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Details needed about ISO1050 and 3,3V CAN Transceivers

clement.pottier
Prodigy 20 points
Other Parts Discussed in Thread: ISO1050, SN6501, SN65HVD255

Hi,
I need precisions about CAN Transceivers:

- for the isolated CAN transceiver ISO1050, I was wondering why this transceiver does not feature any Standby/Sleep/Low Power Consumption Mode and if that meant that I must use the supply current needed when driving the bus recessive to calculate the power consumption of all my inactive/non-sending nodes.

- for the 3,3V CAN transceivers, I would like to know how I can find the minimum allowed load resistance: RL_MIN in order to calculate my maximal number of nodes and my maximal distance regarding voltage drop. Indeed, in all 5V transceiver datasheets that I have read, the minimum allowed load resistance is always stated with the differential output voltage for dominant state (generally, it's 45 ohms or 42,5 ohms). But in 3,3V transceivers datasheets, I haven't succeeded in finding this information.

Thanks in advance for your help

Best regards,
Clément

 

  • 0
    TI__Mastermind 20590 points

    Hi Clement,

    - ISO1050 is a basic isolated CAN transceiver. If you are simply receiving messages as in a normal CAN system then the recessive current of the ISO1050 is the appropriate number to use for non-sending nodes.   There is a very big difference between having low power modes (with or without wake up) and being in the recessive state.  If you use it with the SN6501 transformer driver (isolated power supply) which is controlled via a Vreg with EN/INH circuit you could essentially "shut off" the entire CAN transceiver side of the isolated solution, but that will also disable the receiver path.  

    - 3.3V CAN transceivers were developed prior to any implied load extension down to 45 ohms.  They were only characterized at the CAN standard's 60 ohm load.

    -- Scott

  • 0 in reply to Scott Monroe
    Prodigy 20 points

    Hi Scott,
    first, thank you for the rapidity of your awnser.

    - For the first point, I think I made a confusion between Stand/Sleep modes and Silent Mode (like on the SN65HVD255/6/7 devices), in which the driver is turned off while the receiver remains active (sorry for the mistake). So my question was in fact why does the ISO1050 does not feature any Silent mode.

    - For the second point, I would then like to know which value I should use for my calculations of the maximal number of nodes on a segment
    ( nMAX=1+Rdiff_MIN*((1/RL_MIN)-(2/RT_MIN)) ) ;
    and for the maximal length regarding voltage drop (maximal length of a segment) when using a 3,3V transceiver;
    or if there is another method to calculate these parameters for these transceivers.

    Once again, thanks for your help,
    Clément

  • 0 in reply to clement.pottier
    TI__Mastermind 20590 points

    Hi Clement,

    - The ISO1050 is a basic transceiver and due to the extra cost and power of a 3rd isolated channel to control the mode it doesn't offer one.  There is a device in development, the ISO255B which will offer the S pin on the isolated side of the DW package on pin 14 which is a NC on the ISO1050DW.  Depending on your timing you could plan for that but to make use of the S pin you will need to add another isolation channel such as ISO721D to get the control signal from the MCU over to the isolated transceiver side of the ISO255B.  There is also the SN6501 transformer driver to provide the option of an isolated power supply for the ISO1050 or ISO25xx parts. 

    - 3.3V and number of nodes.  The datasheet features lists this as 120 nodes.  This is based on the Driver Electrical Characteristics spec VOD(D), Differential Output Voltage Dominant with a full common mode range of -2V to +7V via a 167ohm coupling network as shown in figure 2 and 3. A few paragraphs describing this are below.  Using additional calculations of 120 nodes in parallel (40k ohm min) on each plus 2 60 ohm termination resistors you can get to a minimum load from this view point of about 50 ohms to maintain full driver & receiver operation over the common mode range, temp range, Vcc voltage range, etc.

    BUS LOADING, LENGTH AND NUMBER OF NODES:

    The ISO11898 Standard specifies a maximum bus length of 40m and maximum stub length of 0.3m with a maximum of 30 nodes. However, with careful design, users can have longer cables, longer stub lengths, and many more nodes to a bus. A high number of nodes requires a transceiver with high input impedance such as the SN65HVD23x CAN family.

    Many CAN organizations and standards have scaled the use of CAN for applications outside the original ISO11898 standard. They have made system level trade offs for data rate, cable length, and parasitic loading of the bus. Examples of some of these specifications are ARINC825, CANopen, CAN Kingdom, DeviceNet and NMEA200.

    A CAN network design is a series of tradeoffs, but these devices operate over wide –2-V to 7-V common-mode range. In ISO11898-2 the driver differential output is specified with a 60Ω load (the two 120Ω termination resistors in parallel) and the differential output must be greater than 1.5V. The SN65HVD23x devices are specified to meet the 1.5V requirement with a 60Ω load, and additionally specified with a differential output of 1.2V (ISO11898 minimum at all nodes receiving) across a common mode range of –2 V to 7 V via a 167Ω coupling network representing the bus loading of additional transceivers. The differential input resistance of the SN65HVD23x is a minimum of 40KΩ (20kΩ single ended), thus being approximately equivalent to 120 SN65HVD23x transceivers in parallel on a bus while the minimum differential voltage at each node is met across the defined common mode range. Therefore, the SN65HVD23x theoretically supports up to 120 transceivers on a single bus segment with margin to the 1.2V minimum differential input requirement at each node. However for CAN network design margin must be given for signal loss across the system and cabling, parasitic loadings, network imbalances, ground offsets and signal integrity thus a practical maximum number of nodes is typically lower. Bus length may also be extended beyond the original ISO11898 standard of 40m by careful system design and data rate tradeoffs. For example, CANopen network design guidelines allow the network to be up to 1km with changes in the termination resistance, cabling, less than 64 nodes and significantly lowered data rate.

    This flexibility in CAN network design is one of the key strengths of the various extensions and additional standards that have been built on the original ISO11898 CAN standard. In using this flexibility comes the responsibility of good network design.