Design Your Own Simple I2C Isolator

Usually product design time is very tight, and funds for new product design are not generous. However, we must design robust systems that can operate in harsh environments without increasing cost. Typically, this will require the use of galvanic isolation to protect sensitive control Electronic components from external inrush and transient surge currents.

If your design involves many industrial interfaces, you will find yourself like a go-to candy when you see the dazzling array of RS-485, RS-232, CAN and I2C signal isolators on the official websites of major semiconductor manufacturers The kids in the store were just as excited. But when you ask your purchasing manager to approve the purchase of these products, he will throw cold water on you: “Can’t you use some of the standard components you already have? Use them anyway?”

In future situations like this, you can answer enthusiastically “no problem” because this article will introduce you to a small selection of industrial interface circuits that almost exclusively use a standard isolator. Figure 1-4 shows a simplified schematic of the most common digital interfaces in industrial applications.

Design Your Own Simple I2C Isolator

Figure 1 Isolated RS-485 bus interface

Design Your Own Simple I2C Isolator

Figure 2 Isolated CAN bus interface

Design Your Own Simple I2C Isolator
Figure 3 Isolated RS-232 Line Interface

Design Your Own Simple I2C Isolator

Figure 4 Isolated I2C bus interface for multi-master applications

Note that we have omitted bypass capacitors and pull-up/pull-down resistors for ease of illustration. The first three circuits have an asynchronous data transfer mode that uses two data lines and one control line for driver/receiver activation. In this way, only a triple isolator is required 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 and therefore does not require line transceivers. In some multi-master applications, two nodes access the bus at the same time. To prevent the signal from turning back to its source, we use a bidirectional buffer to support receive transfers from R(x,y) to S(x,y) and send transfers from S(x,y) to T(x,y) , rather than a direct loop from R(x,y) to T(x,y).

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

With a single master, the clock signal (SCL) only needs to be transmitted in one direction, reducing clock isolation to one channel. Then, replacing the bidirectional snubber with a crystal diode switch so that each end of the isolation barrier (Figure 5) simplifies the circuit to our standard triple isolator (Figure 6).

Design Your Own Simple I2C Isolator
Figure 5 Isolation of transmit and receive paths with transistor switches

In standby mode, isolator inputs A and C are pulled high through R2 and R4, driving outputs B and D high. Additionally, the master and slave data lines (SDA1 and SDA2) are pulled high through RPU1 and RPU2. When the master initiates communication by pulling SDA1 low, the Q1 emitter junction is forward biased and Q1 pulls input A low. Output B then goes 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 responding from the data line.
Design Your Own Simple I2C Isolator

Figure 6 Isolated I2C bus interface for single host application

Figure 6 shows the final circuit condition. Use at least a 0.1Mf capacitor to buffer the chip power supply. Always connect the active input to each power rail through a 1k to 10k resistor. These resistors control chip inrush current caused by inrush transients into the power line. Using filter capacitors (220pF here) to suppress sensitive CMOS input noise is a good analog design method.

An isolated design is incomplete without an isolated power supply. Figure 7 shows a low-cost, isolated DC/DC converter design to replace expensive integrated DC/DC modules. Both the main and auxiliary power supplies can be varied between 3.3V and 5V. The following table lists the corresponding components for the three power supply combinations.

Figure 7 Isolated DC/DC converter

Stay tuned next time we discuss how to design a low-power, high-accuracy PID temperature control loop using SPICE.

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