TCAN332: Un-intended GND path through CAN H/L pins

Hi Battery Ninja,

Thanks for bringing your question to E2E. 

My first thought here would be that some voltage on the CAN bus would exceed the transceiver's limits and cause it to break. The absolute maximum voltage that the CAN pins can tolerate is ±14V with respect to GND. With GND floating, it's hard to say what the applied voltage would actually be, but with a battery level of 60V it sounds like it would be quite easy to exceed this ±14V range between the two boards. 

To avoid this problem, there are a few approaches that you could take. One common method is to ensure that the GND connection is always made first by using a connector with slightly longer GND than other pins so that it always connects first when plugging and last when unplugging. Similarly, the two boards could also share a chassi ground or shielded ground (often connected to external metal frames) through some high resistance path. This gives both boards a method of discharging to a common ground even when they are not connected directly. Lastly, you may consider including protection devices along the CANH and CANL nets on each board to help dissipate or block transient energy during connection events. This would primarily include TVS diodes, but may also be PTC fuses and bus capacitors. Let me know if you have any questions on these components or how to select appropriate values. 

Regards,
Eric Schott

Hi Ninja,

I'm glad to hear that you are seeing some success with the TVS diode implementation. The important factors to consider when selecting these components is that the breakdown voltage is outside of the normal operating conditions of the system (for CAN > 5V) and that the clamping voltage is less than the abs max of the transceiver pin (14V in this case). For this, your selection of the SMAJ6.0CA is a fine choice, though it sounds like the power presented by the event can still be a bit much for it. 

In addition to shunting the extra energy from the event with a TVS diode, a blocking device such as a PTC fuse can act to protect the TVS diode once it engages. A Positive Temperature Coefficient (PTC) fuse increases in impedance when more current passes through it. This allows it to act as a short circuit during normal operating conditions and an open circuit during fault conditions. Once the fault is removed and the component cools down, it becomes a short circuit again to allow normal operation to resume. These devices can limit the amount of energy a shunting device needs to dissipate as it only needs to conduct until the PTC fuse has time to react and go high-Z. The rest of the energy is dissipated more slower through the fuse and allows more time for all of the components to stay cool. 

Similarly, bus capacitors can help smooth fast transients and decrease the max peak power of a ESD or surge event. This is typically less effective than dedicated protection devices, but it's a relatively simple and inexpensive way to increase immunity to harsh transients. For CAN we typically recommend bus capacitor values between 10pF and 150pF to help filter out high frequency noise and lengthen transient pulses. These should be placed on both CANH and CANL to GND. 

Let me know if you have more questions on these protection methods. 

Regards,
Eric Schott