Optocoupler ICs: Applications and Component Selection

For new designers, an optocoupler sounds like it might have little to do with electronics, but they are important devices for providing isolation between different circuit blocks. An optocoupler IC integrates optical elements that act like a simple switch. They are easy to bring into different circuits and nicely provide isolation between circuit blocks running at different voltages. They are also ideal for use in feedback loops between different circuit blocks, especially where isolation is required. Some optocouplers are also designed to provide switching at high data rates. Here are some options for optocoupler ICs you can quickly import into your next electrical or electro-optical system.

What is an Optocoupler?

Very simply, an optocoupler integrates an infrared LED alongside a photodetector (usually a phototransistor) and acts like an optical switch. When the LED receives an input signal, the LED turns on and supplies photons to the base of the phototransistor. This then turns on the phototransistor, allowing current to flow through a connected circuit. The LED may be running at a different level than the internal phototransistor, which allows some isolation between these two signal levels. This is one way to conduct a low voltage signal into a high voltage circuit block without using an amplifier.

Optocoupler Symbol and Footprint

Because these components are inexpensive optical elements, they nicely provide electrical isolation between different circuit blocks without conducting certain forms of EMI between different circuit blocks. This isn’t to say that they are a cure-all for every noise problem, but the isolation they provide does isolate systems at different voltages, which has a nice benefit of suppressing ground loop noise from two portions of a system. NPN or PNP phototransistors can be found in optocoupler ICs.

Types of Optocoupler ICs

Optocouplers may use other switching elements aside from a phototransistor. Here are other types of optocoupler ICs you’ll find on the electronics market:

• Triode for Alternating Current (TRIAC): An optocoupler IC with a triac as the detector is used in systems that require high output voltage/current. They have slow response speed and are best for high voltage DC systems that require high current output.

• Silicon-controlled rectifier (SCR): These optocouplers also provide high gain, similar to a triac. However, they are also quite slow and are also best used for moderately high voltage/current DC systems.

• Photodiode: An optocoupler with a photodiode as the detector is common in systems that need fast switching. These components can be used when the LED is switched with a stream of digital pulses or with an AC signal. A photodiode will provide a very low output-to-input current transfer ratio compared to a typical phototransistor IC.

• Darlington pair phototransistor: These optocouplers are also useful for their high gain and they offer among the highest output-to-input current transfer ratio.

• Photoresistor: These are less commonly used as they still conduct in the OFF state. They also have low output-to-input current transfer ratio.

Important Optocoupler IC Specifications

You’ll likely start by looking at mounting style for optocoupler ICs; they are available in through-hole DIP packages or as surface-mount components. However, there are some important specifications that should be examined when selecting an optocoupler IC:

• LED forward voltage and trigger current. This tells you how you need to power your input LED to ensure it turns on and provides the desired switching behavior. In optocouplers designed to be switched with a square wave or PWM signal, the peak forward current required to trigger the switch depends on the pulse width of the signal in the ON state. Shorter pulses require larger peak signal current to force triggering.

• Output-to-input current ratio. This tells you the current transfer between each end of the optocoupler. Note that this is dependent on the absolute maximum collector-emitter voltage for a phototransistor optocoupler.

• Forward voltage vs. forward current curve. This specification has the same meaning as that for a standard LED, but it should not be confused with the trigger current.

• Temperature variations. These specifications are quite important for power systems as they can reach high temperatures during operation.

• Safety ratings and IEC/UL certification. If you’re designing for a power system or for data transfer in a high voltage environment near AC mains, IEC 60747-5-2 is one important standard to watch for to ensure high transient voltages can be withstood. You need to follow the safety and insulation guidelines to ensure you’re compliant with safety standards.

• Data rate or switch speed. Components that are intended for use in data networks will normally specify a maximum data rate, although a switching speed or frequency could also be specified.

Here are some example optocoupler ICs you can use in DC systems and low data rate applications.

ON Semiconductor, FODM611

The FODM611 optocoupler IC from ON Semiconductor is a single-channel optocoupler rated for up to 10 Mbps data rate (NRZ, 100 ns propagation delay). This device outputs at 5 V while offering high common mode transient noise immunity, making it ideal in industrial networks (CAN, RS485, and DeviceNet systems) or low-speed automotive systems. Switching is triggered by a photodiode connected to a buffer (see below).

Functional schematic and truth table, from the FODM611 datasheet.

Broadcom, HCPL-7723-300E

The HCPL-7723-300E from Broadcom is designed for higher data rates (50 MBaud with 2 ns maximum PWD). It features an integrated CMOS LED driver, where the input signal triggers the driver on. The detector section consists of a photodiode, high speed transimpedance amplifier, and voltage comparator with an output driver.

Functional schematic and truth table, from the HCPL-7723-300E datasheet.

Renesas, PS2802-4

The PS2802-4 quad-channel optocoupler from Renesas uses a Darlington pair phototransistor to provide high output-to-input current ratio ranging from 2 to 20 (up to 40 V rated collector-emitter voltage). This component provides 4 channels in parallel, making it useful in power management systems requiring isolation between a variety of voltages. Dark current in this component is as low as 400 nA, so very little power is wasted between switching events in a high power system. This component is also available as a single-channel variant (PS2802-1, see below).

Quad-channel and single-channel variants, from the PS2802-4 datasheet.

A variety of systems can benefit from using optocoupler ICs for isolation, and you can find the components you need for your next system with the component search and filtration features from Octopart.

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