MLX90109_06 Datasheet, PDF (7/15 Pages) Melexis Microelectronic Systems – 125kHz RFID Transceiver

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 V_DD (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.
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 V_MODU, at power on, from 5V to less than V_pu 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 Z_ant
should be larger than 1kΩ. This is so called “auto start-up condition”.
8.2 Antenna current
The MLX90109 is specified to drive a maximum 14mA antenna drive current (I_driver).
The AC equivalent supply current (I_DDant) can be calculated as:
(2)
I _DDant
=
2
π
⋅ sin(π
⋅α ) ⋅ I _driver
= 0.63 ⋅ I _driver
with α 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 = Q_ant ⋅ I _DDant
3901090109
Rev 008
Page 7 of 15
Data Sheet
Jun/06