ST7540 datasheet(28/44 Pages) STMICROELECTRONICS

28 / 44 page

Functional description

ST7540

28/44

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

PA_OUT.

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 (RCL) connected between CL and VSS. 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) < CLTH - CLHYSTVoltage Control Loop Acting

–If CLTH - CLHYST < V(CL) < CLTH + CLHYSTNo Gain Change

–If V(CL) > CLTH + CLHYSTThe 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

ALC

VOLTAGE

LOOP

CURRENT

LOOP

Vsense

PA_OUT/TX_OUT

VoutPK

VsenseHYST

VsenseTH

1.865V (Typ)

VCLHYST

VCLTH

AVss

10nF

RCL

Vout

80pF typ.

D03IN1421

OUTPK

--------------------

Vsense

HYST

()

⋅

≅

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