Ultra-low power design for next generation battery-powered devices

From the manufacturing process to all application areas, ultra-low power technology has gradually become an indispensable requirement in all designs. Energy-sensitive applications, especially products that must provide continuous operation for several hours on a single battery, need to incorporate ultra-low power design concepts. To meet these requirements, it is necessary to integrate a microcontroller, which requires not only understanding of battery characteristics, but also in-depth understanding of how to design a device in order to achieve the goal of continuous use of 10, 15 or even 20 years without replacing the battery.

Operating at very low power requires a whole new way of looking at these very low power applications. Traditionally, these complex applications have had their power usage analyzed from a convenient point of view, making assumptions about the application and power supply using a paradigm in order to produce a "typical" power consumption.

However, to achieve, for example, 20 years of continuous operation on a single battery, the application must draw less power than the battery’s self-discharge rate, making each nA even more critical in the overall power budget.

The industry standard CR2032 button cell self-discharge current can be less than 250nA. CR2032 is a common lithium/manganese dioxide battery with a nominal (no load) voltage of 3.0V. In practical applications, to achieve the longest battery life, a highly integrated microcontroller (MCU) must be used. In sleep mode, the MCU must be able to operate well in the range of less than 1μA while providing the right mix of processing power and integrating peripherals and on-chip memory.

Figure 1: Relationship between MCU sleep current and battery life.

When every nA of power consumption matters, simply making assumptions about performance or power consumption is no longer an absolute necessity. To estimate the best choice for a design, it is necessary to look at more parameters that may not seem so critical in less energy-sensitive applications. For example, ultra-low-power microcontrollers with advanced sleep modes are now common, but specifying the same sleep mode power consumption for an entire family of microcontrollers may not be correct. Well-known microcontroller product families can show variations of more than 1,700%. Therefore, for ultra-low-power designs, it is important that the selected microcontroller family can be upgraded with memory without sacrificing low-power performance and must be pin-compatible.

Another important point is to estimate the change of the device battery charge over time. All engineers understand that the voltage change of a primary battery over time depends largely on the battery architecture and load. For example, a pair of AA/AAA alkaline batteries, such as CR2032, have different discharge modes. Therefore, a well-designed application must be able to operate with the same performance under different battery conditions (Figure 2).

Figure 2: The service life of different batteries.

Without considering the battery characteristics, it is not easy for an engineering team to use the same series of microcontroller products in a power-sensitive application and ensure that the product can maintain normal operation for many years with a single battery. The design team's considerations include power consumption characteristics at low voltages and operating performance. At this time, the microcontroller application can operate at 2V or lower voltage to extract more battery power. In addition, under low voltage conditions, the microcontroller must also maintain high-frequency operation to ensure maximum application performance.

As the demand for extremely low power designs grows steadily, the importance of an effective instruction set architecture (ISA) grows. Energy-sensitive applications can spend up to 99% of their time in sleep mode, and inevitably these devices must be awakened periodically, either at predefined intervals or in response to external stimuli. In this regard, the key design consideration is the total energy usage to accomplish the task. Design teams must select microcontrollers that implement an ISA that has a greater percentage of single-cycle instructions to perform a specific task, thereby completing the task with shorter execution times and lower power consumption.

Figure 3: Impact of single-cycle instructions on power consumption. (PIC24 vs MSP430)

For example, if a generic C function memcpy() is used to copy 32 bytes of data from one memory address to another, and compiled for PIC24F and MSP430, the resulting program code requires 790% more cycles than the MSP430 (316 vs. 40). At 3V and 4MHz, this example consumes 230% more energy than the MSP430. This shows the importance of the ISA.

The embedded electronics industry has reached a watershed moment in building future applications, with current integrated component design, estimation and practice methods facing a shift. This shift is so significant that in the next few years, more applications will incorporate extremely low power technology into their designs.