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Pull Up Resistor for SN65HVD251D

frank wong
Prodigy 150 points
Other Parts Discussed in Thread: ISO1050, ISO7521C

I recently replacing ISO1050 with ISO7521CDWR and SN65HVD251D for higher isolation voltage rating.

Would you please advice what value of pull up resistor we should use for CAN TXD to have a fault tolerant network?

Do we need other pull up or pull down resistors on this isolated transceiver combination?

Thank you very much,

Frank Wong

  • 0
    TI__Mastermind 21821 points

    Hi Frank,

    I am a little confused by your statement of switching away from the ISO1050 to the ISO7521C due to isolation ratings. If you were previously using the ISO1050DUB device, then my first recommendation would be to move to the ISO1050DW device. The isolation rating changes from 2500VRMS for the DUB package to 5000VRMS with the DW package.

    If you go with the ISO1050, which is a fully integrated device, you don’t have to worry about failsafe output states, glitch filters, external pull-up resistors, or propagation delay through the isolator channels.  

    However, if you still prefer the discrete solution, I would recommend a pull-up resistor of 4.7kΩ on the TXD pin between the isolator and the CAN transceiver. It looks like the ISO7521C device that you choose has failsafe high outputs which is very important in CAN due to the fact that logic high on the I/O pin in CAN signifies a recessive level on the bus, and if they failsafe logic low, it could indicate a wake-up request or start of frame in the CAN controller on the RXD path. The ISO7521C device has a low enough propagation delay that it shouldn’t cause any signaling issues either.

    The only other thing that I would be concerned about is whether the ISO7521C device has a glitch filter. I will need to check with my team to see if one was added to this family.

    Let me know what you think about the feedback above, and I will check about the glitch filter.

    Thanks.

    John

  • 0 in reply to John Griffith
    TI__Mastermind 21821 points

    Hi Rick,

    I just heard back from an engineer that supports the isolator products, and they verified that the ISO7521C device does not have a built in glitch filter.

    This means that in cases where there is a small pulse (2 edges very close together in time) on the RXD pin that feeds into the isolator input, the isolator may see the first edge, and miss the second edge. This is due to the bandwidth of the receiver, and can cause the device to output the incorrect state. Therefore, we recommend a minimum pulse width of 1µs, which should be fine for all standard CAN applications.

    Thanks,

     John