Designing robust, fully isolated data & power I2C/PMBus data interfaces
The Inter-Integrated Circuit (I2C) bus is a 2-wire bidirectional bus used for low-speed, short-distance communication between integrated circuits. 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.
– Examples of isolated I2C applications include:
– Isolated I2C, SMBus, or PMBus interfaces
– Level-translating I2C interfaces for power supplies
– Networking and Central office switching
– Power-over-Ethernet
– Telecommunication and data communication equipment
– Isolated data acquisition systems
−48 V distributed power systems and modules
The figure 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 analogue 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.
Figure 1 Isolated I2C designs compared: (a) Discrete solution and (b) Integrated solution.
The integrated solution (ADM3260) 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 integrated device is free of glitch and lock-up issues, has UL approved 2.5-kV rms isolation ratings, and comes 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.
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.
About the 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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