Ultra-Low-Power Intelligence and Power Management Devices for IoT Nodes Reliant on Energy Harvesting

Precise definitions of the ‘Internet of Things’ (IoT) vary. Some see it as a logical extension of the existing Machine-to-Machine (M2M) market, enabling remote control and monitoring via the Internet. Some see the term as embracing M2M and more, covering the broader concept of networked objects, whether communicating via the Internet or closed networks. However the definition evolves, the IoT concept is forecast to proliferate the connection of millions, even billions of embedded devices and sensors.

The IoT is expected to stimulate growth in remote monitoring markets such as environmental monitoring, traffic monitoring in the smart city, and structural sensing and industrial monitoring from factories to farms. Increasingly sophisticated technology allows us to install new, or enhance existing, wireless sensor nodes easily and cheaply, with the addition of a sensor, RF device and some intelligence.

Many of the end nodes of IoT will be previously unconnected objects, now enabled to provide small amounts of data on a regular, perhaps infrequent basis. Improved access, via the Internet, to this more frequent, varied and numerous sensor data, will provide better insight into system and network status, improving decision making, and thereby enhancing our lives and saving resources.

Ultra-low-power operation is vital in many of these remote monitoring-type sensor nodes, especially as more intelligence and networking capability is added. These nodes need to be maintenance free, and operate continuously, accurately and reliably for long periods of time. Many will have to rely on energy harvesting and/or long-life batteries for power. They also need to be able to switch to ultra-low-power or hibernation states between data transmissions.

Extremely-low-power, yet smarter, microcontrollers are continuing to evolve to meet these requirements together with networked radio communications devices. Power management circuitry is the third essential element. See Figure 1 below.


Figure 1: IoT wireless sensor nodes, relying on harvested energy, typically require a microcontroller, power conversion, power management and communication devices, all operating at extremely-low power levels (Courtesy of Texas Instruments).

This article will review the state-of-the-art intelligent, ultra-low-power devices designed for IoT nodes, with reference, for illustrative purposes, to the latest complementary ranges from Texas Instruments. These include the MSP430F52xx range of microcontrollers, the TPS62740 buck converter, bq25504/5 boost chargers and CC2541 Bluetooth transceiver.

Ultra-low power

Many microcontroller manufacturers have in their portfolios ultra-low-power devices, featuring extremely-low-power hibernation states, and capable of operating from very small amounts of energy, measured in nanoamps. ARM cores appear in devices from many vendors, including Freescale Semiconductor, STMicroelectronics and Texas Instruments. Other vendors of very-low-power devices include Atmel, Cypress Semiconductor, Microchip Technology, Silicon Labs, and Renesas.

Similarly, ultra-low-power radio designs are readily available, capable of transmitting sensor data across a network, and which, depending on the application, can operate for a decade or more from a coin cell battery, supercapacitor or rechargeable battery topped up from harvested energy. Manufacturers of such devices include: Analog Devices, Atmel, Nordic, STMicroelectronics, and Texas Instruments.

Managing these minute amounts of energy to ensure reliable, maintenance-free operation of sensor nodes over time requires sophisticated devices that can acquire and boost or convert microwatts of power to milliwatts. The devices themselves have to be low power. Key players in this market include Analog Devices, Maxim Integrated Products, Linear Technology, STMicroelectronics, and Texas Instruments.

Microcontroller

Let’s take the Texas Instruments MSP430 family of ultra-low-power 16-bit microcontrollers, which includes variants with selected sets of features and peripherals. The device architecture is designed to optimize battery life in portable applications, whether a remote wireless sensor node or a smartphone.

The MSP430F5249 features four 16-bit timers, a high-performance 10-bit ADC, two universal serial communication interfaces, hardware multiplier, DMA, comparator and real-time clock module with alarm capabilities. Power consumption with all clocks active is 2990 µA/MHz at 8 MHz, 3 V (Flash) or 150 µA/MHz (ram). Standby modes are 1.4 µA or 1.9 µA, with wake up in 3.5 µs. Off mode, with full ram retention is 1.1 µA at 3 V and shutdown mode at just 0.18 µA at 3 V.

The MSP430F5239 variant offers the same functionality without the ADC. With 128 KB Flash incorporated, these devices are ideal for industrial applications requiring larger memory, such as analog and digital sensor systems, and are perfect for wireless sensor nodes in remote or hazardous areas.

