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Volume 48 – July 2014

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Designing Robust, Isolated I2C/PMBus Data Interfaces
for Industrial, Telecommunications, and Medical Applications

By Maurice O'Brien

Introduction
A key requirement for industrial and instrumentation (I&I), telecommunications, and medical applications is a reliable interface for transmitting data. The Inter-Integrated Circuit (I2C) bus is a 2-wire, bidirectional bus used for low-speed, short-distance communication between integrated circuits. Developed by Philips in the early 1980s for ICs on a single board, I2C usage is still increasing. The power management bus (PMBus), a relatively slow 2-wire communications protocol based on I2C, is targeted at digital management of power supplies. The PMBus protocol defines an open-standard, digital-power-management protocol that facilitates communication with a power converter or other connected device.

Figure 1 shows how an isolation barrier galvanically isolates the I2C interface from each system connected to it, allowing digital data to travel between two points but preventing the flow of ground current; this reduces signal distortion and errors by removing noise that gets coupled onto the communications bus.

Figure 1. Isolation Function.

PC boards used in telecommunications applications often include digitally controlled power converters and circuits that operate at different ground potentials. To ensure trouble-free card insertion/removal and robust operation, each interface must be isolated, but isolating I2C interfaces is complicated because the bus is bidirectional. This requirement is not compatible with optocouplers, which are unidirectional. Figure 2 shows a PMBus communications link that isolates the ADM1075 –48-V hot swap and digital power monitor on the primary side from the secondary side, which operates with 12-V and 3.3-V supplies. The ADM3260 dual I2C isolator with dc-to-dc converter isolates the SDA and SCL signals. Its isolated power supply (3.3V_ISO) powers the ADuM3200 2-channel digital isolator that isolates the SHDN and RESTART signals.

Figure 2. Typical isolated PMBus communications link.

Isolation is required because the primary side is referenced to –48 V, while the secondary side is referenced to ground in a low-voltage domain. Isolation prevents permanent damage that could occur if the I2C port was inadvertently connected directly to the –48-V supply. Isolation also provides protection against high voltages or currents caused by the line surges or ground loops that can occur in a system with multiple grounds. The isolated power channel (3.3 V_ISO) allows the primary-side circuitry to be powered from the secondary side, removing the requirement for a separate low-voltage power source, which is not commonly available in the –48-V domain and is problematic to generate. All additional I/O signals crossing the isolation barrier require isolators that can also be powered by the ADM3260. To achieve a robust data communications link, each I2C device connected to the I2C bus must be isolated.

Examples of isolated I2C applications include:

  • Isolated I2C, SMBus, or PMBus interfaces
  • Level-translating I2C interfaces for power supplies
  • Networking
  • Power-over-Ethernet
  • Central office switching
  • Telecommunication and data communication equipment
  • Isolated data acquisition systems
  • −48 V distributed power systems
  • −48 V power supply modules

It is often necessary to bring data from a precision converter (ADC or DAC) across an isolation barrier via an I2C bus. Figure 3 shows two isolated data-acquisition systems. These applications also require an isolated power supply to power the converters and amplifiers on the secondary side.

Figure 3. (a) Isolated I2C ADC and amplifier, and (b) Isolated I2C DAC and amplifier.

Some applications require channel-to-channel isolation, where each channel is isolated from every other channel, as shown in Figure 4.

Figure 4. Channel-to-channel isolated I2C ADC and amplifier.

In larger systems, level translation is required between different voltage domains. An example of this is isolating the PMBus on each line card in a telecommunications rack-mounted system. Figure 5 shows a typical telecommunications application featuring multiple line cards that can be inserted into a –48-V backplane. In this application, the isolators level shift the I2C logic signals from the –48-V backplane to the fully isolated +12-V system.

Isolated power for the I2C communication link is obtained by using an isolated dc-to-dc power supply or isoPower® integrated dc-to-dc converter technology from Analog Devices. Signal isolation is implemented using optocouplers or iCoupler® technology from Analog Devices.

Figure 5. Isolating and level translating PMBus signals in a –48-V application.

