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TCAN1044A-Q1: Allowable GND Shift

Part Number: TCAN1044A-Q1

Hello team,

This question is related to below thread. Sorry for the almost same question.

https://e2e.ti.com/support/interface-group/interface/f/interface-forum/1029925/tcan1044a-q1-gnd-shift?tisearch=e2e-sitesearch&keymatch=tcan1044a-q1%20gnd%20shift#

Above thread says common mode voltage range of the receiver is +/-12V for TCAN1044A-Q1.

Does this mean +/-12V GND shift can be acceptable?

Or this VCM is specified at CANH/CANL pin so we should take into account of the recessive/dominant voltage at the CANH/CANL?

If so I calculated the allowable GND shift in the below attachment and allowable GND shift is +/-7.5V. Is this correct?

Allowable GND Shift for CAN1 and CAN2 (SD).pptx

I would like to know the exact allowable GND shift (and how to derive it from D/S).

So could you let me know?

I appreciate your support.

Regards,

Hirata

  • Hi Hirata-san,

    We define the common-mode voltage on the CAN bus to be the average of CANH and CANL during our characterisation testing. This means that the VCM of ±12V includes when CANH = +12V + (0.9V / 2) and CANL = +12V - (0.9V / 2). (I'm using 0.9V as the receiver dominant state input threshold as-per the ISO spec.) Therefore, the only thing we would need to account for in determining the tolerable ground shift is the nominal VCM of 2.5V of the other transceiver. Assuming this would be 2.5V + Vgnd_shift, the tolerable ground shift voltage would be -14.5V to +9.5V. In reality, this would constrain both sides to ±9.5V of ground shift between nodes. 

    Keep in mind that this common mode range only defines the range in which all other datasheet specification (particularly receiver input threshold) remain true. If a larger common-mode shift is presented, the receiver will likely continue to function, but may drift outside of datasheet specified ranges. 

    Let me know if this clears things up and if you have any more questions.

    Regards,
    Eric Schott

  • Hi Eric-san,

    Thanks so much for the clear answer. Regarding +/- 9.5V, I clearly understood. 

    Let me doublecheck.

    Regarding below underlined portion ("if a larger common-mode shift is presented"), do you mean this is "VCM < -12V" or "VCM > +12V" case, correct?

    or Are there any situation we should care when VCM is within +/- 12V?

    Keep in mind that this common mode range only defines the range in which all other datasheet specification (particularly receiver input threshold) remain true. If a larger common-mode shift is presented, the receiver will likely continue to function, but may drift outside of datasheet specified ranges. 

    I appreciate your support, as always.

    Regards,

    Hirata

  • Hi Hirata-san,

    Yes, you understood this correctly. The underlined portion could also read "if the common-mode shifts outside of the recommended +-12V range...". 

    Regards,
    Eric Schott

  • Hi Eric-san,

    I understood. Thanks for the clarification.

    I appreciate your support.

    Regards,

    Hirata