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SN65HVD25X, SN65HVD23X, CAN EVM

Joy Tan
Expert 1175 points


Hello,

Referring to user guide sllu158.pdf for CAN EVM.

I will like to find on the recommended part number for R9/R14 or L1 for the series resistor and common mode choke?

Thanks,

Joy

  • 0
    TI__Mastermind 28265 points

    The present installation of these resistors is 0 ohms. These resistors are used in combination with the TVS diodes (MMBZ) and have a value of 5 to 10 ohms. The type of resistors should be pulse-proof thick film resistors which limit the clamping current during a transient event. For example suring a transient event when the TVS diodes conduct and divert most of the current to ground, there will be a clamping voltage across these TVS diodes which is often higher than the breakdown voltage of the transceiver's internal ESD structures. When no resistors are used, the internal ESD diodes conduct and their lower clamping voltage drops below the breakdown voltage of the external TVS diodes. The external TVS diodes then become high-impedance again and the entire transient current is flowing through the much weaker internal ESD diodes and most likely destroy the device or cause a latch-up condition.

    In order to prevent this latching-up, the two external resistors (R9 and R14) are used to limit the current into the chip but also to create a voltage drop across these resistors, which is in series with the clamping voltage of the internal ESD diodes. The sum of these two voltages must be higher than the breakdown of the external TVS diodes in order to keep them turned on.

    With regards to the value of L1 I have to ask our CAN team. In fact the entire BOM should be included in the EVM manual.

    regards, Thomas

  • 0 in reply to Thomas Kugelstadt
    TI__Mastermind 20590 points

    For normal applicatuions requireing no advanced filtering for harsh noise or transients on the lines the R9/14 will be 0 ohms and the transceiver will be directly connected to the CAN bus.  TVS diodes / MOVs may be used as additional system level ESD & transient protection.

    If you have known harsh noise the CM choke may be used and a LC filter built and tuned to the necessary filtering by removing the R9/14 and replacing with one of the common CM chokes for CAN systems (TDK (ACT45B family), Epcos, Wurth and others make them).  Common values are 11uH through 101uH.  C2 and C7 are used as the C legs of the filter.  The series element of the CM choke also helps prevent current in rush to the device under transients. 

    For very harsh transients you can use the R9/14 as 5-10 ohms for current limiting to the device in combination with strong external TVS solution to clamp the voltage.  Care must be taken then to allow for the voltage drop in the differntial signal due to the series resistance. 

    The EVM gives you the basic pads on the PCB to develop a filter and protection scheme for the CAN transceivers to meet your specific applicaiton requirements.  It also shows how you can build your PCB to allow for future development of filtering and protection schemes even if they don't seem to be required at the immediate development.  It is much easier to put the pads on the PCB and then only populate if necessary vs finding out later that it may be needed. 

    -- Scott