ST25R3918 datasheet(36/141 Pages) STMICROELECTRONICS

36 / 141 page

Application information

ST25R3918

36/141

DS13490 Rev 2

Figure 8. ST25R3918 power supply system

Regulators have two basic operation modes depending on supply voltage, 3.3 V supply

mode (max. 3.6 V) and 5 V supply mode (max 5.5 V). The supply mode is set by writing bit

sup3V in theIO configuration register 2. Default setting is 5 V so this bit has to be set to 1

after power-up in case of 3.3 V supply.

In 3.3 V mode all regulators are set to the same regulated voltage in range from 2.4 to 3.4 V,

while in 5 V only the VDD_RF can be set in range from 3.6 to 5.1 V, while VDD_A and VDD_D

are fixed to 3.4 V.

Figure 8 also shows the signals controlling the power supply system. The regulators are

operating when signal en is high (en is configuration bit in Operation control register). When

signal en is low the ST25R3918 is in low power Power-down mode. In this mode

consumption of the power supply system is also minimized.

VDD_RF regulator

The purpose of this regulator is to improve the PSRR of the transmitter (the noise of the

transmitter power supply is emitted and fed back to the receiver). The VDD_RF regulator

operation is controlled and observed by writing and reading two regulator registers:



Regulator voltage control register controls the regulator mode and regulated voltage.

Bit reg_s controls regulator mode. If it is set to 0 (default state) the regulated voltage is

set using direct command Adjust regulators. When bit reg_s is asserted to 1 regulated

voltage is defined by bits rege_3 to rege_0 of the same register. The regulated voltage

adjustment range depends on the power supply mode. In case of 5 V supply mode the

adjustment range is between 3.6 and 5.1 V in steps of 120 mV, in case of 3.3 V supply

mode the adjustment range is from 2.4 to 3.6 V with 100 mV steps.



Regulator display register is a read only register that displays actual regulated voltage

when regulator is operating. It is especially useful in case of automatic mode, since the

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

observed.

MS52437V1

VDD_D

regulator

Power-down

support

sup3v

50Ω

VDD_A

regulator

VDD_RF

regulator

BGR

and

AGD

Autoreg

AGDC

VDD_RF

VDD_D

VDD_A

reg<3:0>

VDD

Adjust

regulators

rege<3:0>

VDD_AM

regulator

VDD_TX

VDD_AM

VDD_DR

ST25R3918 Datasheet(PDF) 36 Page - STMicroelectronics

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