Designing robust, fully isolated data & power I2C/PMBus data interfaces

February 12, 2015 // By Maurice O’Brien, Analog Devices
A key requirement for industrial and instrumentation (I&I), telecommunications, and medical applications is a reliable interface for transmitting data.

The Inter-Integrated Circuit (I 2C) 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 I 2C applications include:
- Isolated I 2C, SMBus, or PMBus interfaces
- Level-translating I 2C 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 I 2C designs compared: (a) Discrete solution and (b) Integrated solution.

The integrated solution (ADM3260) provides a fully isolated bidirectional I 2C interface and isolated power with a single IC, plus the decoupling capacitors and pull-up resistors associated with any I 2C 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 I 2C interface, while enhancing reliability. It operates