MLX90109
125kHz RFID Transceiver
3901090109 Page 14 of 16 Data Sheet
Rev 011 Jun/12
13 FAQ
13.1 Is it possible to make proportional modulation (depth less than 100%) with the
MLX90109?
The amplitude of the MLX90109 antenna can be adjusted on the fly by changing the MODU pin level
between V
MODU
= 0.8V and V
pd
. However, the MLX90109 cannot change instantaneously the voltage on its
antenna according to a voltage step on MODU pin, and a transient waveform will appear on the voltage
antenna. This particular waveform may disturb the transponder and in the worst case (modulation depth more
than 20%) the MLX90109 may stop its oscillation.
Using the MLX90109 with proportional modulation (modulation depth less than 100%) is not recommended
and supported by Melexis and must be evaluated case by case.
13.2 How should I read data information from a transponder up to 15cm?
The reading distance depends on the complete system composed by the reader and the transponder. A
reading distance with the MLX90109 transceiver up to 15cm has been demonstrated with a specific reader’s
antenna (diameter = 130mm, Inductance = 44
µ
H, Quality factor Q
ant
= 87.2@125kHz) and a transponder with
a credit card size antenna (80 x 50mm).
13.3 Is it possible to increase the output power of the MLX90109 transceiver?
The current flowing through the antenna (I
ANT
) can be maximized by a careful design, respecting the design
specification of the MLX90109 (Auto start-up impedance, the maximum driver current I
DRIVER
).
The voltage on the antenna cannot be increased as it is limited by the power supply V
DD
(V
ant
≤
V
DD
-
V
MODU
+V
os
). Moreover, as the MLX90109 uses the same connection (COIL ) for the transmission and the
reception, it is not possible to use an external power transistor supplied with a higher voltage than V
DD
.
13.4 Are there any specific coils available for the MLX90109 transceiver?
Melexis has developed an 18mm coil which is used on the evaluation board EVB90109. Please contact your
sales channel if you wish to purchase production quantities.
13.5
What are the recommended pull-up values on DATA and CLOCK pins?
The DATA and CLOCK are open-drain drivers which require external pull-up resistors. The values are not
critical therefore, to reduce the general power consumption, we recommend to use high ohmic (100k ohm)
pull up resistances.
