MLX90109 데이터 시트보기 (PDF) - Melexis Microelectronic Systems

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MLX90109

125kHz RFID Transceiver

Melexis Microelectronic Systems  

MLX90109

125kHz RFID Transceiver

8. System design parameters

The antenna internal driver is switched on as soon as the antenna voltage V(COIL) drops below VDD (see graphical

representation below). The MLX90109 will inject a current Idriver into the antenna to make its amplitude follow the voltage

on the MODU pin.

In order to make the antenna start swinging on the resonance frequency, the chip needs to provide a positive feedback loop.

This loop requires a minimal voltage swing at the COIL pin in order to be operational (typically 100mVpp). Below this value,

the MLX90109 may not be able to retrieve its clock.

V(COIL)

VDD

VMODU

VSS

Idriver

on

Graph:

Antenna voltage and Driver current during normal operation. VMODU=0.8V for VDD=5V. The dashed curve shows the antenna voltage when

the reader has been powered down. The internal driver current is a square wave with a 45% duty cycle.

8.1. Auto start-up condition

Pulling VMODU, at power on, from 5V to less than Vpu will set the internal driver FET on. Provided the voltage drop on the coil

pin is large enough (as explained above), the feedback loop is closed and the oscillation will increase in amplitude.

To obtain the required positive feedback to start-up the oscillation successfully, the antenna impedance Zant should be larger

than 1k. This is so called “auto start-up condition”.

8.2. Antenna current

The MLX90109 is specified to drive a maximum 14mA antenna drive current (Idriver).

The AC equivalent supply current (IDDant) can be calculated as:

(2)

I DDant



2



 sin(

 )  I driver



0.63  I driver

with  the duty cycle which is typically 45%.

The current that the MLX90109 can inject at each oscillation onto the total antenna current is therefore limited to 9mA.

The actual antenna current that generates the magnetic field can be calculated as:

(3)

I ant  Qant  I DDant

A typical coil quality factor (Qant) value is 23, resulting in antenna currents of about 100mA

This current resonance of the parallel antenna allows to build very low power reader base stations, contrary to serial

antenna based versions. Readers using a serial antenna can leverage their voltage resonance to drive bigger antenna’s for

long distance reading up to 1m, whereas the MLX90109 is designed to drive antennas to obtain a reading distance of 1cm up

to 15cm (6”) (depending on efficiency and dimensions).

REVISION 011 - JUNE 19, 2017

3901090109

Page 9 of 17

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