Implementing an Isolated I2C Interface
Bidirectional data must pass between an intelligent device (such as an ADC or DAC) on the primary side to a processor on the secondary side and power must pass from the primary side to the secondary side. To isolate a data link, the data lines and the power supply must all be isolated. For an I2C link, all connected devices must be isolated from the I2C bus, as shown in Figure 6.

Figure 6. Isolated I2C interface.

The Challenge of Isolated I2C Interfaces
Because the I2C interface is bidirectional, providing isolation while avoiding bus glitches and lock-up can be a challenge. Figure 7 shows an optocoupler-based interface. Optocouplers are inherently unidirectional, so each bidirectional I2C line must be split into two unidirectional lines. Isolating a complete I2C interface requires four optocouplers and several passive components. The resulting cost, PC board area, and complexity diminishes the inherent value of the otherwise simple, low-cost, 2-wire I2C interface. Note that an isolated power supply is also required.

Figure 7. An optocoupler-based I2C interface.

Isolation Technology: Data and Power
Figure 8 compares two principal isolation technologies. iCoupler technology (a) uses thick-film processing techniques to build microscale on-chip transformers that achieve 2.5-kV isolation. The older, but widely employed, optocoupler solution (b) uses light-emitting diodes (LEDs) and photodiodes. The LEDs convert electrical signals to light, and photodiodes convert the light back to electrical signals. The intrinsically low conversion efficiency for electrical-to-light conversion leads to relatively high power consumption, the slow response of photodiodes limits their speed, and aging limits their lifetime.

Using wafer-level processing to fabricate on-chip transformers allows low-cost integration of iCoupler channels with each other and with other semiconductor functions. One example is the ADM3260 hot swappable, dual I2C isolator with integrated dc-to-dc converter. iCoupler isolation overcomes the limitations imposed by optocouplers in many ways: these easy to use devices reduce overall solution size, system cost, and power consumption, while increasing performance and reliability. In addition, iCoupler technology does not suffer performance degradation caused by current transfer ratio (CTR) aging of standard optocouplers over time and iCoupler is bidirectional technology, whereas optocoupler technology is inherently unidirectional.

Figure 8. Isolation technologies compared: (a) iCoupler isolation. (b) Optocoupler isolation.

Until recently, creating a low-voltage supply on the isolated side required either a separate dc-to-dc converter, which is relatively large and expensive, or a custom discrete circuit as shown in Figure 9. These approaches were the only viable alternatives, even for I2C data communication or other applications requiring only a small amount of isolated power.

Figure 9. Discrete –48-V-to-5-V power solution to power isolators.

To solve this problem, Analog Devices developed a complete, fully integrated solution that combines signal and power transfer across an isolation barrier using microtransformers. An extension of the well-established iCoupler technology, isoPower is a breakthrough alternative. Achieving up to 5 kV signal and power isolation within a single component, it eliminates the need for an isolated power supply, and significantly reduces PC board area, design time, and total system cost for a typical I2C bus.

Dual I2C Isolators with Integrated DC-to-DC Converter
Figure 10 compares PMBus isolation using discrete components with a fully integrated solution. The discrete approach requires four optocouplers for isolation, an isolated power supply, and complex analog circuits to prevent latch-up and suppress glitches. The isolated power supply uses a transformer driver IC to drive a discrete transformer, along with a simple rectifier and low-dropout regulator to clean up the isolated rail. This design requires eight ICs and several passive components, and burdens the interface with higher cost, increased PC board area, and lower reliability.

The integrated solution provides a fully isolated bidirectional I2C interface and isolated power with a single IC, plus the decoupling capacitors and pull-up resistors associated with any I2C interface. The ADM3260 is free of glitch and lock-up issues, has UL approved 2.5-kV rms-isolation ratings, and is offered in a 20-lead SSOP package. It provides bidirectional isolated data and clock lines and isolated power without the size, cost, and complexity of optocouplers.

This single-chip solution significantly reduces the cost, design time, and PC board area required for an isolated I2C interface, while enhancing reliability. It operates from 3.3-V or 5-V supplies without modification, avoiding the design changes that would be necessary with a discrete design, and provides 150 mW of output power at 5 V or 65 mW at 3.3 V, allowing it to power ADCs, DACs, or other small systems on the isolated side.

