Design Ideas: June 6, 1996
Figure 1. Electrically isolating the sensor allows operation in situations prone to high common-mode voltages; it's also useful in breaking ground loops. IC1 and IC2 comprise a bidirectional isolator. Assume an initial high state on the serial-data (SDA) bus. The normal rest state for both the SDA and SCL (serial clock) lines of the I2C bus is high.
The photodiode in IC1, which has a current path from its anode via R2 and the internal resistor in IC2, is inactive. As a result, the high state of the off collector of IC1's output transistor transmits (via IC2's photodiode in parallel with D2) to the SDA input of temperature-sensor IC5. Grounding the SDA line causes current to flow in IC1's photodiode, thereby activating IC1's output transistor. The transistor's low state transmits to IC5 via the photodiode of IC2. D2, connected in parallel with the photodiode, limits the reverse-bias drop to the typical 0.4V of a Schottky diode. This arrangement prevents the low condition from circulating back across the barrier, causing the circuit to latch.
When IC5 communicates a low state back across the interface, the mirror image of the above scenario occurs. When IC5 pulls the SDA line low, it applies a forward bias to the photodiode in IC2. When IC2's output transistor pulls SDA low, it does so in such a way that it applies a reverse biases to IC1's photodiode, with a current path via D1 (again, to limit the drop to 0.4V) preventing a recirculating condition. In short, when you take the left-side SDA low, the right side goes low. When you take the right side low, the left side goes low. Inputs equal outputs; whatever one does, the other imitates, without latching.
The I2C bus normally requires a dual-optocoupler arrangement, such as IC1 and IC2, for the SCL line. If the LM75, IC5, is the only device on the isolated side, you can use a single-direction isolator, thereby simplifying the circuit. The LM75 is an I2C slave only, and its SCL pin is strictly an input. Optocoupler IC4 is optional, necessary only when you need to monitor the OS output of the LM75. The OS pin is a watchdog output that has user-settable overtemperature and hysteresis values.
The LM75 provides a 9-bit digital output with a 0.5 X C LSB. A0 to A2 on the LM75 allow you to set any one of eight unique addresses. In this application, tie these lines either to ground or to the supply rail. You need an isolated voltage source, either a dc-dc converter or a battery. The LM75 operates from 3 to 5V supplies and typically draws 250 µA. IC1 and IC3 require 7 to 10 mA each, and the photodiodes need approximately 700 µA (but only when active). Total current drain is approximately 21 mA. (DI #1880)