Functional description ST7540
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The Current control loop acts to limit the maximum Peak Output current inside
PA _ OU T.
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 500mrms),
is set by means of an external resistor (R
CL
) connected between CL and V
SS
. 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) < CL
TH
- CL
HYST
Voltage Control Loop Acting
–If CL
TH
- CL
HYST
< V(CL) < CL
TH
+ CL
HYST
No Gain Change
–If V(CL) > CL
TH
+ CL
HYST
The next gain level is decreased by 1 step
Figure 17 shows the typical connection of Current anVoltage control loops.
Figure 17. Voltage and current feedback external interconnection example
Voltage Control Loop Formula
R1
ALC
VOLTAGE
LOOP
CURRENT
LOOP
Vsense
CL
PA_OUT/TX_OUT
Vout
PK
Vsense
HYST
Vsense
TH
1.865V (Typ)
VCL
HYST
VCL
TH
AVss
10nF
R2
RCL
Vout
80pF typ.
D03IN1421
V
OUTPK
R
1
R
2
+
R
2
--------------------
Vsense
TH
Vsense
HYST
±()⋅≅
