Battery Life: How Can We Get More from Each Charge?

Fitness trackers, medical patches, video game controllers, portable speakers — these are among the many internet of things (IoT) devices that are shrinking in size but still expected to offer increased functionality and higher performance. As such, long battery life can be both an enabler and a differentiator. To extend battery life, it’s worthwhile to take a close look at the quiescent current of your battery-powered design’s components.

Many IoT devices sit on store shelves or in storerooms for extended periods of time. Many also have long stretches of inactivity. And some remain in standby mode for a long time, waking periodically to perform some task. But still, these smart, connected things need to perform when needed and expected. No one, after all, would enjoy having to continually charge a device or change its batteries, or even have the gadget stop working at a crucial moment. (Picture participating in a triathlon-style event, and having to pause along the route to charge your fitness tracker.)

Every IoT node requires a battery, and a typical household of two can have anywhere from 30 to 60 batteries in use.¹ Each device will have its own energy use patterns. Active, sleep, and hibernate currents of the central controlling unit, such as a microcontroller, all factor into the calculation of battery life. A system can also have associated sensors and radios that work with the microcontroller. An essential component is the power supply, which delivers energy to all of the system’s functional blocks. Power supplies are usually made up of boost or buck regulators or low-dropout (LDO) regulators, and sometimes power management ICs (PMICs). As sleep/hibernate functions take on larger roles, the power supply’s quiescent current becomes the biggest contributor to a system’s standby power consumption. Quiescent current (I_Q) refers to a circuit’s quiet state. In this state, the circuit isn’t driving any load and its inputs aren’t cycling. While typically nominal, quiescent current can substantially impact a system’s power transfer efficiency during light load operation. As an example, think about a system powered by a 40mAh, 1.55V silver-oxide coin-cell battery with a one-year shelf life. Provided that the current drawn is about 4µA, you could increase the wearable shelf life of the system by about three months by reducing the current by a single microamp.²

Using the right boost converter can extend battery life when the battery voltage drops to low levels. Selecting the right device is critical, because one that has higher quiescent current can backfire and drain the battery faster. Key selection criteria include:

• Quiescent current: the lower this current, the better the converter can preserve battery life at system standby mode. Look for nanoamps, not just milliamps, of current flow.
• True shutdown, which blocks the current output from the input in shutdown to improve efficiency and extend product shelf life  
• Input voltage range: allows running off of an almost “dead” battery
• Efficiency: measured in V_IN, V_OUT, and I_OUT, the higher the percentage, the better for increased battery life (>90% efficiency at uA level is ideal)

Ultra-Low Quiescent Current Boost Converters with True Shutdown

Maxim now offers a family of boost (step-up) DC-to-DC converters with ultra-low quiescent current (300nA) and True Shutdown™ technology, ideal for small battery-powered applications that require long battery life. The MAX17222 device features a 0.5A peak inductor current limit. The converter provides post-startup enable transient protection (ETP), which allows the output to remain regulated for input voltages down to 400mV, depending on the load current. The entire family is available in 0.88 x 1.4mm² 6-bump WLP and 6-pin uDFN packages and features 95% peak efficiency throughout the entire load range.

For a deeper understanding of quiescent current’s role in extending battery, read our white paper, Why Low Quiescent Current Matters for Longer Battery Life.

Maxim Integrated

Sources

¹ http://blog.batterysharks.com/average-household-and-the-number-of-batteries/

² Maxim Integrated. Meeting the Design Challenges of Wearable and IoT Devices. San Jose, 2017.