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Discrete custom design of RS485 driver

Discussion in 'Electronic Design' started by Klaus Kragelund, Dec 21, 2012.

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  1. Hi

    The standard RS485 drivers available has a minimum voltage of 3V and a rarther large drop voltage when loaded with the defined bus load for Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the bus

    So, we are thinking about designing our own driver in discrete components, so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

    We only need 115k baud, so we could use a tiny logic level FET as the output stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

    Backfeed would need to be solved with a beefy diode to a defined clamp voltage.

    So, anyone been down this road, designing your own RS485 driver?


  2. A rough first draft:
  3. Guest

    This is not a Modbus specific issue, but rather RS-485 specific issue
    with a twisted pair bus with characteristic impedance of 100-120 ohms.
    In order to avoid reflections at the open ends of the bus cable,
    termination resistors are typically used at both ends with the same
    value as the cable characteristic impedance.

    For DC, those two resistors are effectively in parallel and hence the
    45 ohm total load.

    However, those termination resistors are needed only to avoid the
    reflections from voltage _transitions_. Thus, putting a capacitor in
    series with the termination resistor(s) should reduce the idle power
    consumption, when no data is being sent. Of course, without DC
    continuity, the end to end signal ground conductor is essential.

    There are application notes describing even more elaborate termination
    methods, describing their advantages and disadvantages. You should
    also look for various termination techniques used on CAN bus (which is
    essentially RS-485).
  4. Joerg

    Joerg Guest

    Just a comment: Diodes are already in the FETs, in the form of body diodes.

    One thought would be whether a hysteretic sync-buck IC could be pressed
    into service here. I haven't needed one this low in voltage yet but they
    should come for very low supply voltages (processor core supplies and such).
  5. The voltage rail for the FET are driven by 2V and I will generate an additional supply voltage to drive the gates, about 3V.

    The majority of the power goes for the bus, driving the 54ohms load (120//120//1500 ohms in parallel, that is two termination resistors and the 32 unit load impedance).

    Right now the implementation is using a standard RS485 driver running at 3Vsupply, but with 54 ohms resistance along with the driver impedance, draws90mW during transmission.

    A low RDSon driver at 2V would reduce that to about 60mW


  6. Yes, I added parallel more sturdy diodes, to direct the current away from the low current body diodes.


  7. Yes, but to conform to the Modbus standard, the termination resistors are added without diodes


  8. Maybe a good point, if I can find a logic device that has low RDSon at 2V.

    The ones I have found have 10ohms RDSon (NC7SZ74), but could parallel some of those to bring down the RDSon to the 2-3 ohms range


  9. Tim Williams

    Tim Williams Guest

    Are you expecting huge common mode transients? MOSFET diodes have been
    rated at, or above, the channel current for ages. FDV301N says 0.29A
    diode, 0.22A channel (both I'm sure depend on thermal resistance, it's
    only an SOT-23). I've never used external diodes in an inductively loaded
    inverter and never found any reason to: the body diodes do a fine job.
    They just aren't good at hard switching (slow recovery).

    Have you considered BJTs for this? They tend to be easier to drive at
    lower voltages. With Vceo as low, you can easily find fast transistors
    with high hFE, so even with saturated operation, you don't have to worry
    about switching speed or error in the current source. You may still need
    a bootstrap (using all NPNs, or a negative bootstrap for the PNP pair),
    but only one at least.

    The TL431 as shown clamps about 5V, which is way more than your supply --
    are you sure about this? If it's for ESD, it's only clamping 100mA, and
    takes a moment to respond. A zener TVS would be a bit sloppier (a 3.3V
    rated device might break down at 5V and carry a heavy load at, say,
    8V...), but much faster and more robust. You could also use a diode back
    to the +2V supply, which is probably as transient-resistant.

  10. Yes, the RS485 line is subjected to hot swapping, termination resistors inserted "live" and must be tested against surges/bursts. I am also worried about injected DC voltages from user wrongful installation.

    The big diodes is used to divert current to the clamp using the 1ohms resistor to allow for the external diodes to draw the biggest portion of the current.
    Yes, could be a good idea, just need to add circuitry to draw the carriers out of the base to switch them off fast.


  11. Joerg

    Joerg Guest

    Usually they are about as sturdy as the channel in the FET, can take a
    similar current.

    Diverting current away from those only works (to some extent) if you
    hang a Schottky of sigifnicant size in parallel. The best method would
    be to steer the FET conductive while some massive surge current shows up
    for some reason.
  12. Guest

    What diodes ? I was suggesting using capacitors.

    What Modbus "standard" ?
    The closest that I can think as electric Modbus standard is the
    "MODBUS over serial line specification and implementation guide V1.02"

    Look at page 28
    Polarization = "Fail safe termination" in RS-485 speak.
  13. Joerg

    Joerg Guest

    Why that? They are often used as regular power current paths. The
    current rating is roughly the same as the FET itself, usually.

    With RF stuff all bets are off, RF transistors can be like the princess
    on the pea. 3V reverse Vbe ... poof ... gone. Or gaoan, as they'd say at
    one client.

  14. I meant to write capacitor, sorry :)
    Yes, the Modbus standard defines that, but the widespread industry standard is 120 ohms and no capacitor. (adopted from the RS485 standard)


  15. Yes, I need to tristate the driver, since it is a 2 wire system, half duplex.


  16. We would need to add protection in any case to reduce the currents, the driver IC would suffer from latchup problems if not.


  17. Tim Williams

    Tim Williams Guest

    And that's not even that big of a deal, really -- if you run 2N4401/3 kind
    of hot (~20mA Ic, 2mA base drive, 680 ohm B-E resistor), you'll see edges
    under 100ns and storage time under 300ns (storage causes skew, but it's
    symmetrical in an H-bridge, so it causes shoot-through and delay).
    115kbaud gives you almost 10us between edges, so there's tons of time for

    The average switching transistor (like the little complementary gate drive
    things, or just a plain old ZTX651 or etc.) is even beefier, maintaining
    hFE > 100 at rated collector current. So, even saturated (where hFE is
    lower and stored charge piles up), they don't take much drive current at
    all, relative to what they're doing. They start looking like low Vgs(th)
    MOSFETs, with an input diode thrown in for convenience.

  18. Joerg

    Joerg Guest

    What's the difference? Whether 0.7V drops across a more or less
    resistive path, who cares? All that counts is total dissipation and that
    it's not too localized.

    I sure hope we can at least keep high-end semiconductor engineering and
    processing in the country for a while, now that Obamacare is smothering
    much of the med device investment climate. We can't afford to lose such
    leadership positions but obviously that doesn't sink in on the hill :-(
  19. rickman

    rickman Guest

    I would love to hear some sort of rational explanation of how that is

  20. Joerg

    Joerg Guest

    Interesting. Mine is still mostly on US soil, or all of it right now.
    But medical has nearly evaporated from a consulting point of view, from
    industrial, oil, gas, aerospace and so on. That does not bode well for
    med-tech in our country.
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