Save space and manage IoT device power with advanced PMICs

This article originally appeared in the Jan'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

The Industrial IoT (IIoT) demands wireless products that run for long periods to minimise maintenance visits. And many IIoT devices might be in remote or difficult-to-reach locations.

Because many smart home and IIoT sensors will be battery powered, yet will need to be available 24 hours a day, seven days a week, efficient power consumption is important to extend battery life to months or even years. Such battery life is important as it offers convenience for consumers and limits maintenance for the owners of IIoT devices.

Maximise battery life

Bluetooth Low Energy (Bluetooth LE), Wi-Fi 6, and cellular IoT bring connectivity to low power IoT devices. And the Nordic Semiconductor SoCs, companion ICs, and SiPs running these protocols are designed to be the most power efficient wireless solutions on the market, extending devices’ battery life.

But there’s more to managing power than just efficient chips. The developer can also use a few design tricks to increase the life of a battery. A key technique is to perform an event-based power consumption analysis on the system. The device’s power consumption is monitored when different system or environmental events occur, with the data collected then being analysed by designers to help refine the application to increase power efficiency and prolong battery life.

Another energy-saving technique is to reduce the power consumption to as low a level as possible when the device is not monitoring or transmitting data. This ‘standby’ mode or ‘sleep’ mode leaves only the most necessary functions switched on. It can be programmed in the device’s firmware, ensuring the device will automatically wake from standby mode when required by the system.

Finally, battery energy can be saved by reducing the frequency of data transmission or reception. For example, there’s little point reporting the temperature of a room once a second when once a minute is satisfactory to keep things stable. The difference in power consumption achieved by doing so would be dramatic.

Figure 1. The nPM1300 PMIC is designed for maximum efficiency and compactness. The PMIC simplifies system design by bringing together the essential functions needed for reliable operation in one package.

Power management ICs

Choosing efficient SoCs or companion ICs and following established design principles is a good start towards power efficient devices. But there’s more to power-frugal design than that. No matter how efficient the processor and radio are, battery life will be compromised if the developer designs an inefficient power management system.

Power management ensures the supply voltage is regulated and distributed over multiple power rails, while with rechargeable applications, it supervises charging from an external source.

Another factor the developer must consider when designing power management is the space that power management solutions can take up. They typically comprise separate voltage regulator, battery charger, fuel gauge, external watchdog, and hard reset functions which all take up valuable real estate.

Power management ICs (PMICs) are chips that supervise and control the distribution of electrical power. While there are a range of commercial solutions, the better products can save space and system complexity by integrating multiple voltage options and safety functions such as undervoltage and overvoltage protection. Superior devices enable better conversion efficiency, a reduced solution size, and an improved heat dissipation.

These advantages have led the PMIC market to grow to $34.5 billion in 2023, a figure expected to reach $42.29 billion by 2028.

Figure 2. The nPM1300 PMIC offers hardware hooks that enable the host processor to measure vital battery parameters. The host processor then uses a free software algorithm to precisely calculate battery state-of-charge. There is no additional hardware cost or impact on power consumption.

Intelligent power management

Nordic Semiconductor has now taken power management a step further. The company has introduced a range of PMICs – which complement Nordic wireless solution but also work well with non-Nordic solutions – to meet the needs of low power and space constrained IoT products.

Designed for maximum efficiency and compactness, the nPM1300 PMIC is the latest introduction to Nordic’s PMIC family. The nPM1300 simplifies system design by bringing together the essential functions needed for reliable operation in one compact package. (Figure 1.)

Figure 3. The top graph illustrates the error between a coulomb-counter reference, a voltage lookup table, and the Nordic Semiconductor nPM1300 and algorithm. The middle graph compares the instantaneous error between the reference and the alternative state-of-charge measurements. The final graph shows the battery temperature over time.

Figure 4. The top graph illustrates the error between a coulomb-counter reference, a voltage lookup table, and the Nordic Semiconductor nPM1300 and algorithm with wide temperature variations. The middle graph again compares the instantaneous error between the reference and the alternative state-of-charge measurements with the final graph showing battery temperature over time.

In a single package, the PMIC incorporates two buck converters, two load switches, a battery charger, a USB-C compatible input, ship mode and fuel gauge. This cuts the part count from up to eight chips plus associated passives for a conventional design to one chip plus a handful of passives.

With the developer working with just one device, interaction with and configuration of the PMIC is greatly simplified. The nPM1300 also incorporates key system management features, which on conventional power management systems are usually added separately. These features make the product the most integrated PMIC on the market. 

The first of these features is failed boot recovery. In such an event, devices start to run through power cycling but hang before the watchdog is enabled – if this happens, the PMIC waits until the host processor indicates everything is normal. If this message is not received, the PMIC will power cycle any connected devices to attempt another boot recovery. 

Another feature of the nPM1300 is a hard reset, which allows one or two buttons to be used to power cycle a hung device. Although this feature can be achieved using external chips, placing the function directly into the PMIC saves space and is more convenient. The Nordic PMIC also features a ‘hibernate’ mode, which powers just the essential parts of the PMIC. A timer, which can be overridden by pressing a button, wakes the PMIC after a predetermined period. 

Also included is a watchdog timer. For example, the watchdog instructs the PMIC to stop charging the battery if the software has crashed. It can also reset the host processor and power cycle the whole system. Finally, there is the power loss warning, used for example, when the battery is close to exhaustion or has been removed, or mains power has been unplugged while a depleted battery is still charging.

There is also an evaluation kit (EK) for the device. The nPM1300 EK allows developers to conduct simple evaluation and code-free configuration of the nPM1300. Using the nPM PowerUP app, the settings of the nPM1300 can be easily configured and exported into the user application software. The major benefit of this is that the hardware engineer does not need to write code – and the software engineer does not need to read the PMIC datasheet. 

Precise fuel gauging

One of the major differentiating features of the nPM1300 is a highly accurate fuel gauge, which requires no external components. The device combines the precision of a coulomb counter with the modest power consumption and simplicity of battery voltage measurement.

The battery voltage measurement is simple to implement and uses little power, but is not very accurate, particularly if the battery is subject to temperature variances. The coulomb counter is more precise but requires additional components and drains more power.

The key to Nordic’s fuel gauge’s accuracy and low power consumption over a wide temperature range is the software algorithm powered by the host processor. The processor uses information such as the current flowing out of the battery, the battery terminal voltage, the system voltage and a battery temperature thermistor reading. (Figure 2.)

Experiments compared a coulomb counter, battery voltage measurement, and the nPM1300’s fuel gauge. Compared to the coulomb counter, the battery voltage calculation peaked at nearly 20% error. In comparison, the nPM1300’s fuel gauge showed a maximum of 2% error at worst compared with the coulomb counter but was mostly within 1%. (Figure 3.)

These common measurement inaccuracies are made worse if the battery experiences wildly fluctuating temperatures. In an extreme test, with a battery temperature swing between -10 and +50˚C over a 20-minute period, the voltage measurement technique showed an error up to 30%, while the nPM1300’s fuel gauge exhibited a maximum error of just 4%, compared to the coulomb counter. (Figure 4.)

Maximising battery life demands taking responsibility for how each part of the wireless product’s system affects the overall power consumption. The developer needs to optimise each element – from the battery all the way to antenna, including radio operation, processing, and power management – to get the most from the power source.

Basing a design on a Nordic SoC or SiP together with a Nordic PMIC, is an excellent foundation for keeping the energy consumption of wireless products as low as possible.