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Non-Isolated Can with selectable Termination

Raphael Jambor
Prodigy 70 points

Other Parts Discussed in Thread: TIDA-01238

Hello, everybody,

i want to make the termination resistor selectable for a can transceiver. I have seen the referencedesing for an isolated input.
There, a photo transistor from Toshiba is used to select the terminating resistor.
Can't I replace it with 2 of your FETs if I don't need an isolated Can?

Regards Raphael

  • 0 in reply to Raphael Jambor
    TI__Guru 62920 points

    Hi Raphael,

    What was your solution?

    In general using FETs can be a little difficult since the CANH/CANL voltage ranges can be fairly wide (for example, in the case of nodes operating at different ground reference potentials).  So, it can be a challenge to control the FET gate voltages in such a way that the FETs remain reliably on or off.  For applications where a ground is shared this becomes less of a concern.

    Max

  • 0 in reply to Miguel Robertson
    Prodigy 70 points

    Hi Max,

    Sorry about the late answer.
    Why can't I just take the back to back fets from the hardware reference design to make the final resistor switchable?
    Then I can leave out Toshiba completely?
    I would have simply controlled JMP 9 and JMP 10 with the controller.

    Regards Raphael

  • 0 in reply to Raphael Jambor
    TI__Guru 62920 points

    This can be done as long as the voltage range of the CAN signals does not exceed the range of the control voltage applied to the FET gates.  That's what I was mentioning before.  Typically CAN buses are specified to accommodate voltage shifts between the ground references of each node, resulting in a common mode voltage range of +/- 12 V on the CANH/CANL bus lines.  If you had just a 0-5 V signal to control the gate of the FET, then, it would be hard to make sure the FETs are reliably "on" or "off" over the full CAN bus voltage range.  If ground shifts are not a concern and your CAN signals will always be in a 0-5 V range themselves, though, then the FET-based solution is workable.

    Let me know if that doesn't make sense.

    Best regards,
    Max