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What are the Best IoT Applications in the New World of Power Management ICs?

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Figure 1. LTC3337 and LTC3336 application circuit.

Diarmuid Carey, Central Applications Engineer

This article explores battery technology for the Internet of Things (IoT). Describes some of the problems designers face with power supply and provides solutions from Analog Devices. These solutions are highly efficient and can help curb other issues with your IoT devices, including size, weight, and temperature.

With the increasing use of IoT devices in industrial equipment, home automation, and medical applications, there is increasing pressure to optimize the power management portion of these devices, whether through a smaller form factor, better efficiency, better current draw or faster charging times (for portable IoT devices). All of this must be accomplished in a small form factor that does not adversely affect thermals or interfere with the wireless communication implemented by these devices.

What is IoT?

This particular IoT application area comes in many different guises. It typically refers to a battery-powered, grid-connected smart electronic device that sends pre-computed data to cloud-based infrastructure. It uses a mix of embedded systems, such as processors, communication ICs, and sensors, to collect, respond to, and send data to a central point or other node on the network. This can be anything from a simple temperature sensor that sends ambient temperature information to a central monitoring area, to a machine health monitor that monitors the long-term health of very expensive factory equipment.

Lately, these devices are being developed to solve a particular challenge, whether it is to automate tasks that would normally require human intervention, such as home or building automation, or perhaps to improve the usability and longevity of equipment in the case of industrial IoT applications, or even to improve security when considering condition-based monitoring applications deployed in structure-based applications such as bridges.

Application Examples

The application areas for IoT devices are nearly endless with new devices and use cases being thought of every day. Applications based on smart transmitters collect data about the environment in which they sit to make decisions about heat control, activation of alarms or automation of particular tasks. Additionally, portable instruments such as gas meters and air quality measurement systems provide accurate measurement via the cloud to a control center. GPS tracking systems are another application. They enable the tracking of shipping containers as well as livestock such as cows via smart ear tags. These comprise only a small area of ​​cloud-connected devices. Other areas include portable healthcare and infrastructure sensing applications.

One significant growth area is industrial IoT applications, which are part of the fourth industrial revolution where smart factories take center stage. There is a wide range of IoT applications lately trying to automate as much of the factory as possible, whether through the use of automated guided vehicles (AGVs), smart sensors like RF tags or pressure gauges, or other environmental sensors. placed around the factory.

From an ADI perspective, the top-level IoT focus has been on five main areas:

  • Smart Health: Supports vital signs monitoring applications at both clinical and consumer levels.
  • Smart Factories: Focus on building Industry 4.0 by making factories more responsive, flexible, and efficient.
  • Smart Buildings/Smart Cities: Uses smart sensing for building security, parking space occupancy detection, as well as thermal and electrical control.
  • Smart Farming: Uses available technology to enable automated farming and resource-use efficiency.
  • Smart Infrastructure: Based on our condition-based monitoring technology to monitor movement and structural health.

More information on these focus areas and the technologies available to support them can be found on the Analog Devices website.

IoT Design Challenges

What are the key challenges facing a designer in the ever growing IoT application space? Most of these devices, or nodes, are being installed post-construction or in hard-to-reach areas, so running power for them is not a possibility. This, of course, means that they are totally dependent on batteries and/or power harvesting for their power source.

Moving power around large facilities can be quite expensive. For example, consider powering a remote IoT node in a factory. The idea of ​​running a new power cable to power this device is expensive and time consuming, essentially leaving battery power or power harvesting as the remaining options for powering these remote nodes.

The reliance on battery power introduces the need to follow a strict power budget to ensure battery life is maximized, which of course has an impact on the total cost of the device. Another disadvantage of using the battery is the need to replace the battery after its useful life has expired. This includes not only the cost of the battery itself, but also the high cost of human labor to replace and possibly dispose of the old battery.

An additional consideration regarding battery cost and size: it is very easy to overdesign the battery to ensure there is enough capacity to achieve the required lifespan, which is often in excess of 10 years. However, overdesign results in additional costs and sizes, so it is extremely important to not only optimize the power budget, but also minimize power usage wherever possible to install the smallest possible battery that still meets your requirements. of design.

