TCAN1042-Q1: TCAN1042-Q1 RTERM package risk

Hi James,

You would typically want to size these components differently based on which fault scenarios need to be supported.

The cap rating for a 12V system where the capacitor may come in contact with the battery voltage is typically 40-50V. This is to cover a load dump situation in which one of the CAN lines is shorted to battery and the bus is being driven dominant. This has a very low likelihood of happening, but it’s better to prepared for it than not. The power rating for the termination resistors will depend on the amount of current passing through the CAN bus and the termination at any given time, and you’ll want to size for the worst possible case, which may depend on your application.

According to the datasheet, the worst-case short-circuit steady state output current on the bus is 100mA, meaning that in a fault case, it is limited to 100mA. Using this calculate the power dissipation: i^2 * R = P, 0.1^2 * 120 = 1.2W, or 0.6W between each resistor. The conditions for this amount of current to be driven are: TXD and thus the bus is driving dominant, CANL is shorted to 40V (assume a high battery voltage), and CANH is open. So in an actual application the CAN bus would have to be shorted to battery, the car battery would have to be going through some sort of load-dump condition, and the bus has to be stuck dominant. In reality, this is why dominant time-out on this device exists, so that in cases where there can be high power consumption due to a short, the bus drivers will disable so that dominant-level current isn’t being driven during a fault condition. Now in a real application, typical bus loading is 50-60%, then 50% of those bits are dominant, so 25% of time even in this worst-case fault condition would be dominant. This would move the average power dissipated under this short circuit condition much lower, around 0.3 – 0.4W over the termination resistance, and again, this is in the worst-case bus fault condition. There is also the unlikely case that either CANH or CANL would get shorted to battery, and the other bus line get shorted to GND. This would drop 12V directly over the 120 ohms, and the resultant dissipation would be 1.2W.

The power rating of the termination resistors will depend on how many faults are being designed for at the same time. The most conservative approach would be using the case I listed last, where the entire battery voltage is being dropped over the termination resistance. In this case, 1.2W would be dropped over both resistors, and thus 0.6W in each 60 ohm resistor. For the case where the bus is shorted to battery and the bus is dominant, 0.3 – 0.4W across two 60 ohm resistors would be 0.15 – 0.2W, which would be covered by 1210 resistors, which can withstand 0.25W of power.

Users will ultimately decide how many faults they expect to happen in their system, and will anticipate less catastrophic power dissipation. This is why you see some CAN applications with terminations resistors in smaller packages.

Regards,
Max