCLTH

AVss

5.6nF

RCL

80 pF

Vout

D03IN1421

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