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Original question:

TCAN4550-Q1: Difference between TCAN4550 and TCAN4551

TCAN4550-Q1: Doubts based on the schematic of the TCAN4550 Evaluation Module

Daniel Flor
Intellectual 430 points
Part Number: TCAN4550-Q1
Other Parts Discussed in Thread: TCAN4550

Hello,

I'm designing an automotive scan tool. I intend to give it the ability to communicate via the CAN-FD protocol. I chose the TCAN4550 CI for this. To have a circuit design reference, I took the board "TCAN4550 Evaluation Module" described in the document sllu281a.pdf, as an example.In my project, I will use a microcontroller that has 5V logic.

Before to implement TCAN4550, I would like to clarify the following doubts :

1) On the "TCAN4550 Evaluation Module" board described in the document sllu281a.pdf, there are the "Signal GND" and GND planes.In section 2.2.3 is said that a shunt can be placed on jumper J15 and a DC               signal can be injected into the "Signal GND" plane.

  1.1)How can I know if is necessary to inject DC signal and where would this DC signal come from? Could I remove this jumper and replace it with a permanent short between the GND plane and the Signal GND                plane, keeping the capacitors of course?

2) As I understand it, the voltage value present on the VIO pin will be the voltage value used on the SPI, I/Os, GPIO1, GPIO2 and oscillator block pins.

  2.1) So, I would like to know if, in my case, where the logic will only be in 5V, will I necessarily need to put, at VIO, the same 5V that feed the MCU, or it is not necessary? In other words, for interface with 5V logic,               can I just connect VIO to a 100nF capacitor and nothing else?
  2.2) Will the 40MHz Crystal work if I do not put voltage at the VIO pin?

3) On the board "TCAN4550 Evaluation Module", document sllu281a.pdf, the split termination technique(two resistors of 60 Ohms to reache the 120 Ohms)  is used between CANL and CANH lines, jumpers are used to use this termination. However, when I finish my project, the user will not be able to open the product to remove or place a shunt at the jumpers, so my question is, can I leave this termination permanent, I mean, with a short instead the jumper?

4) Pin INH of the TCAN4550  is used to enable the VIO voltage at the external regulator. I intend to leave VIO fixed at 5V (this, if it will be really necessary), so, I will have no need for the INH pin, could I leave INH pin  floating or else just connected to a pull-down resistor?

Sorry for so many questions.

Regards,
Daniel.

  • 0
    TI__Mastermind 33140 points

    Hi Daniel,

    Thank you for your interest in the TCAN4550! 

    1) On the "TCAN4550 Evaluation Module" board described in the document sllu281a.pdf, there are the "Signal GND" and GND planes.In section 2.2.3 is said that a shunt can be placed on jumper J15 and a DC               signal can be injected into the "Signal GND" plane.

      1.1)How can I know if is necessary to inject DC signal and where would this DC signal come from? Could I remove this jumper and replace it with a permanent short between the GND plane and the Signal GND                plane, keeping the capacitors of course?

    I'll start off saying that the TCAN4550 EVM has features that were intended to make test measurements easier in lab evaluation that are not needed in a final application design.  One of these additional features is the separate signal and power ground planes.  It can be common to have a DC voltage offset between the GND potentials on different nodes along a wiring harness.  This offset voltage can shift the common mode voltage of the CANH/L signals and lead to EMI and other problems.  The EVM has a separate GND plane for the signals that allows a DC offset voltage to be supplied by test equipment to allow the user to test for communication and EMI errors in their system.  However, in a real application, there is no real benefit to having separate planes and you only need to have one GND plane in your design for all GND connections.

    2) As I understand it, the voltage value present on the VIO pin will be the voltage value used on the SPI, I/Os, GPIO1, GPIO2 and oscillator block pins.

      2.1) So, I would like to know if, in my case, where the logic will only be in 5V, will I necessarily need to put, at VIO, the same 5V that feed the MCU, or it is not necessary? In other words, for interface with 5V logic,               can I just connect VIO to a 100nF capacitor and nothing else?
      2.2) Will the 40MHz Crystal work if I do not put voltage at the VIO pin?


