ISL32458E/59E - Overvoltage Protected RS-485 Interface
This module focuses on fault protected, or more specifically, overvoltage protected (OVP) RS-485 transceivers.
This training module will:
- Explain what an OVP transceiver is
- Explain where and how overvoltage events occur
- Discuss whether the use of a standard transceiver in combination with external components can replace an OVP transceiver
Defining a Fault Protected Transceiver
An OVP transceiver provides protection against overvoltages at the bus terminals that can be far above and below the transceiver supply rails, Vcc and GND.
The upper and lower limits of the overvoltage range are specified as I/O voltage levels in the Absolute Maximum Ratings of the transceiver datasheet.
The overvoltage protection allows for all types of waveforms to be present, including fast transients, AC voltages, such as sine or square waves, and DC potentials, as long as the peak voltage level remains within the overvoltage protected range.
Data and DC Supply Networks in Industrial Automation
In industrial automation (IA) and process control, many analog and digital modules of programmable logic controllers (PLC), distributed control systems (DCS), motor and valve drives, and sensor/actuator interfaces operate from a standardized, nominal 24V DC supply. This supply is commonly distributed as a DC bus throughout the entire system.
Data exchange between these modules is carried out via industrial network protocols such as PROFIBUS, Modbus and INTERBUS—all of which utilize RS-485 as their physical layer.
For both economic and convenience reasons, the 24V bus and RS-485 bus are distributed through the same conduits. This means there is risk of short circuits between supply and data conductors in the case of insulation faults.
Reasons Overvoltage Faults Occur
When data and supply lines are sharing the same distribution network the number of causes for overvoltage occurrences increases. Often times a DC supply shares the same connector or screw terminal block with the data lines of an adjacent bus node circuit. As a result, miss-wiring faults can occur that connect one or more supply conductors with the transceiver bus terminals.
Another failure cause is the layout of the conduit, where sharp cable bends often violate the minimum cable radius specified for data and supply cables. Over time, the increased mechanical pressure on the cable will cause a break in the insulation, thus allowing potential for shorts between power and data lines. A similar situation holds true for machinery and equipment placed against a conduit—thus crunching the cable. The duration of these overvoltage events can last for minutes and days until their causes are eliminated.
Much shorter overvoltage events, such as overvoltage transients, can occur due to load switching activity in the power distribution system or as the result of lightning strikes, which induce high surge currents and voltages into the data lines.
Properly Designing with External TVS Devices
Engineers new to overvoltage protection often question whether adding powerful transient voltage suppressors (TVS) to a non-fault protected, standard transceiver can ensure sufficient protection against short and long term overvoltages. The answer is NO because the peak transient power a TVS can absorb decreases with increasing transient duration. This is shown in the peak pulse power versus pulse width characteristic above.
This diagram is taken from the datasheet of a 600W TVS rated at 1ms pulse width. Note that the time axis ranges from 10μs to 10ms with power levels dropping from 6000W down to 200W respectively. From this characteristic it becomes clear that exposing a TVS to long term overvoltages will burn up the device in an instant.
Therefore, to protect your bus nodes against the wide range of overvoltages you need fault protected transceivers, such as the ISL3245x family. These devices provide protection against long-term overvoltages of up to ±60V, and against transient overvoltages conforming to TIA/EIA-485-A, Section 4.2.6 (for 15μs at a 1% duty cycle) of up to ±80V.
Using Power Transistors to Protect Against DC Faults
Occasionally a question arises: Why not use external power transistors with high voltage breakdown to protect standard transceivers against DC overvoltages?
The answer is simple: A discrete solution consumes more cost, time, and space than an integrated OVP transceiver.
For example, let’s assume the half-duplex OVP transceiver on the left is to be replaced by a discrete design using a standard transceiver plus some external components.
Because the overvoltage protection for a standard transceiver must be implemented in the transmit and the receive paths separately, the design requires a full-duplex transceiver.
In the receive path, you must implement a discrete voltage limiter so the bus voltage during an overvoltage event remains transparent.
The output stage of the transmit path can then be ruggedized with four discrete transistors. Another option is an integrated H-bridge with control inputs that require the conversion from RS-485 bus signals into TTL or CMOS logic levels. This requires a drive logic circuit between the unprotected driver output and the discrete output stage.
The implementation of current limiters is required to limit the power consumption of an active output stage during overvoltage events.
Satisfying all of the above requirements makes the design of a discrete overvoltage protected transceiver solution absolutely foolproof.
