Editor's Choice


Make your small asset tracker last longer

28 March 2024 Editor's Choice Power Electronics / Power Management

This design solution reviews a typical asset tracking solution, and shows how the MAX3864x nanopower buck converter family, with its high efficiency and small size, enables longer battery life in small portables. New, low-power data connections are sparking a proliferation of asset tracking solutions thanks to their low cost of deployment. The effects can be seen in multiple applications, particularly transportation and supply chain management.

In a typical application, a sensor provides updates from a given location, transmitting data about temperature, humidity, pressure, and motion. The sensor needs to transmit only small amounts of data, which results in higher coverage and ultra-low power consumption, enabling far greater device longevity. The sensor’s battery must last from several weeks to a few years. Asset tracking, depending on the application, may require the deployment of several tracker devices. Accordingly, these asset tracker devices must also be small, portable, and cost-effective.

In this design solution, we discuss the power management challenges encountered by a typical battery-operated asset tracker device, and show an example using a small, high-efficiency buck converter.

Edge-to-Enterprise communication

Figure 1 illustrates a typical tracking communication chain. The asset being tracked transmits the data via a beacon, which reaches a server through a dedicated cellular network. From here, the data reach the enterprise portal for asset management and analytics.

Asset tracking networks

A new generation of beacons connects directly to dedicated cellular networks (LTE-M, NB-IoT), eliminating the use of Bluetooth to communicate with a gateway. These technologies can be very different, but are all characterised by low power consumption, enabling several years of battery life (table 1).

Typical asset tracker system

Figure 2 shows a typical asset tracker block diagram. The three-series alkaline battery supplies a charge of 2000 mAh. A step-down regulator powers the onboard controller, sensors, and radio.


Figure 2. An asset tracker block diagram.

For demanding asset tracking applications, the system must last for a year on three alkaline batteries, drawing only 100 µA in deep sleep, and transmitting 100 mA once per day for about two minutes (figure 3). While it is true that, depending on power level and other options supported in the LTE-M or NB-IoT asset trackers, currents can be higher, for our discussion, we will stick to the 100 µA to 100 mA range.


Figure 3. Asset tracker current profile.

High-use performance requires careful selection of each block for minimum power consumption. The buck regulator must be efficient over a wide range from 100 µA to 100 mA. For instance, a 4% average loss of efficiency by the buck converter translates into a field deployment reduction of about two weeks.

Ultra-low quiescent current

The buck converter’s quiescent current is especially important since the device is in deep sleep or quiet mode most of the time, consuming only 100 µA or less. With VOUT = 1,8 V, the output power during deep sleep is POUT = 1,8 V x 100 µA = 180 µW. With η  = 90%, the input power PIN is 180 µW/0,9 = 200 µW.

If the buck converter is not carefully chosen, and has a typical quiescent current of 3 µA and a 3,6 V input voltage, there is an additional power dissipation AIN of 3 µA x 3,6 V = 10,8 µW.

The final buck converter efficiency is:

A quiescent current of 3 µA robs the buck converter of four efficiency points, draining the battery significantly faster. On the other hand, a buck converter with 300 nA quiescent current will barely reduce the efficiency, lowering it by only half a percentage point. For asset tracking applications, it is critical to select a buck converter with ultra-low quiescent current, as the system spends the majority of the time in quiet mode and relies on a battery.


Figure 4. An integrated buck converter.

Nanopower buck converter

As an example of high efficiency, the nanopower ultra-low 330 nA quiescent current buck (step-down) DC-DC converter shown in figure 4 operates from a 1,8 to 5,5 V input voltage and supports load currents of up to 175 mA, with peak efficiencies of 96%. While in sleep mode, it consumes only 5 nA of shutdown current. The device is housed in a space-saving 1,42 x 0,89 mm, six-ball wafer-level package. If higher currents are desired based on the power level in the NB-IoT or LTE-M networks, sister parts can deliver higher currents.

Efficiency advantage

Figure 5 shows the efficiency curve of the buck converter with a 3,6 V input and a 1,8 V output. Synchronous rectification at high load, and pulsed operation at light load and ultralight load, assure high efficiency across a wide operating range.

An 87,5% high efficiency operation at 100 µA, and 92% efficiency at 100 mA, make the IC ideal for asset tracking applications. This buck converter has the advantage of several efficiency points compared to alternative solutions.


Figure 5. MAX38640 efficiency curve.

The benefits of high efficiency and smaller footprint go hand in hand, resulting in less heat generation. This helps in designing a smaller, cooler asset tracker, easing concerns of device overheating.

Conclusion

Asset trackers, depending on their specific application, must operate in the field for several weeks to a few years, powered only by small batteries. This type of operation requires careful selection of each block for minimum power consumption. The buck regulator must operate efficiently over a wide input current range, from tens of microamps to hundreds of milliamps. The MAX3864x nanopower buck converter family, with its high efficiency and small size, provides an ideal power solution for asset tracking applications.


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