Power in IoT

For the purposes of this power discussion, power sources for IoT applications can be viewed as three scenarios:

  • Devices that rely on non-rechargeable battery power (primary battery).
  • Devices that require rechargeable batteries.
  • Devices that use energy harvesting to provide power to the system.

These fonts can be used individually or alternatively combined if required by the application.

Primary Battery Applications

You all know about the different primary battery applications, which are also known as non-rechargeable battery applications. These are geared towards applications where only occasional power is used, meaning the device turns on occasionally before returning to a deep sleep mode where it consumes minimal power. The main advantage of using them as a power source is that it provides a high energy density and a simpler design, since it does not need to accommodate battery charging/management circuitry, as well as lower cost, since the batteries are cheaper and less electronics required. They are a good fit for low-cost, low-power applications, but because these batteries have a finite life, they are not suitable for applications where power consumption is a bit higher, so this incurs as much cost. for a replacement battery and for the cost of the service technician required to replace the batteries.

Consider a large IoT installation with many nodes. If you have a technician on site replacing the battery for a device, very often all the batteries will end up being replaced at once to save labor cost. Of course this is wasteful and only adds to our overall global waste problem. On top of that, non-rechargeable batteries provide only about 2% of the energy used to make them in the first place. ~98% of the energy wasted makes them a very uneconomical power source.

Obviously, these have a place in IoT-based applications. Their relatively low initial cost makes them ideal for lower power applications. There are many different types and sizes available, and since they don't require a lot of additional electronics for charging or management, they are a simple solution.

From a design perspective, the key challenge is to make the most of the power available from these small power sources. To that end, a lot of time needs to be spent creating a power budget plan to ensure battery life is maximized, with 10 years being a common lifetime goal.

For primary battery applications, two parts of our family of nanopower products are worth considering: the LTC3337 Nanopower Coulomb Counter and the LTC3336 Nanopower Buck Regulator, shown in Figure 1.

The LTC3336 is a low power DC to DC converter that operates from up to 15V input with a programmable maximum output current level. The input can go up to 2,5V making it ideal for battery powered applications.

The quiescent current is exceptionally low, 65 nA, while regulating without load. As DC to DC converters go, they are pretty easy to set up and use in a new design. The output voltage is programmed based on how the OUT0 to OUT3 pins are connected.

The companion device to the LTC3336 is the LTC3337, a nanopower primary battery health monitor and coulomb counter. This is another user-friendly device in a new design: simply position the IPK pins according to the maximum current required, which is in the region of 5mA to 100mA. Run some calculations based on the selected battery, then place the recommended output capacitor based on the selected maximum current, which is stated in the datasheet.

This is definitely a fantastic device pairing for IoT applications with a limited power budget. These parts can accurately monitor primary battery power usage and efficiently convert the output to a usable system voltage.

Rechargeable Battery Applications

Let's move on to rechargeable applications. These are a good option for higher power or higher drain IoT applications where frequent replacement of the primary battery is not an option. A rechargeable battery application is a higher cost implementation due to the initial cost of the batteries and charging circuitry, but in higher drain applications where the batteries are frequently depleted and charged, the cost is justified and soon paid back.

Depending on the chemistry used, a rechargeable battery application may have a lower initial energy than a primary cell, but in the long run it is the most efficient option and is generally less wasteful. Depending on power needs, another option is storage with capacitors or supercapacitors, but these are more for short-term backup storage.

Battery charging involves several different modes and specialized profiles that depend on the chemistry used. For example, a lithium-ion battery charge profile is shown in Figure 2. At the bottom is battery voltage, and charge current is on the vertical axis.

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Figure 2. Charging current vs. Battery voltage.

 

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Figure 3. Charging voltage/current vs. weather.

When the battery is severely discharged, as on the left in Figure 2, the charger must be smart enough to put it into pre-charge mode to slowly increase the battery voltage to a safe level before going into constant current mode. .

In constant current mode, the charger pushes the programmed current into the battery until the battery voltage rises to the programmed float voltage.

Both the programmed current and the voltage are defined by the type of battery used: the charging current is limited by the c-rate and the charge time required, and the float voltage is based on what is safe for the battery. System designers can lower the float voltage a bit to help with battery life if required by the system, as with anything to do with power, it's all about trade-offs.

