Datasheet
ST7538Q Functional description
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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 < VCL
TH
- VCL
HYST
The next gain level is increased by 1 step
If VCL
TH
- VCL
HYST
< Vsense < VCL
TH
+ VCL
HYST
No Gain Change
If Vsense > VCL
TH
+ VLC
HYST
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 (R
CL
) 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) < CCL
TH
- CCL
HYST
Voltage Control Loop Acting
If CCL
TH
- CCL
HYST
< V(CL) < CCL
TH
+ CCL
HYST
No Gain Change
If V(CL) > CCL
TH
+ CLC
HYST
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
R1
ALC
VOLTAGE
LOOP
CURRENT
LOOP
Vsense
CL
ATOP/ATO
VR
PK
VCL
HYST
VCL
TH
1.865V (Typ)
CCL
HYST
CCL
TH
AVss
5.6nF
R2
RCL
80 pF
Vout
D03IN1421