ST25R3912 datasheet(28/130 Pages) STMICROELECTRONICS

28 / 130 page

Functional overview

ST25R3912

28/130

DS11794 Rev 9

Figure 7. ST25R3912 power supply

Figure 7 shows the building blocks of the ST25R3912 power supply system and the signals

that control it.

The power supply system contains three regulators, a power-down support block, a block

generating analog reference voltage (AGD) and a block performing automatic power supply

adjustment procedure. The three regulators are providing supply to analog blocks (VSP_A),

logic (VSP_D) and transmitter (VSP_RF). The use of VSP_A and VSP_D regulators is

mandatory at 5 V power supply to provide regulated voltage to analog and logic blocks that

only use 3.3 V devices. The use of VSP_A and VSP_D regulators at 3 V supply and

VSP_RF regulator at any supply voltage is recommended to improve system PSRR.

Regulated voltage can be adjusted automatically to have maximum possible regulated

voltage while still having good PSRR. All regulator pins also have corresponding negative

supply pins that are externally connected to ground potential (VSS). The reason for

separation is in decoupling of noise induced by voltage drops on the internal power supply

lines.

Figure 2 and Figure 3 show typical ST25R3912 application schematics with all regulators

used. All regulator pins and AGD voltage are buffered with capacitors. Recommended

blocking capacitor values are detailed in Table 4.

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

sup3 V in the IO configuration register 2. Default setting is 5 V, hence this bit has to be set to

one after power-up in case of 3.3 V supply.

MS42462V1

Power-down

support

sup3V

50 Ω

RV<3:0>

VSP_RF

REG

BGR

and

AGC

AUTOREG

VSP_A

1 kΩ

VSP_D

AGD

reg 2Bh

reg 2Ah

VSP_A

REG

VSP_D

REG

adjust

VSP_RF

VDD

Table 4. Recommended blocking capacitor values

Pins

Recommended capacitors

AGD - VSS

1 μF, in parallel with 10 nF

VSP_A - VSN_A

VSP_D - VSN_D

2.2 μF, in parallel with 10 nF

VSP_RF - VSN_RF

2.2 μF, in parallel with 10 nF

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