HOW TO DESIGN LOW POWER DEVICES

Power management is an important concern when dealing with small, battery-powered devices such as sensors or wearables. The successful design of low-power devices requires a concerted team effort between the firmware, mechanical and electrical engineers to ensure client performance, quality, and design requirements are met. Let’s explore the process more closely.

 

Architecture

We start thinking about power during the architecture phase of the product development process. This is an area where the electrical and firmware engineers will work closely together because they each have responsibilities for parts of the system that affect battery life.

First, you need to consider power management use cases and requirements to determine the expected behavior of the system during normal use, during low-power, and during charging. How long should the battery last between charges? What does the device have to do during that time? Are there idle periods where the device doesn’t have to do much? If so, when does it need to start doing things again?

Processor Selection and Battery Capacity

Given those requirements, the team needs to choose a processor that can support low power modes (sleep modes) and can wake up at appropriate times or based on specific events.

Based on the current draw of that processor when it is active and when it is in its low power state, along with other components in the system, we'll work up a rough power budget, which is essentially a big spreadsheet that estimates the total current draw in the system.

These numbers will determine the battery capacity needed. The electrical engineer will then identify a battery with that capacity and physical size and weight to meet the mechanical requirements of the system.

Firmware Design

Once the design is complete and we have initial boards to work with, the firmware engineer is responsible for implementing the code that puts the processor or other peripherals into their low power states at the appropriate time.

There are a surprising number of challenges in getting power management correct. Debuggers don't work in low power mode, and there are often special settings to allow some parts of the processor to stay active. Additionally, you need to consider memory retention during sleep and how to wake up from sleep. Are there only certain pins that can wake you up from sleep?

Prototyping and Testing

Finally, the team will have prototype hardware with firmware that will tell the processor to go to sleep. At that point, the electrical and firmware engineers will work together to check that the battery model spreadsheet matches reality. They test that the sleep current is as low as expected. They also test that the firmware does indeed go to sleep at the periods estimated earlier.

Prioritizing Tradeoffs

Throughout this process, there are several tradeoffs to be considered that are often worked through with the client including:

1. Tradeoffs between battery life and battery size which can impact the size of the device itself.
2. Tradeoffs between responsiveness and battery life.
3. In systems with wireless communications, there are tradeoffs between communication range and response times with battery life.
4. Processor capability affects both battery life and the size of the printed circuit board.
5. The choice of wireless connectivity has a big impact on power consumption.
6. The battery and processor size have an impact on the mechanical and industrial design which are usually trying to reduce the size and weight of the device.

Many of those tradeoffs aren't engineering decisions, but product decisions, and we need to work closely with the clients to make those decisions and refine the requirements if necessary.

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Example:

Consider a wearable heart rate monitor. Since it is wearable, it needs to be small and light with a rechargeable battery. The client specifies that it needs to last 2 days between charges, and it needs to do a heart rate measurement every 10 seconds. Each measurement requires 5 seconds of data collection. The data needs to be transmitted to a phone via Bluetooth Low Energy (BLE), so it needs to store the data for upload. The time delay for uploads is not critical for this application.

During the architecture phase, the team would choose a processor. For example, the Nordic 52840 has low current consumption and several low power modes. It also has non-volatile memory that is retained during low power. Since we need to maintain our BLE connection, we cannot use the lowest power mode, System OFF, which turns off BLE. We will wake up based on time, so we need to ensure the low power mode can be woken from the Nordic’s real-time clock. Additionally, the requirements around timing for BLE uploads are not critical. Based on those criteria, we can use the low latency mode of System ON when it is not doing ECG measurements.

The team would then use the power consumption numbers for System ON-low latency from the data sheet in the initial battery model estimates and determine the necessary battery capacity. The EE would then provide the options for battery pack sizing to the mechanical engineer. If the size is too large for the mechanical requirements, the team would need to work with the client to consider what tradeoffs to make.

Once the team has received the first prototypes and enough firmware has been written to measure the heart rate, communicate over BLE, and enter the appropriate low power mode, the electrical and firmware engineers would test actual current consumption. They would use equipment capable of measuring and graphing very low currents such as a Source Measurement Unit (SMU). The firmware engineer would put the device into the expected low power mode to confirm it meets the expected current draw and timing estimated in the original battery model.

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Team Effort

Power management is a critical part of the design for small low-power devices and impacts nearly all aspects of the system design. It's a team effort that needs to be planned for and worked through from the very beginning of the project to ensure a successful final product design.

Our clients rely on Simplexity’s experienced design engineering teams to deliver thoughtful, well-engineered designs with smart solutions in power management. Contact our team today to find out how we can help you with your next project.