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SN65HVD72: Reduce the power usage

Dharmesh Joshi
Expert 1895 points
Part Number: SN65HVD72

Tool/software:

Hi,

This is my circuit design.

Are the pull-up resistor (R11) and pull-down resistor (R16) necessary? If so, can their values be increased significantly? I am working on battery-powered applications, and the current draw is a bit high. Also, do we need these resistors on both sides of the communication?


  • 0
    TI__Guru 72181 points

    Hi Dharmesh,

    Normally these external fail safe resistors are used on one of the nodes, you normally don't place them one both.

    These resistors (external fail safe) help to generate more noise immunity during idle states and generate a known level during open conditions. The Sn65HVD72 already has an internal fail safe feature so if your receiver has this feature it's technically not required. You can remove them by just having them set to do not populate. 

    Note: normally the pull up resistor is on A pin and the pull down is on B pin, doing it backwards makes the known state output R as a low during idle conditions.

    -Bobby

  • 0 in reply to BOBBY
    Expert 1895 points

    Thanks for the feedback

    Typically, architectures follow a master-to-slave type of configuration. Therefore, for future releases of these PCBs, we plan to remove the pull-up and pull-down resistors from the master board but will retain them on the slave PCBs.

    Regarding the existing PCBs, if the resistors have been swapped, what impact could this have? So far, we haven't observed any noticeable differences.

  • 0 in reply to Dharmesh Joshi
    TI__Guru 72181 points
    Dharmesh Joshi said:

    Typically, architectures follow a master-to-slave type of configuration. Therefore, for future releases of these PCBs, we plan to remove the pull-up and pull-down resistors from the master board but will retain them on the slave PCBs.

    Regarding the existing PCBs, if the resistors have been swapped, what impact could this have? So far, we haven't observed any noticeable differences.

    The only main difference is the R output would be a low during the idle bus state for all receivers on the bus. For some people who use UART are the software protocol, this can look like a break condition and flag an error. It depends on the system and the software protocol but if you're system is okay with R output being LOW during idle states then there isn't a problem.

    -Bobby

  • 0 in reply to BOBBY
    Expert 1895 points



    Is it recommended that a TVS be added to the data lines?

  • 0 in reply to Dharmesh Joshi
    TI__Guru 72181 points

    Hi Dharmesh,

    It's not a requirement to have one. This device has ESD/EFT ratings which I would assume would work for most customers, it kind of depends on the customer's end system and if they think the device will see an transient that is outside the ratings. 

    Is there a particular ESD level your system needs to pass? 

    If you're not sure you could always put some in your schematic as do not populate and populate them when necessary. 

    -Bobby