2 Switching Performance

Isolators are extensively used in many industrial and automotive applications where isolation of data, control or status signals is needed. To enable processing of the isolated data, control, or status signals in a timely manner, it is critical for the isolator to have optimum switching characteristics, minimizing the impact on the overall system timing performance. Optocouplers fare very poorly when it comes to switching characteristics whereas digital isolators offer one of the best switching characteristics in the industry, enabling more systems to meet their performance requirements.

General purpose optocouplers usually do not have any supported data rates mentioned in their data sheets, making it difficult to know their suitability for a given application. Most of these optocouplers also have an open-collector output, due to which they are only characterized to a few select pullup or load resistor values. One of TI’s latest digital isolators, ISO6741, has its maximum supported data rate clearly specified in the data sheet as 50Mbps, which makes it easy to know its suitability for a given application. Unlike optocouplers, digital isolators do not require any external pullup resistors for operation and the maximum data rate is not heavily dependent on external components.

Table 2-1 compares timing specifications of a general purpose optocoupler with TI digital isolators. The information in the table also estimates the asynchronous and synchronous data rates that are achieved using the data sheet timing specifications. Table 2-1 shows that the data rate achieved using a general purpose optocoupler is much lower than what can be achieved using digital isolators. The two pullup resistor options listed with R_L = 100 Ω and R_L = 1.9 kΩ for optocoupler consume significantly higher current compared to digital isolators, making them unsuitable for many applications.

High-speed optocouplers offer better switching characteristics compared to general-purpose optocouplers. Table 2-2 compares a typical high-speed optocoupler with TI digital isolators in which the asynchronous and synchronous data rates for the devices are estimated using the timing specifications given in their respective data sheets. As shown in the comparison table, digital isolators still support much higher data rate compared to the high-speed optocoupler.