ST25R3910 datasheet(33/65 Pages) STMICROELECTRONICS

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ST25R3910

Functional overview

opposite case, when the resonance frequency is lower than target, the result of A/D

conversion is lower than 0111 1111.

Execution of the command Check Antenna Resonance is fast and can be used frequently to

check whether system settings are correct.

Calibrate Antenna Resonance

In order to implement the antenna LC tank calibration binary weighted trimming capacitors

have to be connected between the two coil terminals to the pads TRIM1_3 to TRIM1_0 and

TRIM2_3 to TRIM2_0. In case single driver is used only the pads TRIM1_3 to TRIM1_0 are

used, pads TRIM2_3 to TRIM2_0 are left open.

Figure 11 shows connection of the trim capacitors for both single (left side) and differential

(right side) driving. The TRIMx_y pads contain the HVNMOS switch transistors to VSS.

During trimming procedure the resonance frequency is adjusted by connecting some of the

trimming capacitors to VSS and leaving the other ones floating.

Figure 11. Connection of trimming capacitors to the antenna LC tank

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 of off). The

breakdown voltage of the HVNMNOS switch transistors is 30 V, this limits the maximum

peak to peak voltage on LC tank in case trimming is used. The on resistance of TRIM1_0

and TRIM2_0 switch transistors (to be connected to LSB trimming capacitor) is typically

50 Ω at VSP = 3 V, the on resistance of other pads is binary weighted (the on resistance of

TRIM1_3 and TRIM2_3 is 6.25 Ω).

ST25R3910 Datasheet(PDF) 33 Page - STMicroelectronics