ST25R3920 datasheet(22/152 Pages) STMICROELECTRONICS

22 / 152 page

Application information

ST25R3920

22/152

DS13261 Rev 9

possible to operate only one of the two drivers by setting the IO configuration register 1 bit

single and selecting which RFO/RFI to be use on bit rfo2.

Output resistance

Each driver is composed of eight segments having binary weighted output resistance. The

MSB segment typical ON resistance is 4 Ω. When all segments are turned on, the output

resistance is typically 2 Ω. Usually all segments are turned on to define the normal

transmission (non-modulated) level. It is also possible to switch off certain MSB segments

when driving the non-modulated level to drive the circuitry with a higher impedance driver.

The bits d_res<3:0> in the TX driver register define the resistance during the normal

transmission. The default setting is minimum available resistance.

When using the single driver mode, the number and therefore the cost of the antenna LC

tank components is halved, but also the output power is reduced. In single mode it is

possible to connect two antenna LC tanks to the two RFO outputs and multiplex between

them by controlling the IO configuration register 1 bit rfo2.

To transmit data, the transmitter output level needs to be modulated. AM and OOK

modulation principles are supported. The type of modulation is defined by setting bit tr_am

in the Mode definition register.

Driver TX modulation

During the OOK modulation (e.g. for ISO14443A) the transmitter drivers stop driving the

carrier frequency. As a consequence the amplitude of the antenna LC tank oscillation

decays, the time constant of the decay is defined with the LC tank Q factor.

AM modulation (for example ISO14443B) is done via an additional regulator providing the

supply voltage VDD_AM, used as the driver supply voltage during the modulation state.

The AM modulation level is set by am_mod3:0 bits in the TX driver register.

AM modulation has to be manually enabled and the level to be set correctly for the following

protocols:

•

ISO14443B

•

FeliCa

•

ISO15693 (if not OOK)

•

NFCIP-1 212 and 424 kb/s initiator or active target.

Depending on the applicable standard the modulation index can be set in a range between

5 and 30% in the TX driver register.

Passive load modulation

The ST25R3920 enables passive load modulation using two different methods

•

Internal driver load modulation

•

Load modulation with an external MOS transistor and a diode that directly loads the

antenna circuit

The driver load modulation is selected by bit lm_dri and the external MOS modulation is

selected by lm_ext option bits.

Normally, the internal driver or the external load modulation should be used exclusively, but

the device also allows simultaneous modulation.

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