LF transmitter chip enables innovative start / immobiliser functions

November 12, 2014 // By Christoph Hammerschmidt
For communications applications in areas like automotive, safety and building automation where power-efficient operation has a high priority, Melexis has developed a low-frequency (LF) initiator chip. It consists of two high-power LF drivers which can be programmed independently along with an immobiliser.

One of the main applications for Melexis' MLX74190 is automobile remote passive start, where it is used to transmit a high power LF signal to wake up the key inside the car. The key then transmits its encrypted identification number to the car via an UHF signal, enabling the engine to start. The device can also serve for similar purposes with other vehicles, such as motorcycles, scooters, all-terrain vehicles (ATVs), jet skis, snowmobiles and motorboats, where the key holder can be detected when approaching the engine and will be able to activate it from a 'Start' button. Conversely it can serve as a ‘dead man switch’ so that if the driver falls off then the engine is automatically cut. In addition, it may be incorporated into building access systems or tire pressure monitoring system (TPMS) initiators.

Both of the LF drivers can be programmed to generate sine (1A peak maximum) or square (2.5A peak maximum) waves with frequencies between 109 and 140 kHz and an output amplitude from 0.25V to 32V peak-to-peak. These drivers can be used in either full-bridge or half-bridge configurations and have sophisticated protection mechanisms which safeguard them against over-current and over-temperature conditions. Amplitude-shift keying (ASK) or phase-shift keying (PSK) modulation can be used to transmit LF telegrams to the key or LF receiver.

The two built-in immobilizers support ASK and frequency-shift keying (FSK) modulations. These can be employed should the key’s battery become depleted and therefore unable to generate a signal. Under such circumstances passive RFID communication can be initiated when the key comes into close proximity of the initiator device. The immobilizers are compatible with the most popular types of passive transponders

The chip's diagnostic capabilities make it possible to detect various forms of misconnection with the LF antenna such as short to GND, short to Vbat, open load or incorrect resonance frequency.

More information: http://www.melexis.com/General/General/MLX74190-742.aspx