When the float voltage is reached, the charging current can be seen to drop to zero and this voltage is held for a time based on the termination algorithm.

Figure 3 provides a different graph for a 3-cell application showing behavior over time. The battery voltage is shown in red and the charging current is in blue. It starts in constant current mode, reaching a maximum of 2 A until the battery voltage reaches the constant voltage threshold of 12.6 V. The charger maintains this voltage for the time defined by the termination timer, in this case, one 4 hour window. This time is programmable in many parts of the charger.

For more information on battery charging, as well as some interesting products, I would recommend the article from Analog Dialogue  "Simple Battery Charger ICs for Any Chemistry. "

Figure 4 shows a good example of a versatile battery charger, the LTC4162, which can provide charging current up to 3.2 A and is suitable for a variety of applications, including portable instruments and applications requiring larger batteries or multi-cell batteries. . It can also be used to charge from solar power sources.

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Figure 4. The LTC4162, a 3,2 A buck battery charger.

energy harvesting applications

When working with IoT applications and their power sources, another option to consider is power harvesting. Of course, there are several considerations for the system designer, but the appeal of free power cannot be underestimated, especially for applications where power requirements are not overly critical and where installation must be hands-off, i.e. no maintenance technician. service can reach it.

There are many different power sources to choose from, and you don't need to be an outdoor application to take advantage of them. Solar energy and piezoelectric or vibrational energy, thermoelectric energy and even RF energy (although this has a very low power level) can be harvested.

Figure 5 provides an approximate energy level when using different harvest methods.

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Figure 5. Approximate power sources and levels available for various applications.

As for the disadvantages, the initial cost is higher compared to the other power sources discussed above, as you need a harvesting element such as a solar panel, piezo receiver or Peltier element, as well as the power conversion IC. and associated enabling components.

Another drawback is the overall size of the solution, particularly when compared to a power source like a coin cell battery. It is difficult to achieve a small solution size with an energy harvester and conversion IC.

Efficiency wise, this can be tricky to manage low power levels. This is because many of the power supplies are AC so they need rectification. Diodes are used to do this. The designer must deal with the loss of energy resulting from its inherent properties. The impact of this lessens as the input voltage increases, but that's not always possible.

The devices that appear in most power harvesting discussions are from the ADP509x family of products and the LTC3108, which can accommodate a wide range of power harvesting sources with multiple power paths and programmable load management options that offer the highest design flexibility. A multitude of power sources can be used to power the ADP509x, but also to draw power from that power source to charge a battery or power a system load. Anything from solar power (both indoor and outdoor) to thermoelectric generators for extracting thermal energy from body heat in portable applications or engine heat can be used to power the IoT node. Another option is to harvest power from a piezoelectric source, which adds another layer of flexibility; this is a good option for extracting power from an operating engine, for example.

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Figure 6. Block diagram of the ADP5090 in a power harvesting application.

Another device that is capable of being powered from a piezo source is the ADP5304, which operates with a very low quiescent current (260 nA typical with no load), making it ideal for low power energy harvesting applications. The datasheet shares a typical power harvesting application circuit (see Figure 7), powered by a piezo source and used to provide power to an ADC or RF IC.

Energy management

Another area that should be a part of any discussion regarding applications with a limited power budget is power management. This starts with developing a power budget estimate for the application before looking at different power management solutions. This essential step helps system designers understand the key components used in the system and the amount of power they require. This affects your decision to select a primary battery, rechargeable battery, power harvesting, or a combination of these as your power delivery methodology.

How often the IoT device collects a signal and sends it back to the central system or the cloud is another important detail when looking at power management, which has a big impact on overall power consumption. A common technique is to duty cycle power usage or stretch the time between waking up the device to collect and/or send data.

Making use of standby modes on each of your electronic devices (if available) is also a useful tool when it comes to managing system power usage.

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Figure 7. ADP5304 piezoelectric source application circuit.

Conclusion

As with all electronic applications, it's important to consider the power management part of the circuit as early as possible. This is even more important in power constrained applications such as IoT. Developing a power budget early in the process can help the system designer identify the most efficient path and the right devices that meet the challenges posed by these applications while achieving high power efficiency in a small solution size.