• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 32 subscribers
  • Views 322 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN65HVD251-Q1: Dominant current flow

Kurumi Hasegawa
Expert 7281 points
Part Number: SN65HVD251-Q1
Other Parts Discussed in Thread: SN65HVD251

Hi team,

 

One of my customer have questions on SN65HVD251-Q1, and I’d like to ask your help.

The datasheet state the supply current of dominant state is 65mA.

From the below blog, in the dominant state the transceivers are ON, so the current flows from Vcc to GND.

https://e2e.ti.com/blogs_/b/industrial_strength/posts/the-inner-workings-of-a-can-bus-driver

 

The questions are:

  1. How does the dominant current flow from Vcc in real situation?
  2. When cable is connected between SN65HVD251-Q1 and communicating device (like MCU), does the dominant current flow into the MCU as well?
  3. From the datasheet, the output current is ±50mA, does it contain the dominant current?

 

Best regards,

Kurumi

  • 0
    TI__Guru 52325 points

    Hi Kurumi,

    These are good questions. I'm glad you were able to find John's blog to look over.

    Kurumi Hasegawa said:
    How does the dominant current flow from Vcc in real situation?

    The main current path will be from Vcc through the CANH driver, through the termination resistors on the CAN bus (can be in multiple locations in the network), into the CANL driver and then to ground. Small leakage paths will allow some current to flow through the receiving transceivers, but most of the current will pass through the termination. The current across the termination load is what creates the voltage differential between CANH and CANL that each CAN transceiver recognizes as a dominant signal. 

    Kurumi Hasegawa said:
    When cable is connected between SN65HVD251-Q1 and communicating device (like MCU), does the dominant current flow into the MCU as well?

    The communication between an MCU and CAN transceiver will consist of different current paths from the CAN bus. The transceiver digital output pins (RXD primarily) can be sourced from Vcc or Vio (if device supports). Since this is seperate from the CAN bus, the current path is simply from Vcc/Vio out of the driving pin (RXD) and through the MCU input pin's leakage path to ground. This will be far less current (-4mA) than we'll see on the CAN bus (-50mA). The CAN bus itself should only be connected to other CAN transceivers. The transceiver will then translate the differential signal into a digital one that the MCU can manage and recognize. 

    Kurumi Hasegawa said:
    From the datasheet, the output current is ±50mA, does it contain the dominant current?

    The recommended high- and low-level output currents refer to different parts of the device. The "Driver" specification refers to the current through the CANH/CANL bus pins. The "Receiver" spec refers to the digital output pins (RXD). Keep in mind that the "Supply Current" specification will include all of these output currents as well as the needed supply for device logic. 

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 7281 points

    Hi Eric,

    Thank you so much for the detailed explanation. I was mixing the CAN bus and the communication between MCU-CAN.

    Could you tell the leakage (dominant) current that flows though the receiving transceivers, from Vcc->CANH?

    I’ve searched datasheet but could not find the number. If the exact number is not available, could you tell me the estimate?  

    Best regards,

    Kurumi

  • +1 in reply to Kurumi Hasegawa
    TI__Guru 52325 points

    Hi Kurumi,

    Kurumi Hasegawa said:
    Could you tell the leakage (dominant) current that flows though the receiving transceivers, from Vcc->CANH?

    The load presented by a receiving transceiver (and subsequent leakage current) is determined by the device's input resistance. For SN65HVD251, this value is 20k-ohm min. If the CANH bus pin was shorted to Vcc (fault case) the resulting wore-case current through the receiving device would be 5.5V / 20k-ohm = 275uA. More typical operational leakage will be closer to 75uA (recessive bias and Rin typical).

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 7281 points

    Hi Eric,

    I understand. Thank you so much for the help!

    Best regards,

    Kurumi