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Designing single-battery apps

Posted: 05 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:battery? alkaline? lithium? boost converter?

The market for battery-powered applications is huge, lending itself to a large number of products in the consumer, medical, personal-care and entertainment market segments. Successful battery-powered products in all of these markets need intelligent control, maximized battery life, as well as minimal size and weight that support device portability. Rechargeable batteries can be a good option in devices that are used frequently and operate at higher drain rates, but for many applications, primary batteries are the best solution because they support simpler and lower-cost implementation options that enable a truly portable device. Whether the device is a blood-glucose meter, computer accessory, camera or wireless headphone, battery-powered products continue to become physically smaller as each iteration of their design enters the market.

There are a number of popular battery technologies, such as alkaline and lithium batteries, that are available to developers who are designing primary batteries into their projects. Each battery option targets different types of devices and use cases. Lithium coin batteries (typically Li/MnO2) feature packages in 13 different sizes that fit within small and lightweight devices, such as timers and watches, which use relatively low amounts of energy and need a long shelf life on the order of seven to 10 years.

Two primary battery technologies that are available in a cylindrical form factor are alkaline and lithium (LiFeS2). Alkaline cylindrical batteries are suitable for a wide variety of portable devices that exhibit a low to moderate drain rate. They are widely available and inexpensive, compared to the other primary battery technologies, and they are available in three compact form factors: AA, AAA, and the smaller AAAA package (as well as larger cylindrical and 9V forms that are less relevant for this article).

Lithium cylindrical batteries are available in AA and AAA form factors, and are best suited for applications that exhibit a medium to high drain rate or need to be used in cold temperatures. They provide high-reliability operation, are 33% lighter than alkaline batteries, and have a long shelf life of up to 15 years (2 to 3 times longer than alkaline batteries).

Single-battery approach
While portable, battery-powered products keep shrinking in physical size, their overall size is becoming more and more limited by the size and shape of the battery cavity, which may house two or more batteries. Therefore, the ability to operate a microcontroller on a single 1.5V battery is becoming increasingly valuable in a growing number of applications, in order to reach even smaller sizes.

Using a battery boost converter in a design makes it possible to operate the system on a single battery, where earlier designs relied on two or more batteries. A battery boost converter is a power supply that steps an input voltage up (boosts it) to a higher, regulated voltage, such as 2.0 to 5.0V. By incorporating a battery boost converter, a device can startup with an input voltage that is significantly lower than the operating voltage.

For example, many microcontrollers cannot operate with a voltage that is lower than 2V, and the battery boost converter can provide the voltage that the microcontroller requires, with a startup voltage as low as 0.65V (figure 1). This enables an application using a boosted single alkaline battery to start up at any point in its discharge curve, provided that it has enough remaining capacity to operate the device. While a battery boost converter may be able to deliver a consistent output voltage from very low input voltages (for example 0.35V), battery suppliers, such as Energizer, do not recommend discharging alkaline or lithium batteries below 0.8V, as it can damage the battery.

 startup waveform

Figure 1: Here's a typical startup waveform, where the low-voltage startup begins to charge the output voltage up to the input voltage. Once the output voltage is charged, the N-Channel begins to switch, pumping the output voltage up, after which the internal bias switches from the input to the output.

Using a single-battery implementation offers different advantages, depending on what alternate battery configuration you are comparing it to. The most straightforward comparison is a single alkaline battery compared to two alkaline batteries. The most obvious difference is the volume savings by eliminating one battery. Another obvious difference is the weight saved by not including the second battery or its extra housing. There is a small amount of area that is taken up by the converter and supporting components, which is discussed in the tradeoffs later in this article. Using a battery boost converter also maximizes the system power efficiency by providing regulated power over the battery's entire operating range. This regulated voltage can make the microcontroller operate more efficiently, by enabling it to run at a lower and flatter voltage. For example, reducing the microcontroller operating voltage from 3.3V to 2.2V provides 1.8 times lower power consumption on the microcontroller side (voltage difference squared). Additionally, the boost converter provides short-circuit protection through current limiting.

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