Powering Embedded Systems

Embedded systems are ubiquitous. They control our home appliances, track our pets, manage industrial processes, and take soil samples on distant planets. Initiatives to improve industrial performance, such as Industry 4.0, launched the Industrial Internet of Things (IIoT), which accelerated the need for low-power sensors and actuators across geographically dispersed production sites. Remote monitoring of crops and other agriculture applications took the IoT into the field and challenged engineering to deliver suitable wireless connectivity and long-life power supply methods.

Of the many design considerations when architecting a new embedded-based product, one of the most critical is establishing how to power the product, since this determines the overall power budget. The amount of available power will also influence some of the product's capabilities and functions. For example, large LCD screens consume a lot of energy during use, limiting usage for a battery-powered application. Likewise, high-bandwidth wireless protocols such as Wi-Fi are also power-hungry compared to those designed for low-power use cases such as Bluetooth Low Energy.

Mains (line)-powered applications offer the optimal power supply solution but introduce design constraints such as isolation, power conversion, heat dissipation, and the available enclosure space.

Power choices

The power supply of an embedded system has several facets, each with a specific set of technical attributes. The power source(s), power conversion method(s), power management, and power delivery network are the crucial discrete functions of a power supply architecture. Although the power requirements of each design will vary considerably, the following are key topics engineers should review:

Power sources

There are several popular methods of powering an embedded system, including mains, non-rechargeable or rechargeable batteries, and energy harvesting (single or a variety of sources). Is a secondary supply stipulated to cater for power outages for a line-powered application? If so, how long should the backup supply be available? Likewise, a battery-powered application should state the desired service duration between battery charges or replacements. Energy harvesting techniques are a popular method of extending battery life by trickle-charging. For some ultra-low-power applications, replacing a battery with a supercapacitor and an energy harvester power management IC (EH PMIC) might be possible.

Power conversion

What voltage(s) are required to power the embedded system? Is a single 3.3VDC supply sufficient, or does the design need additional rails such as 1.8VDC or 5VDC for specific devices or peripherals? What are the current consumption specifications for each supply rail, and is any consumption profiling data available? Line-powered systems will require at least one voltage conversion stage, for example from 240VAC to 3.3VDC. A DC/DC converter could provide additional DC rails, for example, from 3.3VDC down to 1.8VDC or up to 5VDC.

Power delivery network

A more complex embedded application might require a power delivery network (PDN) such as the multiple voltage rails highlighted above. Power distribution around a PCB might require specialist skills since some ICs are particularly susceptible to EMI and high dV/dt transients. Designers must also consider whether to place DC/DC converters close to the point of load. Additionally, sophisticated ICs may require sequenced power rails.

Power architecture constraints

Some embedded products, such as medical safety devices, may be subject to regulatory compliance. Isolation is a crucial aspect when selecting an AC/DC or DC/DC converter; for medical and healthcare applications, this aspect is covered by the internationally recognised IEC 60601 safety standard. Energy efficiency is also a regulated characteristic, for example, the Level VI requirement, with the no-load power consumption of a line-powered supply stipulated to be below 0.3 watts. Power converters and power supplies are also subject to compliance with electromagnetic compatibility (EMC) and EMI standards.

Power management and technical specifications

If the product features a rechargeable battery, a power management IC (PMIC) monitors the battery's state of charge (SOC) and sets the charge current as needed. The PMIC also manages the battery's discharge performance and decides when to isolate the load to prevent damage and erratic system operation. PMIC and converter communication with the host microcontroller typically utilises the PMBus protocol, SPI or I²C interfaces.

Power delivery implementation

The engineering team will choose a module or integrated component for most applications, some of which are highlighted in the next section. However, a discrete approach might be viable if the team cannot accommodate the required supply voltages with off-the-shelf components.

Popular power supply components

An example of an AC/DC power supply is the single-output PBO-15C series from CUI Inc. (Figure 1). The 15 watt PBO-15C series is packaged in an open-frame, single in-line package (SIP) arrangement for PCB mounting horizontally or vertically. Accommodating the universal wide input range of 85VAC to 305VAC, the series is available with the popular nominal voltages from 3.3VDC to 24VDC. The output is galvanically isolated from the input up to 3,000VAC for one minute. The PBO series is suitable for industrial and smart home applications, conforming to IEC 62368 requirements, and is a Class II (no protective earth) design.

