DESIGN SOLUTION 7590

Extending Battery Life in Medical Wearable Devices

By: Elnaz Shayesteh

Abstract:

Portable medical equipment requires both long battery life and small form factor to ensure successful patient use and positive medical outcomes. The MAX16164, nanoPower on/off controller addresses both longevity and portability while offering design flexibility with programmable sleep time.


Introduction

Health is fundamental to happiness and fulfillment of life. The recent COVID-19 pandemic further highlights the need for and importance of a healthy lifestyle, and the importance of continuous health monitoring. Medical wearables such as activity trackers, blood pressure monitors, biosensors, and continuous glucose monitors (CGMs) offer the ability to regularly monitor health valuable information, such as movement to check steps achieved in a day, detection of a fall, heart rate tracking to check for atrial fibrillation, or monitoring for blood glucose levels. These devices continuously collect valuable data for diagnostic and therapy purposes in an unobtrusive way. Recent innovation in semiconductor technology and an increased consumer demand for health monitoring has generated a booming need for medical wearables.

Medical wearables are becoming part of everyday life. With more people wearing health monitoring devices, more companies are investing and innovating in this field. As technology evolves, such devices become more reliable, precise, and convenient. Figure 1. Medical wearables are becoming part of everyday life. With more people wearing health monitoring devices, more companies are investing and innovating in this field. As technology evolves, such devices become more reliable, precise, and convenient.

Battery-Powered/Medical Wearable Technology Challenges

Among the most important considerations when designing a medical wearable device are efficient, reliable operation and a small form factor. For example, CGMs track a user’s glucose levels and indicate if such levels are too high or too low. Patients with Type 1 or Type 2 diabetes must respond to these variations, which, if left untreated, lead to life-threatening dehydration and diabetic coma. Before CGMs, users would prick their fingers and draw blood multiple times a day to measure sugar levels. Fortunately, wearable CGMs reduce the need to finger prick for blood analysis. Since CGM patches are battery-powered, the longer the battery lasts, the fewer times a user needs to apply the patch. This is clearly a highly desirable feature.

System designers now have many tools for extending battery life in battery-operated devices. In addition to battery type and size, many low-power devices reduce system power consumption, including low-power microcontroller, low-power signal conditioning devices, buck/boost converters, load switches, and LDOs. First, low quiescent current significantly extends battery life by reducing system standby power consumption. Second, nanoPower devices such as nanoPower buck, boost, and signal conditioning permit new solution architectures to further reduce system power consumption.

In many medical applications such as continuous glucose monitoring and ECGs, the systems need to regularly and automatically monitor and report information. This means the devices need to be powered on all the time yet consume as little power as possible when not measuring or reporting. Microcontrollers and microprocessors may perform this task, yet they often consume too much power, even in low power or sleep modes. Another solution for periodically waking up a system would be to use a real-time clock and load switch. This solution results in extra ICs and less reliability.

On/Off Controller System Diagram

A novel solution is the on/off controller to wake the system microcontroller, when necessary, and to minimize power consumption in standby mode. On/off controllers with programmable sleep time offer the benefits of lowest power consumption, no software dependency, and the smallest form factor.

The MAX16164, a nanoPower on/off controller with programmable sleep time, can be programmed with a resistor or through the I2C bus. The device consumes 30nA of current in sleep time and only 10nA in shutdown mode.

How does a nanoPower on/off controller help save energy and extend battery life? Assume a battery-powered medical device with current consumption as shown below is like the system block diagram shown above.

Based on Cypress PSoC 6 CPU
Average active current of 4.7mA
Duty cycle: 0.1%

This assumes the battery voltage is consistent, and that there are microcontrollers responsible for turning the system on and off and have a sleep mode with an RTC on consumption of 7µA.

Current consumption:

CPU Active Current x CPU Duty Cycle + CPU Sleep Current x (1 - CPU Duty Cycle) = 4.7mA x 0.001 + 7µA x 0.999 = 4700nA + 7000nA = 11700nA.

If the MAX16164 is used, the part and system have a 30nA sleep time power consumption.

Power consumption with MAX16164:

CPU Active Current x CPU Duty Cycle + CS28 Sleep Current x (1 - CPU Duty Cycle) = 4.7mA x 0.001 + 30nA x 0.999 = 4700nA + 30nA = 4730nA.

As a result, power consumption with the MAX16164 vs. CPU with sleep mode = 4730/11700 = 40%. So, using the MAX16164 will extend battery life by 60%.

Further, the MAX16164 comes in both tiny WLP and small µDFN packages, providing all the benefits of reliability and power savings with minimal footprint.

Summary

People are more aware and cautious of their livelihood and looking for ways to better monitor and control their health. Now, more than ever, small, reliable, and long-running medical devices are critical for health monitoring. Using the nanoPower MAX16164 on/off controller will extend battery life in medical wearable devices that require continuous monitoring of health signals.