Design your own simple I2C isolator

Often product design timelines are tight and funding for new product designs is limited. However, we must design robust systems that can operate in harsh environments without increasing costs. Typically, this requires the use of galvanic isolation to protect sensitive control electronics from external inrush and transient surge currents.

If your design involves many industrial interfaces, you will find yourself as excited as a kid in a candy store when you see a wide variety of RS-485, RS-232, CAN and I2C signal isolators on the official websites of major semiconductor manufacturers. However, when you want your purchasing manager to approve the purchase of these products, he will immediately pour cold water on you: "Can't you use some existing standard components? No matter what method, use them all?"

In the future, you can answer this question with an enthusiastic “no problem”, because this article will introduce you to a small number of industrial interface circuits, almost all of which use only a standard isolator. Figure 1-4 shows a simplified schematic diagram of the most common digital interface in industrial applications.

Note that bypass capacitors and pull-up/pull-down resistors have been omitted for ease of illustration. The first three circuits all have an asynchronous data transfer mode that uses two data lines and one control line for driver/receiver activation. This requires only a triple isolator between the node controller and the standard-compliant transceiver chip.

The isolated I2C (inter-integrated circuit, IIC) shown in Figure 4 represents a special case because it supports short communication links that are only a few inches long, so no line transceiver is required. In some multi-master applications, two nodes access the bus at the same time. To prevent the signal from being transferred back to its source, a bidirectional buffer is used to support the receive transmission from R(x,y) to S(x,y) and the transmit transmission from S(x,y) to T(x,y), rather than the direct loopback from R(x,y) to T(x,y).

Fortunately, multi-master designs are a minority of cases, and most are single-master applications. Therefore, we can greatly simplify the circuit shown in Figure 4.

Since it is a single master, the clock signal (SCL) only needs to travel in one direction, reducing the clock isolation to one channel. Then, replacing the bidirectional buffer with a crystal diode switch on each side of the isolation barrier (Figure 5) simplifies the circuit to our standard triple isolator (Figure 6).

Figure 5 Using transistor switches to isolate the transmit and receive paths

In standby mode, isolator inputs A and C are pulled high through R2 and R4, pushing outputs B and D high. In addition, the master and slave data lines (SDA1 and SDA2) are pulled high through RPU1 and RPU2. When the master begins communication by pulling SDA1 low, the Q1 emitter junction is forward biased and Q1 pulls input A low. Output B follows low and forward biases D2. D2 pulls SDA2 low. At the same time, the Q2 emitter junction is reverse biased and Q2 remains high impedance. The switching sequence is the same, only reversed when the slave data line responds.

Figure 6: Isolated I2C bus interface for single host application

Figure 6 shows the final circuit. Use at least 0.1μf capacitors to buffer the chip power supply. Always connect the activation input to each power rail through 1k to 10k resistors. These resistors control the chip inrush current caused by surge transients entering the power line. It is a good analog design practice to use filter capacitors (here 220pF) to suppress sensitive CMOS input noise.

An isolated design is incomplete without an isolated power supply. Figure 7 shows a low-cost, isolated DC/DC converter design that replaces expensive integrated DC/DC modules. Both the primary and secondary supplies can vary between 3.3V and 5V. The following table lists the appropriate components for the three power supply combinations.

Figure 7 Isolated DC/DC converter

Next time, we will discuss how to use SPICE to design a low-power, high-precision PID temperature control loop. Stay tuned.