Can Opto-Emulators Serve as Alternatives to Traditional Optocouplers?

Circuit designers are very familiar with optocouplers, also called optoisolators (and sometimes called optorelays, if configured for on/off mode). They contain a photon-emitting LED with a phototransistor as receiver, thus providing galvanic (ohmic) isolation while allowing a signal’s information to pass. They’re used extensively to prevent ground loops, protect systems from high voltage, and implement level shifting, as well as for many other circuit functions.

Among their many virtues is that they provide one function, do it well, are easy to use, and are tiny—usually in a 4-pin package where further dimensional shrinkage is constrained by regulations and mandates for isolation-barrier performance. While alternative isolation technologies use transformer, capacitive, and even RF technologies, optocouplers are a very handy component in the designer’s kit, as they address and solve some otherwise intractable proems and do so with minimal hassle.

TI’s New ISOM8600 Opto-Emulator

Nonetheless, as with any component, they have limitations that can impact their suitability in some applications. Acknowledging this concern, Texas Instruments introduced the ISOM8600 opto-emulator, an 80-V, 150-mA functionally isolated, normally open, opto-emulator switch with integrated FETs (Fig. 1).

1. The Texas Instruments ISOM8600 opto-emulator looks like a conventional optoisolator with respect to pins and packaging. The MOSFET output structure enables it to source and sink current.

A pin-compatible drop-in replacement for many traditional optocouplers, the ISOM8600 joins others in the company’s opto-emulator family that use capacitive isolation and a silicon-dioxide (SiO₂) barrier. Representative applications include factory automation and control, building automation, appliances, and even some test and measurement situations.

In opto-emulators, the input signal is transmitted across the isolation barrier using an on-off keying (OOK) modulation scheme (Fig. 2). Via capacitive coupling, the transmitter sends a high-frequency carrier across the SiO₂ barrier to represent one digital state and sends no signal to represent the other digital state.

2. The opto-emulator uses a silicon-dioxide barrier for galvanic isolation between transmit and receive functions.

Why even consider this non-optical emulator? Since there’s no need to boost LED current to compensate for presumed LED-aging effect, the emulated diode-input stage consumes less power than optocouplers that do have LED aging concerns and may require higher bias currents over the device lifetime. The ISOM8600 switch output can be controlled by just 0.8-mA current through anode/cathode pins over the lifetime of the device, for cumulative system-level power savings.

A Reliability and Performance Edge Over Optocouplers

Furthermore, TI maintains the ISOM8600 opto-emulator switch offers significant reliability and performance advantages compared to optocouplers, such as wide operating-temperature range along with tight process controls, resulting in tighter part-to-part variations.

Key specifications include:

Single-pole, normally open, symmetrical 80-V output switch.
Primary-side current-controlled switch, with no additional isolated high-voltage supply required for 80-V switching.
Ultra-low off-state leakage at V_OFF = 70 V: < 250 nA at an operating temperature of 25°C; < 1 μA across an operating temperature range of –55 to 125°C.
Fast response time: 10 μs (typical) at IF = 5 mA, V_CC = 20 V, R_L = 200 Ω, C_L = 50 pF.
Ultra-low input trigger current of 800 μA (at 25°C).
Functional isolation: 500 V_RMS working voltage.
Supports industrial temperature range: –55 to 125°C.
Small SO-4 package (same as standard optocouplers).

In addition to non-degradation and wide operating temperature, TI claims many other benefits for this opto-emulator technology compared to optocouplers. For example, the company cites more favorable performance specifications for higher common-mode transient immunity (CTMI), stability of the unit’s specification that parallels current transfer ratio (CTR) in optical devices, and higher data-transfer rates. (Of course, optical units from other sources do excel in one or more of these, but at a price.)

Are there limitations to the ISOM8600 when compared to optoisolators? Most likely, it’s the 500-V_RMS functional isolation value. Many opto-based devices cite figures of 2, 3, and even 10 kV for that parameter, and such high isolation may be required to meet regulatory or safety mandates. However, many applications require isolation levels that are much lower, such as ground loops or level translation, and lower-voltage situations where higher voltages aren’t a concern.

The ISOM8600 is supported by a 19-page datasheet with the necessary performance graphs, min/max specifications, and design-in guidance. For those who want to better understand the iso-emulator technology, TI also has two informative application notes (see References).