ST25R3916 datasheet(41/157 Pages) STMICROELECTRONICS

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DS12484 Rev 4

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ST25R3916/7

Application information

156

actual regulated voltage, which is result of direct command Adjust regulators, can be

observed.

The VDD_RF regulator includes a current limiter that limits the regulator current to

350 mArms in normal operation. The i_lim in the Regulator display register is set when the

VDD_RF regulator is in current limiting mode.

If a transmitter output current higher than 350 mArms is required the VDD_RF regulator

cannot be used to supply the transmitter. VDD_RF and VDD_DR have to be externally

connected to VDD_TX (connection of VDD_RF to supply voltage higher than VDD_TX is not

allowed).

The voltage drop of the transmitter current is the main source of the ST25R3916/7 power

dissipation. This voltage drop is composed of a drop in the transmitter driver and of a drop in

the VDD_RF regulator. Due to this it is recommended to set the regulated voltage using direct

command Adjust Regulators. It results in good power supply rejection ratio with relatively

low dissipated power due to regulator voltage drop.

In Power-down mode the VDD_RF regulator is not operating. VDD_RF pin is connected to

VDD_TX through a 1 kΩ resistor. Connection through resistors assures smooth power-up of

the system and a smooth transition from Power-down mode to other operating modes.

VDD_AM regulator

This regulator is used to support the transmitter AM modulation. Its output voltage is used as

transmitter supply during modulation phase. The output is internally connected to the

transmitter. It requires decoupling capacitors (2.2 µF + 1 nF) at VDD_AM pin.

VDD_DR is used as reference voltage, resulting in correct VDD_AM voltage and modulation

index at supply voltage between 2.4 and 5.5 V.

The output voltage and thus modulation setting is controlled by am_mod<3:0> option bits

from 5 to 30% in 16 steps.

In Power-down mode the VDD_AM regulator is not operating. VDD_AM pin is connected to

VDD_TX through 1 kΩ resistor, as in the VDD_RF regulator.

VDD_A and VDD_D regulators

VDD_A and VDD_D regulators are used to supply the ST25R3916/7 analog and digital blocks

respectively. In 3.3 V mode VDD_A and VDD_D regulator are set to the same regulated

voltage as the VDD_RF regulator, in 5 V mode VDD_A and VDD_D regulated voltage is fixed to

3.4 V.

The use of VDD_A and VDD_D regulators is mandatory in 5 V mode since analog and digital

blocks supplied with these two pins contain low voltage transistors which support maximum

supply voltage of 3.6 V. In 3.3 V supply mode the use of regulators is strongly recommended

to improve PSRR of analog processing.

For low cost applications it is possible to disable the VDD_D regulator and to supply digital

blocks through external short between VDD_A and VDD_D (configuration bit vspd_off in the

IO configuration register 2).

Power-down support block

In the Power-down mode the regulators are disabled to save current. In this mode a low

power Power-down support block that keeps VDD_D and VDD_A below 3.6 V is enabled.

Typical regulated voltage in this mode is 3.1 V at 5 V supply and 2.2 V at 3 V supply. When

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