Internet connectivity

Complementing the ultra-low-power microcontroller, TI offers a range of wireless network processor modules, designed specifically to implement Internet connectivity with minimal software and memory overheads for the MCU. The CC3000 module is a complete platform solution to reduce component count and ease certification. It comes with software driver, sample applications, and full documentation, and supports IEEE 802.11b/g.

A step-by-step guide to designing a basic Wi-Fi application using both the CC3000 and the MSP430 microcontroller is available on the Texas Instruments website.¹ Designers can find a CC3300 basic Wi-Fi example application using the MSP-EXP430G2 Launchpad development board package. 

The development kit is supplied with two 16 MHz MSP430 microcontrollers, on-board emulation for programming and debugging, buttons, LEDs and sockets for add-on modules for wireless connectivity and more. A recently published user guide² for the LaunchPad Evaluation Kit is also available.


Figure 2: Texas Instruments’ MSP430G2 Launchpad evaluation board uses the CC3000 wireless network processor module.

Power management

The definition of ‘ultra-low power’ evolves with technology, and comparing devices can be tricky as there are a number of low-power states. A useful measure of the low-power efficiency of a power supply is its quiescent current (I_Q). See this informative White Paper, I_Q: What it is, what it isn’t, and how to use it.³ 

To convert harvested energy into usable power, a wireless sensor node needs a power management device such as a buck or step-down converter. Designed to power microcontrollers such as the MSP430 and communications devices like the CC3000 (Wi-Fi) and CC2541 (Bluetooth), TI has introduced the TPS62740. Targeted at 300 mA output current designs, this step-down converter provides just 360 nA of quiescent current during active operation and 70 nA in standby mode. Efficiency is better than 90% down to 10 µA. This tiny device operates from rechargeable Li-ion batteries. The output voltage is selectable via four V_SEL pins within a range from 1.8 to 3.3 V in 100 mV steps.

To help designers evaluate this device and test out its operation and functionality, the TPS62740EVM-186 evaluation board is supplied with a user’s guide. The guide explains how to set up the efficiency measurement, avoiding common errors.

For lower current designs, the TPS6273 is aimed at 50 mA output current circuits, providing a 370 nA quiescent current during active operation and 15 nA in sleep mode, while still achieving 90% efficiency at output currents lower than 15 µA.

Boost charger

A popular type of device for extracting power from low-voltage output harvesters, such as thermoelectric generators or piezo devices, is a boost charger with integrated DC/DC buck converter. TI’s range includes three specifically targeted at what it calls nano-power applications, such as battery-free wireless sensor nodes. The boost converters also support a variety of energy storage elements, such as rechargeable battery, supercapacitor or conventional capacitor.

The bq25504 can be started with V_IN as low as 330 mV, and can then continue to harvest energy down to 80 mV. Quiescent current is 330 nA. The device implements a programmable maximum power point tracking (MPPT) sampling network to optimize the transfer of power into the device. Alternatively, an external reference voltage can be provided by an MCU to produce a more complex MPPT algorithm. A more detailed explanation of how it works can be found in a previously posted TechZone article.⁴

The evaluation board, bq25504EVM-674, demonstrating the ultra-low power boost converter with battery management for energy-harvesting applications, is compatible with most MCUs and 3 V coin-cell batteries.

The bq25505 boost charger, with active quiescent current of 325 nA, adds an autonomous power multiplexer gate drive that ensures constant power is available when the system needs to operate, even when no energy is available from the harvester. Two storage elements can be multiplexed to provide a single rail to the system load. The device is ideal for applications where scavenged energy can be exploited some of the time, thereby extending the lifetime of the battery.

Finally, the bq25570 adds buck output regulation to ensure the system is provided with an externally-programmable regulated supply. Active quiescent current is 488 nA and efficiency is better than 90% down to 10 µA. Further, there are independent enable signals that allow the application to control when to run the regulated output, or to put the device in an ultra-low quiescent current sleep state, down to 5 nA.

Conclusion

Much of the massive growth forecast for the Internet of Things movement will come from the proliferation of smarter wireless sensor nodes in a wide range of remote monitoring applications. Such nodes are likely to rely entirely or partially on energy harvesting to fuel the ultra-low-power devices that provide the intelligence (MCU), the communication (RF network processor) and the power management (booster and converter) required.

References:

1. Texas Instruments Design Guide: Basic Wifi Application with CC3000 and MSP430
2. Texas Instruments White Paper on quiescent current
3. Texas Instruments User Guide for MSP-EXP430-G2 LaunchPad Evaluation Kit
4. DigiKey Techzone Article covering bq25504