The Tech Blog

Continuing with the isolator series, digital isolators were designed to remove the 3 major flaws of optocouplers: slower transmission rate, higher power consumption and LED derating problem for longer life. Digital isolators(although it’s an overarching word) are usually built on CMOS technologies and use magnetic or capacitive coupling to transfer digital signals across the isolation barrier. The input and the output side, are separated by an isolation barrier.

Let’s see how they transfer data. Inside the IC, The input and output sides are made on 2 separate dies(Check images) on the same IC and there is a physical distance(>0.5mm) providing isolation. Capacitive coupling uses two conductive plates separated by an insulating material to transfer electrical energy between two circuits. The input digital signal is modulated to a higher frequency and is passed through via isolation capacitors(one on each side) on the silicon dies. On the output side, these are demodulated to get the input signal back. The two sides are powered with separate voltage sources with no common feedback loop. Wirebonds run the signal between two halves. The use of SiO2 as the dielectric material provides great. isolation. Another type of digital isolator comes with integrated transformers built into the ICs providing data transfer via magnetic coupling across the isolation.

When to use them? From a performance standpoint, digital isolators outperform optocouplers in terms of faster transmission rates, lower power consumption, longer lifespan, and ability to operate in harsh environments. Although digital isolator technology is relatively new (around the last 15 years), compared to opto-based technology which has been around for a long time, the relative cost of optocouplers is cheaper than other digital isolators. Therefore, during the design stage, it is important to balance the benefits of digital isolators against their cost. If cost is not a major concern, it may be worth considering digital isolators for your application.

A few folks from the last post wanted to know more about the design considerations of optocouplers. Let’s briefly go over them. The important consideration is the forward LED current, If. Its current-limited by a resistor Rf, which is straightforward to calculate based on the maximum current which can pass through the LED from the datasheet and the LED forward voltage. Standard KCL will give you the current. The question here is should you be driving the LED at its max current? Problem is that LED driven at its maximum current loses its brightness over time. For any LED application where it needs to last a long time, a common rule of thumb I personally follow is to limit the current through the LED to half the maximum current possible. I don’t push the LED to its limits and derate itself. Yes, this means that I leave a larger margin which can effectively increase the cost but it can help the circuit perform normally over a long time.

The second is ambient temperature. Like the current through the LED, choose optocouplers based on the operating temperature. It does have a severe impact again on the lifetime failure of the device. Obviously, lower is better for longevity. In good datasheets, you will find a curve which shows current and temp deratings.

The next consideration is to figure out what sort of isolation voltage you need from the input to the output. It heavily depends on the voltage applications you use this device. The important parameter in this is also the creepage distance between the input and output side to prevent the arcing over the isolator for higher voltage application.

The current Transfer Ratio(CTR, refer prev post for more) is another parameter in design. Its ratio of output to input current. For analog applications, try not to push the transistor to saturation by increasing the If current. If it saturates, the input current has relatively no role in determining the output side current(It will only be decided by the max current limits on the output). On datasheets, you will see CTR vs If curves, which increase till a point and then drops down. Usually(not always) you are looking for a high CTR to transfer max current at the output for a small change in input.

I have only glanced through a few points for more read App Notes ANO006(Wruth, Agilent’s OptoCoupler Design Guide and Toshiba’s Photocoupler App Note.