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SN65HVD233: Transceiver Supply Current draw with many CAN nodes

Caitlin Slayden
Prodigy 80 points
Part Number: SN65HVD233

Hello, 

This question is about the SN65HVD233/234/235.  I have read the app note SLLA337 (Overview of 3.3V CAN Transceivers) which has a table of supply current each part draws on a 2 node CAN bus.  How does the current draw scale as more nodes are added to the bus? 

If both transceivers are recessive, each draws roughly 7.2mA.  If both transceivers are dominant, each draws about 26mA.  If one is dominant and the other is recessive, the dominant transceiver draws 38.4mA and the recessive transceiver draws 7.2mA. 

If more than 2 nodes are on the bus and one transceiver is dominant while the others are recessive, what is the current drawn by the dominant transceiver?  Does this value continue to scale as more nodes are added?  

Why does the dominant transceiver draw a higher current when the 2nd transceiver is recessive? 

Thank you

  • 0
    TI__Guru 62910 points

    Hi Caitlin,

    Good questions!  When a transceiver is driving a dominant state, it will consume some current from VCC in order to establish a positive differential voltage on the CAN bus (i.e., by pulling CANH high and CANL low).  The current required to create this voltage depends on the resistance between CANH and CANL.  CAN buses will typically have about 60 Ohms of loading due to termination (i.e., via two 120-Ohm resistances, one at each furthest end of the bus), and this will make up the majority of the output loading.  Each transceiver that is in the recessive state will have an effective input resistance as well that will contribute to this loading, but this input resistance is fairly large (tens of kOhms).  So, as more and more transceivers are added there will be some increase in current, but it will typically be negligible unless the number of nodes becomes very large.

    When multiple transceivers drive dominant, the required bus current is shared among these transceivers.  This reduces the current draw per transceiver, but does not reduce the total current being sourced to the bus.

    I hope this all makes sense - please let me know if not.

    Regards,
    Max