1.    Introduction

 

Basic functionality of TPMS (Tire Pressure Monitoring System) is to monitor tire inflation pressure and temperature.

Most of TPMS are attached to the valve. Transponder coils acts as the media to transmit information to wake-up the system. Tipically all information concerning pressure and temperature are transmitted to “central electronic box) through RF interface (tipically 433MHz).

Next, is an application note of RTPMS from GE, that use 2D transponder coils, in this case it can be achived using two single coils 90º oriented.
 

Figure 1. NPX-C0XXXX block diagram

The NPX-C0XXXX device w/o accelerometer (NPX-I) consists of two chips: a piezoresistive absolute pressure sensor and a CMOS-based ASIC, assembled in a standard surface-mount SOIC 14L package. Whereas the NPX-C0XXXX device w/accelerometer (NPX-II) consists of three chips: a piezoresistive absolute pressure sensor, a piezoresistive accelerometer and a CMOS-based ASIC, assembled in a standard surface-mount SOIC 14L package.

The ASIC is powered by a low power 8 bit RISC processor, performing signal conditioning and data framing according to the application needs. A number of general purpose I/O’s are provided to sense and control external circuitry, like a UHF transmitter or additional sensor circuitry. The device incorporates the following features:

 

 

• 2D wake up LF interface
• Front end Low Noise Amplifier (LNA), 12 Bit ADC
• Temperature sensor and Battery voltage detection
• 4 Kbyte ROM, 4 Kbyte Flash EROM, 128 Byte EEPROM, 128 Byte RAM

2.    Low Frequency Interface (how to calculate LCR circuit).

Premo offers to market several possibilities for TPMS application (different range insductance and size):

 

TP0602: 6.6 x 2.3 x 1.75mm / 2.38 – 10.8 mH

 

TP0702: 7.7 x 3 x 2.2mm / 2.38 – 10.8 mH 
 

TP1103: 11.5 x 2.9 x 2mm / 0.34 – 16.2 mH   
 
SDTR1103: 11.8 x 3.6 x 2.5mm  / 0.34 – 16.2 mH   

LF wake up function is accomplished with the LF interface of NPX-C0XXXX, and LF receiving transponder. NPX has 2 LF channels. Each LF input channel is designed with a RLC parallel circuit like the following chart.
 

Figure 2 NPX LF Interface-Parallel RLC Circuit

L and C have to be resonant at LF carrier frequency of 125 kHz in order to maximize the sensitivity.

 

               
f (carrier frequency): 125kHz
here L = 4,77mH (optional 1-9mH), then 

 

 

Another tipical value for TPMS application is 7.2mH, and the correspondance capacitor value will be C=220pF

The voltage coil V produced by the LF receiver in the magnetic field:

 

Figure 3 Transponder orientation vs Magnetic field

           
Fc  is the carrier frequency and  F0 is the resonance frequency of parallel RLC circuit.
N is the number of coil turns;
Q is the quality factor of the parallel RLC circuit;
S(m2 ) is the effective area of the coil;
α is the angle between the magnetic line and the coil’s S


The LF wake-up data transfers at a rate of 3.9kbps in Manchester code. Therefore the required data band should be not less than 7.8kbps.

Resistor R here is to reduce the Q (Quality Factor) of this LC circuit to reach enough bandwidth (7.8kHz).

According to the specification, the LF data rate of 4kpbs, the quality of the resonant circuit should not exceed 15.


 

To calculate resistor value close to this “Q factor”, using following equation:


 




Figure 4 NPX & Premo LF Receiving Coils



IC included in this application note is courtesy from GE.

 

PREMO is a leading company in designing, manufacturing and marketing custom designed and inductive components for electronic market. Its product range includes RFID antennas, EMC filters, accessories and PLC components, planar transformers, power supplies, transformers, current transducers, chokes for automotive, renewable energies and railway, among other markets.