The series also conforms to the EN55032 EMC and EMI standards for conducted and radiated immunity. Line regulation, the measure of how the output voltage varies when the input changes, is better than +/-0.5% at full load. Load regulation, the metric of how the load changes impact the output voltage, is +/- 1.0% for a load of 0 to 100% for the 5VDC model. The no-load power consumption at 230VAC input is 0.25W, and the energy efficiency is typically better than 82% for the 9VDC to 24VDC output models.

For battery-powered applications or those that already have an AC/DC converter, a linear regulator may provide a convenient, low-cost, and compact method of providing a regulated DC rail. An example of a three-terminal linear regulator is the L78L12ACZ-TR from STMicroelectronics. Constructed in a TO-92-3 package suitable for through-hole mounting, the L78L12 supplies a regulated 12VDC output of up to 100mA from an input voltage of 14VDC to 35VDC. The L78L regulator family offers all the popular nominals from 3.3VDC to 24VDC. The regulator consumes a quiescent (no load) current of 6.5mA. The design simplicity of linear regulators provides a low-noise characteristic, making them suitable for use cases where conducted noise disrupts system performance (for example, for accurate analogue-to-digital conversion).

Another type of low-noise linear regulator is the low dropout regulator (LDO) such as the Analog Devices ADP151 ultra-low-noise 200mA linear regulator. An LDO exhibits minimal input and output voltage difference.

The ADP151 features a dropout voltage of 140mV compared to the 2V attribute of the L78L12 highlighted above. Capable of delivering up to 200mA with a 2.2VDC to 5.5VDC input voltage, the ADP151 suits low-noise, battery-powered analogue conversion and noise-sensitive RF applications. The quiescent current with no load is just 10μA. Figure 2 illustrates the internal architecture of the ADP151, highlighting the use of an op-amp, internal voltage reference, and a PMOS pass transistor.

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Figure 2: The internal architecture of the Analog Devices ADP151 ultra-low-noise linear regulator. (Source: Analog Devices)

Linear regulators are extremely popular; however, they cannot provide an output voltage greater than the input. Depending on the topology (that is, how the conversion occurs), switching DC/DC converters can perform this function and deliver an output lower than the input voltage. DC/DC converters offer a practical, cost- and space-efficient approach to provisioning single or multiple power rails for an embedded design. There are several different types of DC/DC converter topologies. Some topologies provide input to output galvanic isolation, while others are non-isolated. The majority are derived from or are iterations of two types: buck (step down) or boost (step up).

An example of a compact, single-output DC/DC converter is the IZB series from XP Power (Figure 3). These isolated and regulated 3 watt converters are packaged in an industry-standard SIP8 encapsulated format and available in various input (2:1) and output voltage combinations covering the popular nominals from 3.3VDC to 24VDC. Efficiency is typically 68% to 84% and device dependent. Line regulation is better than 0.5% for a 1% change of input voltage and load regulation within 1% up to full load. The switching frequency is 250kHz across all load conditions.

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Figure 3: The XP Power IZB series of isolated SIP8-packaged DC/DC converters. (Source: XP Power)

Another compact, switching DC/DC converter is the non-isolated Traco TSR 1.5E series (Figure 4). Capable of supplying up to 1.5A from a 7VDC to 36VDC input (15VDC to 36VDC for the 12V output model), 3.3VDC, 5VDC, and 12VDC models are available. Efficiency is typically 95% across the range.

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Figure 4: The Traco TSR 1.5 E series offers a compact, highly efficient DC/DC converter for point-of-load applications. (Source: Traco)

As mentioned, some battery-powered handheld designs require circuitry to charge the battery and deliver a regulated supply voltage. For this type of circuitry, one solution may be the Maxim Integrated MAX77654 ultra-low-power, highly integrated PMIC. As a three-output, single-inductor, multiple-output (SIMO) buck-boost regulator, the MAX77654 can deliver programmable outputs from 0.8VDC to 5.5VDC. Dual separate 100mA LDO outputs are also available.

Mouser Electronics