AS3955 datasheet(13/111 Pages) AMSCO

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[v1-01] 2015-Apr-29

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AS3955 − Detailed Description

Demodulator is used for communication NFC device to NFC

tag. It detects AM modulation of the NFC device magnetic field.

The demodulator is designed to accept modulation according

to NFC-A specifications ([NFC Analog] [NFC Digital]).

Clock Extractor extracts a digital clock signal from the PCD

carrier field frequency which is used as clock signal by logic

blocks.

HF_PON enables operation of the AFE and the logic when the

supply voltage is sufficiently high. A buffer capacitor and

HF_PON hysteresis guarantee that there is no reset during NFC

device modulation.

Internal Regulator provides regulated voltage VP_INT to the

AFE and in most cases also to EEPROM and logic blocks. Typical

regulated voltage VP_INT is 2.0V. A buffer capacitor is also

integrated.

External Regulator provides regulated voltage on external pin

VP_REG where it can be used to supply some external circuitry.

The regulated voltage and output resistance can be adjusted

using EEPROM settings. Appropriate external buffer capacitor

is needed in case VP_REG is used in the application. Current

which may be provided depends on reader field strength,

antenna size and Q factor, but it is limited to maximum 5mA.

Bias provides bias currents and reference voltages to AFE

analog blocks.

Power Management

AS3955 power management comprises of four different modes

to fit requirements of different applications. AS3955 supports

two power sources, whose activation depends on the selected

power mode.

Power Mode 0

In this power mode, the Power Manager is controlling the

supply of the PICC Logic, EEPROM and SPI / I²C Interface (VDD).

Its inputs are VP_INT (rectified and regulated supply extracted

from RF field) and the VP_IO (supplied by external battery).

In standby mode, when AS3955 is not in the RF field (the

condition is that rectified supply voltage is below HF_PON

threshold) and the SPI / I²C is not active (/SS is high), the VDD

supply is disconnected. The only current consumption in this

state is leakage on VP_IO, mainly due to level shifters and SPI /

I²C pins.

When AS3955 is placed in a RF field, VDD is connected to VP_INT.

This happens once the VP_INT level is above the HF_PON

threshold.

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