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Original question:

SN65HVD1781: RS422/485 Termination under Software Control

SN65HVD72: CAN termination using MOSFETs

Richard Webb71
Prodigy 10 points
Part Number: SN65HVD72
Other Parts Discussed in Thread: TIDA-01238

We wish to implement software selectable termination using the circuit below.

The problem is, when voltage transients appear on the CAN bus, this could change the voltage level on the Source nodes of the MOSFETs and therefore the FETs could change state due to the VGS(th) being crossed.

How can we adjust this circuit to be resilient to transients of up to 60V, so that the FETs remain On or Off through these events?

Regards,

Richard.

  • 0
    TI__Guru 62920 points
    Hi Richard,

    It's challenging to support a wide bus voltage range with this kind of solution, since you would need a gate control signal that is either reliably greater than the bus voltages (when termination is enabled) or reliably less than the bus voltages (when termination is disabled). Without a high-voltage reference voltage that could be used for gate control (and suitably high-voltage FETs) I'm not sure this is possible.

    The TIDA-01238 reference design that is mentioned in the previous thread that you referenced does show an alternate solution using a solid-state relay rather than discrete FETs. Since that component uses optical isolation between the control voltages and the signals being passed, the control voltage range can remain independent from the bus voltage range. This is a better solution, although it would come at greater expense. For CAN termination the currents in many cases are fairly low and only flow in a single direction (CANH to CANL), and so as an alternative to the solid-state relay you might want to evaluate whether a simple opto-coupler could be used. (We haven't had a chance to evaluate such a solution ourselves, but I wanted to offer it up as a suggestion for feedback or for deeper investigation.)

    Regards,
    Max