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THVD1500: Unregulated supply for isolated RS-485 circuit

Greg
Expert 2031 points
Part Number: THVD1500

How important is it to use a regulated supply for RS-485?  I am planning on using an unregulated isolated DC-DC supply for a basic low-speed RS-485 circuit.  The power supply noise would be greatest when the data line switches state, which could be mitigated by including a fair amount of decoupling capacitance on the supply.  I plan on using a 100 to 120-Ohm termination resistor at each end.  Since the data is asynchronous and the data rate is low, the switching noise shouldn't interfere with the data transmission since the data sampling point is in between the switching events.  Am I losing very much noise margin by using an unregulated supply?  At worst case, assume the supply could range from 5.5V at no load, to 4.7V at full load.

Thanks,

Greg

  • 0
    TI__Guru 62920 points
    Greg,

    The THVD1500 transceiver will meet RS-485 standard requirements across the full specified VCC range (4.5 V to 5.5 V), and so I would not expect any issues with respect to communication (even if the supply voltage varies dynamically during operation).

    One thing to consider is that noise on the power supply tends to couple through to the "high" half of the differential output (assuming the driver is enabled) but not the the "low" half. Since the noise isn't perfectly balanced, it is more prone to radiate or conduct to other system components. So, if EMI is a concern in your application I'd recommend to make sure the power supply is well-filtered across the frequency range of interest. (This may be as simple as a decoupling capacitor, as you've mentioned, or may require something more like ferrite bead or CLC pi filter to augment the decoupling capacitance - it depends on the power supply implementation and EMI targets for your application.)

    Also it may be useful to note that the biggest change in supply current occurs when the driver is enabled; it doesn't change much when switching output states (high/low). So, if a change in current induces a problematic amount of noise on the power rail, you might choose to implement a delay from when a given node activates its driver to when it actually uses it to communicate on the bus (i.e., to allow some time for the noise to die off).

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Expert 2031 points
    Hi Max,

    Thank you very much for your detailed answer to my question. I enjoyed reading what you wrote about the EMI potential for noise radiating through the "high" half of the differential output. Fortunately I don't think I'll have to worry about it very much because it will be connected to a Variable Frequency Drive which will likely be kicking out much more radiation than the RS-485 line.

    I will experiment with the decoupling capacitance needed to tame the driver enable event. Thank you very much for your help in confirming the circuit should function well without a regulator.

    Regards,

    Greg