Driver Stage with Double Fold-back Current Limiting
OVP transceivers with wide common-mode ranges require double fold-back current limiting within the driver stage. The above diagram shows the current limiting function of the ISL3245x family of OVP transceivers that operate over a ±20V common-mode range.
Here the first fold-back current level of 63mA ensures that the driver never folds back when driving loads within the 40V common-mode range. The very low second fold-back current setting of 13mA minimizes power dissipation if the driver is enabled when a fault occurs.
This current limiting scheme ensures that the output current never exceeds the RS-485 specification, even at the extremes of the common-mode and fault condition voltage range.
Adding Lightning Protection to OVP Transceivers
The energy of lightning strikes can easily exceed a transceiver’s fault protection and must be absorbed by external TVS diodes. Two conditions must be satisfied when adding external TVS devices to an OVP transceiver:
- The TVS breakdown voltage must be 1V higher than the highest common-mode voltage of the application or the maximum DC-supply, whichever is greater. For applications only exposed to the standard common-mode range of -7V to +12V, VBR-min ≥ 13V, for bus lines running adjacent to DC-power lines with 24V nominal supply, VBR-min should be ≥ 31V, as 24V systems are known for excursions of up to 30V.
- The peak clamping voltage of the TVS must be below the transceiver’s maximum fault protection levels: VCL-max < VOVP-max.
This slide shows the respective circuit and TVS switching characteristics with breakdown and clamping voltages, VBR and VCL, and compares them to the maximum common-mode range, DC-voltage, and fault protection levels.
3V/5V OVP Transceivers in Comparison
Intersil is the only supplier that offers OVP transceivers with wide supply and common-mode ranges at very attractive pricing.
Competing devices, such as Competitor T above, claim operation down to 3V yet already show an inferior output drive capability of VOD = 0.85V across a differential load of RL = 54Ω at VCC = 3.3V.
In strong contrast the ISL3245x family of OVP transceivers provides almost twice as much differential output voltage with VOD = 1.5V (Figure a).
Figure b shows the ISL3245x maintaining a VOD close to 1.5V across the standard common-mode range while providing high output drive towards the outer limits of ±20V.
Although specified for a much narrower common-mode range, Competitor T’s VOD comes nowhere near the 1.5V minimum (dotted line) for the entire range.
Furthermore, Competitor T doesn’t provide true 3V operation but stops operating at 3.15V (Figure c). This is only 5% below the nominal 3.3V level, thus calling for a much tighter output tolerance of the linear voltage regulator providing the transceiver supply.
The ISL3245x transceivers outperform the competition by operating down to a minimum supply of 2V, which ensures true 3V operation and allows for a wider tolerance of the regulator output voltage.
- A whitepaper discussing the causes of overvoltage faults and their remedy through integrated OVP transceivers.
- A datasheet listing the parametric differences between the individual devices of the entire ISL3245x OVP transceiver family.
- IBIS models, consisting of tables with current, voltage, and timing information. IBIS models simulate much faster than SPICE models, with only a slight decrease in accuracy. Non-convergence issues, often encountered with SPICE, are eliminated in IBIS simulation. IBIS models are provided for all of our transceiver products. They are easy to use and are freely available from our web site.
OVP Transceiver Portfolio
Our portfolio of OVP transceivers includes multiple families that provide fault protection from ±40V to ±60V. All transceivers operate over extended common-mode ranges which exceed the RS-485 minimum requirement.
Some device families offer 5V operation only with integrated hot-plug capability, while others simply provide a wide supply voltage range with true 3V operation.
Transceivers are available for data rates ranging from 0.25 up to 20Mbps and in packages from standard 8 and 14-pin SOIC to 8 and 10-pin MSOP.
Both devices are half-duplex transceivers supporting data rates of up to 20Mbps.
Performance Features of OVP Transceivers
Here is a detailed list of the Intersil OVP transceivers and their main features.
The ISL32459E is the world’s fastest OVP transceiver with cable-invert function, allowing the automatic correction of cross-wiring faults of bus nodes.
Note that Intersil’s patented cable-invert function is superior to that of competing devices because the receiver’s full fail-safe function is maintained. That means the receiver output turns high for VID = 0V, independent of the cable-invert pin’s logic state. Competing devices under the same conditions change their output states, depending on the logic state of their cable-invert pin.
ISL3245x OVP Transceiver Summary
What is so special with the ISL3245x family of OVP transceivers?
It offers the widest range of overvoltage protection, the widest common-mode operating range, and the widest supply range with true 3V performance.
It offers the widest choice of data rates and is the world’s only OVP transceiver family that provides a transceiver with cable-invert function.