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TCAN4551-Q1: RGYRQ1

Huajian Li
Prodigy 70 points
Part Number: TCAN4551-Q1
Other Parts Discussed in Thread: TCAN4550

Tool/software:

Hi,everyone,

Rencently when we recently used the chip TCAN4551, we found that the waveform would shake after adding the common mode inductor AMCW4532B-2-220T, and the waveform would be normal after shorting the common mode inductor with 0 ohm resistance. But this common mode inductor used in the TCAN1044 chip waveform is normal. So is it the configuration of the chip that causes the output current capacity of the chip to weaken, which leads to the oscillation of the waveform?

  • 0
    TI__Mastermind 33030 points

    Hi Huajian,

    There is not any ringing (or oscillation as you are calling it) in the Differential (Math) waveform for either of the two device plots you have shared.  Therefore the ringing seen with the TCAN4550 is not differential and will not impact the CAN communication or bit sampling.

    This type of ringing comes from the physical properties of the CAN bus itself and the impedance setup by the Resistance (R), Inductance (L), and Capacitance (C) of the components that make up the complex impedance.  I see that the termination resistors in your schematic have been uninstalled.  Is the bus you are testing with properly terminated with 2 sets of termination resistors?  Testing with an incorrectly terminated bus can create some reflections that will propagate along the bus and create some ringing.

    Are you using the same CAN bus for testing both transceivers?  Also, does the PCB use the same components and same PCB layout?  You can observe there is a little difference between the Differential input capacitance and resistance specs for the two transceivers, so some variance in the signal waveform is not unexpected.

    Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    Prodigy 70 points

    Because this part is not a terminal node, the terminal resistance is removed. When this component was tested separately, a 60ohm resistor was connected in parallel at the end of the cable. The same test method and test device is used for other component . But  the waveforms of other component do not ring. Maybe the PCB layout  is a little different. But I think the ringing is too bad, because the waveforms of other component are not so bad.  So is it the configuration of the chip that causes the output current capacity of the chip TCAN4551 to weaken, which leads to the ringing of the waveform?

  • 0 in reply to Huajian Li
    TI__Mastermind 33030 points

    No, the output current capacity of the chip does not weaken.  The ringing comes from changes in the characteristic impedance of the bus which is generally a ratio of the inductance and capacitance.  (Zo = √L/C).  For a CAN bus, the ideal impedance is Zo = 60 ohms. 

    When the impedance of all the components in the signal path are matched, then there will not be any reflections.  However when the signal crosses a change in impedance, then some of the energy is reflected proportional to the difference of the impedance.

    The TCAN4551 and the TCAN1044 are different transceivers with different source impedance, and there may be a difference between the capacitance and characteristic impedance of the boards and wires used in the different test setups.

    You may need a CMC with lower inductance to get a better impedance match.  Common values are 51uH or 100uH.

    Regards,

    Jonathan