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Designing Robust, Isolated I2C/PMBus Data Interfaces
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:
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
Figure 6. Isolated I2C interface.
The Challenge of Isolated I2C Interfaces
Figure 7. An optocoupler-based I2C interface.
Isolation Technology: Data and Power
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
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.
2.5-kV Isolation Protection and Approvals
Figure 11. ADM3260 recommended printed circuit board layout.
ADM3260 Applications and Benefits
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.
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