AN2768| Application Note
Maxim/Dallas > App Notes > AUTOMOTIVE Keywords: Ultra-Low Quiescent Supply Current 125kHz ASK Receiver
Oct 01, 2003
APPLICATION NOTE 2768
Ultra-Low Quiescent Supply Current 125kHz ASK Receiver
There is an emerging automotive market: Tire Pressure Monitoring Systems (TPMS). These systems, which will soon be mandated for new cars, monitor the tire pressure and alert the driver when pressure is outside specifications. Proper tire pressure enhances fuel economy and vehicle safety. Powering these systems is a complex challenge. Components are built into the wheel, so the batteries used must be light in weight. Batteries must offer a seven-year life, as they are permanently molded into the modules and cannot be replaced. Because of the difficulty in providing a connection between the vehicle and a rotating wheel, wireless systems are a common solution. Pressure information is transmitted via a RF channel at a frequency that varies with the country (usually, 315MHz, 434MHz or 868MHz). Transmitters are generally switched on and off, to prevent excess interference and improve battery life. Some systems use a mechanical switch, others use an algorithm which cycles the transmitter based on rotation speed. Ma
ny designs use a LFRF (low-frequency RF) solution: A 125kHz magnetic field wakes the tire-mounted TPMS module, which transmits the pressure-related signal and returns to sleep mode. The module is controlled by electronics on the vehicle. The TPMS module needs a LFRF receiver with very low power consumption. This application note describes an ASK receiver at 125kHz with a 2.1V supply voltage requirement and only 4uA typical supply current when there is no LFRF trigger signal. Sensivity is adjustable and is typically around 5mVpeak-to-peak. The design is based on the best speed/consumption comparator on the market : the MAX9075. It is available in a tiny SC70 package
Figure 1.
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