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ISO1042: CAN series current limiting resistor value

BOOM
Prodigy 145 points
Part Number: ISO1042

Tool/software:

I am using ISO1042QDWRQ1 for CAN communication. I have been using a series current limiter resistor of 100E/0.75W on the output CAN_H and CAN_L. I also have used split termination for 120E impedance (2 x 60.4 ohm). When I measure the total impedance on CAN_H and CAN_L, it comes out to be 320 -330 ohm, adding the series resistor values too. The resistor values were not calculated as such and is way beyond the suggested termination impedance of 120-130ohm.

Referred this note from TI : www.ti.com/.../slla486a.pdf

I got this on Page 6 of the note:
A low value series resistors (~10 ohm) can be used on both CANH/CANL lines in case lightning surges (tested per IEC61000-4-5) are expected on the cable to reduce the current that flows to internal clamping structures of the device during surge events. These series resistor will attenuate differential signal as it forms a divider with the 60 ohm loading on the bus.
  1. Any calculation for why 10 ohm?
  2. How do I calculate the resistor wattage?
  3. My total termination will become 120+10+10=140 ohm. J1939 standard limits the resistance value to max 130 ohm. Is it okay?
  4. Is there any reference formula to calculate the series resistors that should be used for any such interfaces RS232, RS422, CAN?

  • 0
    Guru 317260 points

    1. It's pretty much a rule of thumb. 10 Ω does not affect the signal too much.

    2. This depends on what overvoltage you expect.

    3. The values of the series resistors behind the termination resistor cannot simply be added. (For a receiver, you can pretty much ignore them; for a transmitter, they must be small enough to not limit the current too much.)

    4. For RS-422/485/CAN, just use 10 Ω or less. RS-232 transceivers have the 5 kΩ termination built in, and usually are used without series resistors.

  • 0 in reply to Clemens Ladisch
    Prodigy 145 points

    Thanks Clemens. 

    Can you also help me with how to calculate the wattage of these resistors? Any equations or notes that can help?

  • 0 in reply to Clemens Ladisch
    Prodigy 145 points

    Also,

    What is the recommended series resistance value from any MCU (STM32 3.3V) to ISO1042QDWRQ1 TXD and RXD pins? 

  • 0 in reply to BOOM
    TI__Mastermind 29375 points

    Hello Joseph, 

    In general, you would not need a series resistance for the logic I/O pins for a short distance connection to the MCU. For long PCB traces, series resistances are also around 10-ohms.

    As for the power rating of the surge resistors, you will need to know the surge, voltage, current and deration to select a surge-proof resistor. These are specific resistors that are designed to meet IEC61000-4-5. You can look at the TI reference design TIDA-010253's schematic and BOM which uses a surge-proof resistor (a 0-ohm jumper is in this design, but the resistor datasheet provides a curve to select a resistor based on the transient's power). The following app note goes over this surge protections and resistors in detail IEC ESD, EFT, and Surge RS-485 Bus Protection Design Guide (Rev. B).

    Best,
    Andrew