ST25R3911B datasheet(49/116 Pages) STMICROELECTRONICS

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The on resistance of TRIM1_0 and TRIM2_0 switch transistors to be connected to LSB tuning capacitor is 50 Ω

typ. at 3 V VSP_D, the on resistance of other pins is binary weighted (the on resistance of TRIM1_3 and TRIM2_3

is 6.25 Ω typ.) The breakdown voltage of the HVNMOS switch transistors is 25 V, putting a limit to the maximum

peak to peak voltage on LC tank in case tuning is used.

During tuning procedure the resonance frequency is adjusted by connecting some of the tuning capacitors to VSS

and leaving others floating. The switches of the same binary weight are driven from the same source and are both

on or off (the switches TRIM1_2 and TRIM2_2 are for example both either on or off).

Antenna tuning can be automatically performed by sending direct command Calibrate Antenna or by an algorithm

implemented in external controller by performing phase and amplitude measurements and controlling the TRIM

switches using Antenna calibration control register.

Figure 25. Connection of tuning capacitors to the antenna LC tank

TRIM1_2

TRIM1_3

TRIM1_0

TRIM1_1

RF01

RF02

RFI1

RFI2

TRIM2_1

TRIM2_0

TRIM2_3

TRIM2_2

Antenna

coil

TRIM1_2

TRIM1_3

TRIM1_0

TRIM1_1

RF01

RF02

RFI1

RFI2

TRIM2_1

TRIM2_0

TRIM2_3

TRIM2_2

Antenna

coil

Antenna tuning using calibrate antenna direct command

In order to perform the antenna LC tank using direct command Calibrate Antenna binary weighted tuning

capacitors have to be connected between the two coil terminals to the pins TRIM1_3 to TRIM1_0 and TRIM2_3 to

TRIM2_0.

During automatic procedure, started by sending the direct command Calibrate Antenna, the ST25R3911B finds

the position of TRIM switches where the phase difference between the RFO output signal and RFI input signal is

as close as possible to the target phase defined in the Antenna calibration target register.

In case the antenna LC tank is directly connected to RFO pins (see Figure 25, where the cases of single and

differential driving are reported, respectively on the left and on the right) there is 90° phase shift between signal

on the RFO outputs and the voltage on the RFI inputs when antenna LC tank is in resonance. In case additional

EMC filter is inserted between RFO outputs and antenna LC tank the phase shift in case of resonance depends

on additional phase shift generated by EMC filter.

During execution of the direct command Calibrate Antenna the ST25R3911B runs several phase measurements

and changes configuration of TRIMx_y pins in order to find the best possible setting. Due to this the format of the

Antenna calibration target register is the same as the format of direct command Measure Phase result.

The TRIMx_y pin configuration that is the result of the direct command Calibrate Antenna can be observed by

reading the Antenna calibration target register. This register also contains an error flag that is set in case the

tuning to target phase was not possible.

After the execution of direct command Calibrate Antenna the actual phase can be checked by sending direct

command Measure Phase.

ST25R3911B

Application information

DS11793 - Rev 6

page 49/116

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