MLX90109_06 MELEXIS [Melexis Microelectronic Systems], MLX90109_06 Datasheet - Page 7

• Manufacturer Part Number: MLX90109_06
• Description: 125kHz RFID Transceiver
• Manufacturer: MELEXIS [Melexis Microelectronic Systems]

8 System design parameters
The antenna internal driver is switched on as soon as the antenna voltage V(COIL) drops below V
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. V
antenna voltage when the reader has been powered down. The internal driver current is a square wave with a 45% duty
cycle.
Pulling V
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
should be larger than 1k . This is so called “auto start-up condition”.
The MLX90109 is specified to drive a maximum 14mA antenna drive current (I
The AC equivalent supply current (I
(2)
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)
3901090109
Rev 008
8.1 Auto start-up condition
8.2 Antenna current
MODU
I
I
DDant
ant
, at power on, from 5V to less than V
Q
ant
2
sin(
I
DDant
)
DDant
I
driver
) can be calculated as:
. 0
63
Page 7 of 15
I
pu
driver
will set the internal driver FET on. Provided the voltage
MODU
with
=0.8V for V
the duty cycle which is typically 45%.
125kHz RFID Transceiver
DD
=5V. The dashed curve shows the
driver
).
MLX90109
Data Sheet
Jun/06
DD
(see
ant