Ally Winning, European Editor, PSD
One of the major trends over the last two decades is to replace mechanical systems with electronic systems wherever possible. Deploying an electronic system usually has the advantage of offering faster and more accurate responses, while being more reliable due to having no physical wear and tear. However, those electronic systems are dependent on power and even though today’s power sources are robust, they sometimes fail. Fortunately there are ways of ensuring that when power does fail, it need not be catastrophic and the whole system, or critical subsystem can keep on functioning as long as necessary. For example, emergency power could ensure that the data from a telematics system operates for a few seconds after a vehicle collision to capture vital information and write it to memory when the battery has been disconnected. It could also be used to keep a plant operating until back-up generators kick in, ensuring that production is unaffected, or alternatively to shut down operations safely without damaging the equipment or the product.
Usually supercapacitors or batteries are used to keep the power flowing after an outage, or in some case both. Batteries are more useful for longer term power. They do have some downsides, including that lithium-based batteries can be flammable, and as such are subject to restrictions in transportation and use. Supercapacitors generally are much safer and can operate in a more diverse spread of environments, but they can only really supply energy for a maximum of a few minutes. However, that is more than enough for many applications, especially ones where only a single subsystem or power rail is required to be supplied to safeguard data or shut an important application down safely. This type of use is currently gaining in popularity with the increase in electric systems along with the need to capture data for IoT applications.
To implement a supercapacitor solution isn’t easy and usually requires some complicated design work. For instance, a voltage drop is usually needed from the supply voltage to the battery voltage for charging and a corresponding boost from the battery to supply rail voltage for its discharge. Control circuitry is required to switch the supercapacitor onto and out of the circuit, and to seamlessly switch over to longer term battery back-up if required. Different thresholds have to be set depending on the application, and a low IQ mode is needed to stop the battery constantly draining.
With more companies wishing to design a supercapacitor back-up circuit into their products, Analog Devices (ADI) saw an opportunity to create a supercapacitor control IC that could be easily integrated into a wide variety of applications, while being flexible enough to differentiate its output using a small amount of passive components to suit different applications. A circuit like this would save design time for customers and cut down on the number of individual components they require. There are many traits that are different in supercapacitor implementations, and just as many that are similar. As a first step, ADI looked at a variety of supercapacitor circuits and made a list of the most common requirements.
ADI Subject Matter Expert, Frederik Dostal explained the findings of that research and the main requirements of a general purpose supercapacitor circuit by saying, “The 5V rail has to be accurate, usually within plus or minus 5%. If it's more than that, then an additional DC to DC converter would be required and that would make the circuit larger and more expensive. A high discharge output current is important, even if the supercapacitor is only used to backup the system for a few seconds, the current has to be high enough to accomplish the task and the DC to DC boost converter has to handle that. Last, but not least, a lot of legacy applications have been designed without power backup and if you retrofit it, there is no room to increase the size of the device. So, power density and a low system cost is also mandatory for a large percentage of these types of applications.”
With those findings in mind, the company developed its Continua range of supercapacitor controllers. The latest of these controllers is the MAX38889, which supports a peak inductor current of up to 3A. Depending on capacitor charge voltage and output voltage, this will typically allow output currents up to 2A. It has highly efficient buck and boost stages for charge and discharge respectively, along with a low IQ idle mode. The chip operates off a 2.5V to 5.5V supply rail and supports single cell and multi-cell supercapacitor configurations and power path management with automatic switch over. It has been designed to be compact and easy to integrate into a wide range of new and existing devices with the minimum of engineering time and supporting components. The company has also produced a demonstration board to allow for easy evaluation of the device.