Source | Vehicle Terminal
Function of the Termination Resistor
The CAN bus termination resistor serves three purposes:
1. Enhance anti-interference capability, allowing high-frequency low-energy signals to dissipate quickly.
2. Ensure the bus quickly enters a dormant state, allowing the energy of parasitic capacitance to dissipate faster;
3. Improve signal quality, placed at both ends of the bus to reduce reflected energy.
What is impedance? In electrical terms, the resistance to current flow in a circuit is referred to as impedance. The unit of impedance is ohms, commonly represented by Z, which is a complex number Z = R + i(ωL – 1/(ωC)). Specifically, impedance can be divided into two parts: resistance (real part) and reactance (imaginary part). Reactance includes capacitive reactance and inductive reactance, where the current obstruction caused by capacitance is called capacitive reactance, and the obstruction caused by inductance is called inductive reactance. Here, impedance refers to the modulus of Z.
The characteristic impedance of any cable can be determined experimentally. One end of the cable is connected to a square wave generator, and the other end is connected to a variable resistor, with the waveform observed on an oscilloscope. The resistance value is adjusted until the signal on the resistor is a clean, ringing-free square wave; at this point, the resistance value can be considered consistent with the cable’s characteristic impedance.
By using two typical automotive cables twisted into a twisted pair, the characteristic impedance can be determined to be approximately 120Ω using the method described above. This value is also the termination resistance value recommended by the CAN standard, so this 120Ω is measured, not calculated, and is derived from the actual characteristics of the harness. Of course, this is also defined in the ISO 11898-2 standard.
Why Choose 0.25W Power Rating?
This must also take into account certain fault conditions. All interfaces of the automotive ECU need to consider short circuits to the power supply and ground, so we also need to consider the situation where the CAN bus node is short-circuited to the power supply. According to the standard, we need to consider a short circuit to 18V. Assuming CANH is short-circuited to 18V, the current will flow through the termination resistor to CANL, while due to current limiting inside CANL, the maximum injected current is 50mA (as indicated in the TJA1145 datasheet). At this point, the power of the 120Ω resistor is 50mA * 50mA * 120Ω = 0.3W. Considering derating under high temperature conditions, the power rating of the termination resistor is 0.5W.
(Image source from the internet)
