ST7570 datasheet(15/26 Pages) STMICROELECTRONICS

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ST7570

Analog front end (AFE)

Doc ID 17526 Rev 2

15/26

5.4

Current and voltage control

The power amplifier output current sensing is performed by mirroring a fraction of the output

current and making it flow through a resistor RCL connected between the CL pin and VSS.

The following relationship can be established between V(CL) and I(PA_OUT):

Equation 2 V(CL) vs. I(PA_OUT)

The voltage level V(CL) is compared with the internal threshold CL_TH. When the V(CL)

exceeds the CL_TH level, the V(TX_OUT) voltage is decreased by one TX_GAIN step at a

time until V(CL) goes below the CL_TH threshold.

The current sense circuit is depicted in Figure 6.

Figure 7.

PA_OUT current sense circuit

The RCL value to get the desired output current limit I(PA_OUT)LIM can be calculated

according to Equation 3:

Equation 3 RCL calculation

Note that I(PA_OUT)LIM is expressed as peak current, so the corresponding rms current

value shall be calculated according to the transmitted signal waveform.

The RCL value to get 1 A rms output current limit, calculated with typical values for CL_TH

and CL_RATIO parameters, is indicated in Table 7.

Table 7.

CL resistor typical values

Parameter

Description

Value

Unit

RCL

Resistor value for I(PA_OUT) MAX = 1 A rms (1.41 A peak)

133

VCL

()

I PA_OUT

()

⋅

CL_RATIO

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

CL_TH

I(PA_OUT)

LIM

CL_RATIO

⁄

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

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