This application note shows how to use the MLX90109 transceiver to obtain 100% modulation of the magnetic field (on-key modulation). The MLX90109 datasheet will help you understand the basic operating principles and specifications before reading this document.

On-off keying modulation involves switching the current through the transceiver antenna MLX90109 on and off. This can be achieved by switching the modulation pin MODU (pin 4) between V(ModuMin) and V(DD).

Turning on the magnetic field depends on the internal driver of the MLX90109 and takes less than 5 carrier cycles as long as the “auto-start impedance” condition is considered. Turning off the magnetic field will depend on the quality factor (Q) of the parallel antennas connected to the transceiver. In the free swing state, approximately Q carrier periods will be required. Depending on the antenna Q and the protocol used, this fall time may be a limitation that will prevent the modulation depth from reaching 100%.

An additional transistor can be used to reduce the fall time of the magnetic field, ensuring compatibility with the on-off keying fast protocol.

Reduce the off-time of the antenna

Protocols that use on-off keyed modulation (such as the HITAG protocol) need to turn off the magnetic field fairly quickly to minimize the time when no energy is transferred to the transponder (such as the HITAG2 protocol uses a field-off pulse of typically 48 µs). The fall time of the MLX90109 mainly depends on the quality factor of the parallel antennas. Using the antenna MLX90125CZA-A used for the measurement, the coil swing requires approximately:

[tex] t_ {shutdown} = Qcdot frac {1} {f_ {carrier}} [/ tex]

[tex] t_{off} = 24.8 hspace{1mm}μs = 192 hspace{1mm}μs [/tex]

The application circuit below shows how the fall time can be reduced by using a transistor and a Schottky diode in parallel with the MLX90109 antenna.

fast decay circuit

Transistor T1 will be controlled by reverse command from the microcontroller port for field management (100% modulation). In the off state (5 V on the grid), T1 is blocked and other systems are disconnected from the antenna. D1 prevents the parasitic diode of transistor T1 from turning on.

When switched on (0 V on the grid), T1 is on. This will short the antenna for a negative swing and dissipate the remaining energy through transistor T1. Hence the fall time will be reduced.

Using two inexpensive components, the MLX90109 is able to properly manage custom protocols, such as HTAG2, that require the use of short on/off keying.

Edit: hfy

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