    The TCAN4550 requires two supply voltages to operate properly.  The VSUP which is considered a High Voltage supply is the automotive battery potential and must be greater than 5.9V.  The VIO is the Low Voltage Digital IO supply and it is usually either 3.3V or 5V to match the MCU operating voltage.  You need to supply both supplies in order for the TCAN4550 to operate properly.  Therefore since you are using a 5V MCU, you will need to supply 5V to the VIO pin and we recommend a 100nF decoupling capacitor on the TCAN4550 pin to help with noise and supply current stabilization.  And since the Clock Oscillator circuit is supplied by the VIO rail, the 40Mhz Crystal will not work when VIO is NOT supplied.  But it is OK and typical to use the same 5V supply voltage for both the MCU and VIO which ensures they are always present at the same time.

    3) On the board "TCAN4550 Evaluation Module", document sllu281a.pdf, the split termination technique(two resistors of 60 Ohms to reache the 120 Ohms)  is used between CANL and CANH lines, jumpers are used to use this termination. However, when I finish my project, the user will not be able to open the product to remove or place a shunt at the jumpers, so my question is, can I leave this termination permanent, I mean, with a short instead the jumper?

    A functional CAN bus requires a 120ohm termination resistance at each end of the wiring harness, but not at every node.  Just like with the GND planes, termination resistance has been added to the EVM to allow test measurements in a lab to be done by simply connecting two EVMs together.  If you added a third evm or connected the EVM to an existing CAN bus that already had the required termination, then you would not want to have the termination on the EVM and it can be removed from the circuit.  Since you are designing a scan tool that I am assuming will be connected to an existing CAN bus, you will not want to have termination resistance included in your module because the CAN bus you are connecting to will already have the proper termination at each ends of the wiring harness.  You may want to include a single resistor pad between the CANH and CANL pins of your module that could allow a 120ohm resistor to be installed if needed, but if my assumptions are correct, you would not want the 120ohm resistor installed by default.  I find it is always good to have an option to install it later if you need to do lab testing or debug on your design.

    4) Pin INH of the TCAN4550  is used to enable the VIO voltage at the external regulator. I intend to leave VIO fixed at 5V (this, if it will be really necessary), so, I will have no need for the INH pin, could I leave INH pin  floating or else just connected to a pull-down resistor?

    If you do not need to use the inhibit feature of the device, you can leave the INH pin floating.  I would not connect a pull up or pull down resistor to it if unused because that will only draw unnecessary current.

    I hope these are clear answers to your questions. 

    Regards,

    Jonathan

  • 0
    Intellectual 430 points

    Thank you very much! Your answers will help me to design my prototype and final product. But I have one more question, about the choke , it is present in the layout datasheet at section 11.2, but it is as DNI, (do not install) in the EVM sllu281a.pdf schematic. So, do I need to use it ?

  • 0 in reply to Daniel Flor
    TI__Mastermind 33140 points

    The common mode choke is not required for the TCAN4550 to function properly.  Many automotive and OEM customers have requirements for chokes in their systems and it is very common to find them in designs.  Therefore there is an option to add a common mode choke to the EVM if a customer would like to evaluate the performance of their system and the TCAN4550 with the choke of their choice. 

    The reason to use or not use a common mode choke is based on the system requirements and not on the TCAN4550's specifications.  As I said, the TCAN4550 does not require a choke to function and the datasheet specifications are not based on the device itself.  But because I am not familiar with your particular system requirements or application, I cannot speak to what the system level requirements you have may be.  You should determine what system level requirements, testing, and certifications your product will need to pass. 

    If those certifications and tests require a choke then you will need to add one in your design.  If they do not, then you can probably safely leave it out.  If you are unsure, you could always place the footprint for a common mode choke on the board and install two 0 ohm resistors across the terminals as is shown on the EVM schematic.  If you chose to follow the example in the EVM, you do not necessarily need to use the large resistors (physical package size) because those were used to allow for a lot of other tests such as power shorts on the bus that might cause large amounts of current to pass through the resistors and I didn't want them to become damaged for those sorts of tests.  You could use standard 0402 or 0603 package sizes depending on your preference.

    Regards,

    Jonathan