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TCAN1043-Q1: Resistor between Vsup and Vbat

user4979899
Expert 3080 points
Part Number: TCAN1043-Q1
Other Parts Discussed in Thread: TCAN1043

Hi Sir,

Reference from TCAN1043 datasheet,

There is a resistor R8 on Vsup pin which connect to Vbat. May I know the purpose of R8?

And what is your recommend value of R8?

Thanks

  • 0
    TI__Guru 62910 points

    Hi Anne,

    Some applications use a series resistance (e.g., 1 kOhm) on this pin in order to help further protect against transients that may occur on the automotive battery line.  Since the maximum current sourced to VSUP is just 70 uA, it is possible to introduce this level of series resistance without creating a substantial loss of voltage.

    Please let us know if you have any further questions.

    Regards,
    Max

  • 0 in reply to Max Robertson
    TI__Expert 3080 points

    Hi Max,

    Another question, the picture in TCAN1042 datasheet page 25 as below, do you know the termination resistors can be another value, not 120 Ohm?

    Because I find some applications that termination resistor were not 120 Ohm, but I don’t know why, is it about operation frequency?

  • 0 in reply to user4979899
    TI__Guru 62910 points

    Anne,

    In some cases the values may vary slightly in order to better match the characteristic impedance of the cabling.  In other cases, large values (in the kOhm range) may be used at certain nodes to help dampen reflections that might otherwise be generated.

    While it can be OK to deviate from the "typical" 120-Ohm values, you should keep in mind that termination serves two main purposes in a CAN application:

    1. To prevent reflections from being generated when the signals propagate to the end of the line.  (Otherwise, these signals will see an abrupt difference in impedance, reflect back, and possibly interfere with the intended signal.)

    2. Help to drive the dominant-to-recessive transition.  (In this transition the CAN driver will go high-Z, and so the external resistances are needed in order to help pull the CANH and CANL signals close to one another so that the differential voltage is close to 0 V in the recessive state.)

    Note that in part because of reason #2, many of the performance parameters of a CAN driver (such as the transition timing and output signal levels) are specified based on an expected load of 60 Ohms (the equivalent resistance of a bus with two parallel 120-Ohm termination resistances).  Changing the overall loading could impact these parameters.

    For more information on CAN termination, you can reference this blog:

    https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/07/14/the-importance-of-termination-networks-in-can-transceivers

    Regards,
    Max