10.3.7 CAN Bus Short Circuit Current Limiting

The device has several protection features that limit the short circuit current when a CAN bus line is shorted. These include CAN driver current limiting (dominant and recessive). The device has TXD dominant time out which prevents permanently having the higher short circuit current of dominant state in case of a system fault. During CAN communication the bus switches between dominant and recessive states, thus the short circuit current may be viewed either as the current during each bus state or as a DC average current. For system current and power considerations in the termination resistors and common mode choke ratings the average short circuit current should be used. The percentage dominant is limited by the TXD dominant time out and CAN protocol which has forced state changes and recessive bits such as bit stuffing, control fields, and interframe space. These ensure there is a minimum recessive amount of time on the bus even if the data field contains a high percentage of dominant bits.

The short circuit current of the bus depends on the ratio of recessive to dominant bits and their respective short circuit currents. The average short circuit current may be calculated with the following formula:

Where:

• I_OS(AVG) is the average short circuit current
• %Transmit is the percentage the node is transmitting CAN messages
• %Receive is the percentage the node is receiving CAN messages
• %REC_Bits is the percentage of recessive bits in the transmitted CAN messages
• %DOM_Bits is the percentage of dominant bits in the transmitted CAN messages
• I_{OS(SS)_REC} is the recessive steady state short circuit current
• I_{OS(SS)_DOM} is the dominant steady state short circuit current

The short circuit current and possible fault cases of the network should be taken into consideration when sizing the power ratings of the termination resistance and other network components.