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Does digital isolator replace optocoupler? What are the advantages?

elecfans | 06/07/2021

Does digital isolator replace optocoupler?

As more and more industrial and communication equipment become intelligent, providing more high-power system control and feedback information, so as to achieve higher efficiency and reduce system costs, the demand for isolators in the market today is also increasing.

On the whole, despite the tremendous growth in market demand, the price of digital isolators will remain stable. Although there has always been price pressure on isolator components, today’s isolator solutions are still adding new features, such as low-power modes and safety capabilities. Therefore, isolator solutions on the market today will continue to increase value while maintaining long-term lower prices.

Isolators are generally divided into three categories in principle: photoelectric isolators, inductive isolators and capacitive isolators. It is customary to call the first type an optocoupler, and the latter two types are called isolators. These three types of isolators are widely used and each have their own advantages and disadvantages. Their major manufacturers are constantly investing in new research and development to gain a larger market share. In terms of optocouplers, Avago, Vishay, Toshiba, Panasonic, NEC, as well as Taiwan Guanxi, Baihong, etc. are industry leaders, especially Avago dominates the market. The isolator market is dominated by manufacturers such as ADI, NVE, TI, and Silicon Labs.

Optocoupler is an isolation device developed in the 1970s. It has a good isolation effect on input and output electrical signals. It has become one of the most diverse and versatile optoelectronic devices, including transistor couplers and high-speed integrated circuit output couplers. , Triac couplers and light-controlled relays, etc., widely used in electrical insulation, level conversion, inter-stage coupling, drive circuits, switching circuits, choppers, multivibrators, signal isolation, inter-stage isolation , Pulse amplifier circuit, digital instrument, long-distance signal transmission, pulse amplifier and solid state relay (SSR), instrumentation, communication equipment and microcomputer interface.

The main advantage of optocoupler is that the signal is transmitted in one direction, the input end and the output end are completely electrically isolated, the output signal has no effect on the input end, the anti-interference ability is strong, there is no contact, and the service life is long. In this case, speed and power consumption have become the focus of procurement. Different applications have different speed requirements for optocouplers. For example, in communication applications, DeviceNet specifies relatively low data rates, including 125kBd, 250kBd and 500kBd, and propagation delay requirements are less than 40ns; CAN bus specifies 125kBd low speed and 1MBd High-speed data rate, but there are no strict requirements for propagation delay; Profibus sending data requires within 12MBd, and specifies the total delay of the isolator, transceiver and the PWD of the connection itself. Avago has recently launched two automotive-grade high-speed and low-power digital CMOS optocoupler products ACPL-M71T and ACPL-M72T, both of which are designed using LED technology with lower drive current and lower power consumption, suitable for high-speed 15MBd and Low-speed digital applications. Vishay recently launched two high-speed analog optocouplers VOM452T/453T in a standard SOP-5 package. In addition to the speed of 1MBd, the propagation delay has been reduced to 1μs. This is for those requiring faster switching speeds than standard phototransistor optocouplers. Application is very important.

In terms of low power consumption, take Avago’s latest ultra-low power optocoupler product ACPL-M61L/061L/064L/W61L/K64L as an example, they save 90% of the current standard optocoupler. These optocouplers can use unique integrated circuit design and thick insulating layer materials, which can greatly save power consumption without affecting isolation and insulation performance. The target market includes communication interfaces such as RS485, CANBus and I2C, and microprocessor system interfaces. , And digital isolation for analog-to-digital conversion applications such as A/D and D/A. These optocouplers integrate high-efficiency light-emitting diodes LED and high-gain photodetectors, bringing a 1.6mA low forward drive current that allows design engineers to directly connect the optocoupler input to the microcontroller output, saving the use of buffers. LED demand.

In addition to the requirements for speed and power consumption, optocoupler products are also trending towards low voltage characteristics. As more and more products are gradually turning to lower power supply voltages, this also requires optocouplers with low voltage to meet the needs of the system. In addition, with the continuous improvement of system integration and the increasingly strict working environment, the volume and working temperature of optocoupler products have received unprecedented attention. Still taking Avago products as an example, ACPL-570xL/573xL/177xL series sealed optocouplers, operating temperature range from -55 to +125℃, these new optocoupler products can work at a supply voltage as low as 3.0V , Will not affect the performance of the parameters.

What are the advantages of using digital isolators over traditional optocouplers?

Since the optocoupler uses light from the LED to transmit data through the isolation barrier, in data transmission applications, when the LED is turned on, it is logic high (Logic HIGH), and when it is turned off, it is logic low (Logic LOW), which means At the same time, power will be generated during the transmission.

Digital isolators have faster transmission response, and can use codes to express logic high and logic low levels.

Therefore, the digital isolator can reduce power at high levels for a long time, and can handle complex two-way interfaces such as USB and I2C.

In addition, digital isolators have better "Common Mode Transient Immunity (CMTI)".

CMTI is very important because high slew rate (high frequency) transients can disrupt the data transmission between the isolation barriers. CMTI is usually measured in kilovolts per microsecond (kV/us) and refers to the ability of an isolator to reject noise between the input and output. High CMTI means strong isolation channel.

The CMTI of digital isolators can reach 200kV/us, while optocouplers are generally low.

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