ST25R3918 datasheet(22/141 Pages) STMICROELECTRONICS

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Application information

ST25R3918

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DS13490 Rev 2

Output resistance

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

all segments are turned on, the output resistance is typically 8 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



ISO15693 (if not OOK)

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 ST25R3918 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.

The driver load modulation is based on the change of driver impedance. Typically, a high

impedance during non-modulated state and a lower impedance for the modulated state is

used. This yields modulation phase equal to passive tag modulation. It is also possible to

reverse the polarity of the driver load modulation by using low impedance during

non-modulated state and higher impedance for the modulated state.

ST25R3918 Datasheet(PDF) 22 Page - STMicroelectronics

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