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Digital isolators provide data, power isolation in single package

Posted: 22 Mar 2006     Print Version  Bookmark and Share

Keywords:ADuM5240  ADuM5241  ADuM5242  iCoupler chip  digital isolator 

The iCoupler chips from Analog Devices Inc. (ADI) integrate very small transformers, using magnetic technology rather than optical methods that are used in optocouplers.

The company's latest iCoupler family of digital isolators employs ADI's proprietary technology called isoPower, which enables the transformers not only to isolate data or signals, but also power in a single package. The extension of isoPower has been done in response to customers needs to reduce cost and size, mainly in the industrial sector.

"This is an industry first," said Ronn Kliger, ADI's product line manager for iCoupler products. "Up to now, designers are really forced to configure signal isolation and power isolation solutions independently with different components and different technology."

There are three devices that incorporate the isoPower technology—the ADuM5240, ADuM5241 and ADuM5242. The only difference between the products is the direction of the signal flow. The products output 10mA of 5V of isolated power that the designers can use to power anything they desire on as isolated portion of the system.

The dual-channel products have two channels of signal isolation, which transmit data at a rate of 1Mbps to 100Mbps. Although the products are capable of data rates as high as 400Mbps, the company chose to cap it at 100Mbps because it hasn't seen demand for anything above that level.

Systems have typically used optocouplers to act as the isolation barrier, supporting the flow of signals between two points in a system. The problem with optocouplers, as well as other conventional devices, is not only sending signals but power across an isolation barrier. Power can't be taken from point A and run through a wire to point B—it must be sent over in an isolated fashion, Kliger explained.

Using traditional methods, designers have typically built an isolated power supply that is based on discrete transformers that takes input power and sends it magnetically across that transformer. That's a mature and costly way of handling isolation. There are also vendors that sell power supplies in an integrated fashion. It is a smaller, design-your-own method way of sending power across the isolation barrier. In both cases, the designer still needs two devices—one product, such as an optocoupler, to send signals across the isolation barrier and another device to send power. ADI has found a way to integrate both functions in one component.

Compared with existing products, such as a two-channel optocoupler and an isolated DC/DC converter, using ADI's dual-channel saves designers 80 per cent in board space and 70 per cent in cost, he said.

"Our first isoPower product is really optimised for low-power applications. The output of this product is 50mW, which is a relatively small amount of power compared with an isolated DC/DC converter, which are typically 1W or 2W," Kliger said. "That was a conscious decision that we made on these first products. We're targeting certain applications where the amount of power is sufficient and our focus here is to minimize cost and size."

The company plans to develop iCoupler products that operate in the sub-watt area, he said.

isoPower technology
The parts have three transformers inside—two are used for signals and one for power. Using transformers to provide isolation instead of LEDs like optocouplers is a more flexible method since they support both signals and power. With CMOS technology, ADI has been able to surround the power transformer with the right amount of circuitry to integrate the functions in one device. The transformers are very small and rated for 2.5kV insulation strength.

The transformers are two coils stacked above each other separated by some insulation. When there's a signal or power present in one coil, it couples magnetically to the second coil and the spacing between the two is only 20μm.

Pricing between the devices doesn't vary. With the stacked configuration, signals can communicate in either direction. As a result, the products can go into a variety of applications. Kliger cited several applications, including data acquisition and RS-232 interface, but power supplies is one of the major areas where it saw a pull for this kind of technology, he said.

One of the trends is to put the control function on the secondary side of the power supply so that's it is closer to the load and can respond more quickly and accurately to changes in the load. One of the problems with this architecture is during system start-up, Kliger explained.

The controller requires power to start operating; however, there's no power available until the power supply is up and running. The power supply can't start operating until the controller is operating. Power supply companies resolved this by adding an auxiliary supply, which adds cost, board space and complexity.

The ADuM5242 eliminates that problem by sending power directly from the input to the output, triggering the controller. When the controller gets going, it sends two digital signals back to the primary side to start the power-supply loop operating. The ADuM5242 supports the two PWM signals coming back from right to left, allowing the power supply to start running. The isoPower portion of the product can be disabled if the user desires. And the power supply functions by itself.

The company has started to sample its isoPower-based iCoupler products to power supply companies. It expects to ship in full production starting in July 2006.

- Ismini Scouras

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