Figure 10. Isolated I2C designed compared: (a) Discrete solution and (b) Integrated solution.

Transient Protection
To allow the isolated interface to operate under the harsh operating conditions found in industrial applications, iCoupler and isoPower isolation technologies provide >25-kV/μs common-mode transient immunity. This specifies the maximum slew rate on the rising and falling edges of the potential difference between primary and isolated sides, ensuring that transients coupled onto the bus will not damage devices connected to the bus or corrupt the transmitted data and enhancing the reliability of the data link.

2.5-kV Isolation Protection and Approvals
The isolated solution specified 2.5-kV rms isolation between the primary and isolated side of the device. This isolation rating ensures that current can’t flow from the primary side to the I2C bus, and that voltages or transients coupled onto the bus won’t reach the logic side. The 2.5-kV isolation protection also means that people and equipment on the logic side are protected from high voltages or transients on the bus side. Approval is pending for the 2.5-kV isolation rating of the ADM3260 at the following agencies: Underwriters Laboratories (UL), Verband Deutscher Elektrotechniker (VDE), and Canadian Standards Association (CSA). UL 1577 approval requires the isolation barrier of all devices to be 100% production tested. The ADM3260 provides:

  • UL recognition
  • 2500 V rms for 1 minute per UL 1577
  • VDE certificate of conformity
  • IEC 60747-5-2 (VDE 0884, Part 2)
  • VIORM = 560 VPEAK
  • CSA Component Acceptance Notice #5A

PCB Layout
Proper PCB layout is critically important to ensure that the specified 2.5-kV isolation is achieved in an actual design. The principal considerations are creepage (shortest distance along the surface between two conductors) and clearance (shortest distance through the air) between the logic-side GND and the bus-side GND. The ADM3260 requires no external circuitry for its logic interfaces. Power-supply bypassing is required at the input and output supply pins, as shown in Figure 11. Further information on PCB layout guidelines for controlling electromagnetic interference (EMI) can be found in AN-0971 Application Note, Recommendations for Control of Radiated Emissions with isoPower Devices.

Figure 11. ADM3260 recommended printed circuit board layout.

ADM3260 Applications and Benefits
The ADM3260 hot swappable isolator provides both data and power isolation. Two nonlatching, bidirectional communication channels support a complete isolated I2C/PMBus interface, and an integrated dc-to-dc converter provides up to 150 mW of isolated power at 3.15 V to 5.25 V. The bidirectional channels eliminate the need for splitting I2C/PMBus signals into separate transmit and receive signals for use with standalone optocouplers, and the integrated dc-to-dc converter enables a complete isolated I2C/PMBus interface to be implemented in a small form factor. The ADM3260, shown in Figure 12, is available in a 20-lead SSOP package with 5.3-mm creepage, operates from –40°C to +105°C, and is priced at $2.99 in 1000s.

In addition to isolating I2C buses for hot-swappable central-office line cards, the ADM3260 can be used to isolate data-acquisition equipment in harsh industrial environments, to provide power and level translation over Ethernet, and in many other applications.

Figure 12. ADM3260 isolated I2C/PMBus Interface.

Conclusion
Isolated I2C/PMBus links in industrial and instrumentation, telecommunications, and medical applications need to be small, robust, and inexpensive. By integrating chip-scale transformer isolation, a single chip can implement a fully isolated I2C/PMBus data link including isolated power. The ADM3260 hot-swappable, dual I2C isolator with integrated dc-to-dc converter can provide a compact, reliable, low-cost, high-performance solution for these demanding applications while significantly reducing circuit complexity and design time.

References

I2C Digital Isolators

Digital Isolator Product Selection and Resource Guide

iCoupler® Products with isoPower™ Technology: Signal and Power Transfer Across Isolation Barrier Using Microtransformers

 

Author
  Maurice O'Brien[maurice.obrien@analog.com] joined Analog Devices in 2002, following his graduation from the University of Limerick, Ireland, with a bachelor’s degree in electronic engineering. He currently works as a product marketing manager in the Power Management product line. In his spare time, Maurice enjoys horse riding, outdoor sports, and travel.
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