Extending Battery Life of Wireless Medical Devices

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Extending Battery Life of Wireless Medical Devices  
Wireless medical devices are becoming increasingly more prevalent for remotely monitoring and logging  
vital signs to assist in the detection and treatment of diseases and medical anomalies. An example is  
shown in Figure 1. Wireless body sensors upload vitals via an internet hub or personal server such as the  
patient’s smart phone. To continuously monitor and upload vital data, wireless medical devices need to  
maintain long-term connectivity to the cloud. Key to continued rapid growth of medical wearables is  
reduced device size, longer battery life and ubiquity of smart phones.  
Monitoring Site  
Internet Hub  
Auto Alerts  
Body Area Network  
RF Li  
Medical device  
Medical device  
Data Analysis  
Medical device  
Medical device  
Internet Cloud  
Figure 1: U-Healthcare system overview  
Wearable medical monitoring devices are designed to collect and compress data (metadata), send it to the  
cloud via internet hub devices in short bursts, and then go to sleep to conserve power. Battery life depends  
in part on the power consumption of the wireless radio and interface protocol deployed in the design.  
SiTime Corporation  
Extending Battery Life of  
Wireless Medical Devices  
Designers of wearable sensor devices can choose from the following PAN (personal area network) low  
power wireless communication standards.  
Bluetooth® low energy (BLE)  
These radio standards are designed to satisfy demands of short-range wireless applications. Using a few  
passive components, any of these transceivers can be interfaced with a low-cost microcontroller via  
UART, SPI or USB and can fit a small footprint ideally suited for monitoring human vitals.  
Which Wireless Standard has the Lowest Power?  
Independent studies comparing the top three wireless standards have shown that BLE has the lowest  
power consumption in a cyclic sleep scenario typical of a network of wireless body sensors [1], [2]. The  
cyclic sleep scenario is a typical use case of these battery powered devices wherein the device core is  
shut down for a pre-set time called “sleep time” typically in the range of 2 to 10 seconds and “woken” when  
it needs to transmit vitals during a short burst lasting a few milliseconds. This translates to a low duty cycle  
activity scenario which leads to lower average power consumption as illustrated in Figure 2.  
SLEEP TIME (Tsleep)  
Figure 2: Cyclic sleep activity scenario; Average power is directly proportional to duty cycle (Ton/Tsleep)  
In one experiment, average power consumption was measured across various sleep intervals on three  
wireless modules [1]. The results of the power consumed across the various RF modules, shown in Figure  
3, indicate that the BLE protocol consumes the least amount of power compared to ANT and ZigBee  
irrespective of sleep intervals. The data also show power consumption scales inversely with sleep interval  
across all three RF standards in a cyclic sleep activity scenario.  
Given the ubiquity of the smart phone and its native support for Bluetooth 4.0, BLE is ideally suited for  
wearable medical devices. In certain medical environments, where smart phone use is prohibited, the use  
of a BLE-to-Internet bridge may be used as an alternative.  
SiTime Corporation  
Extending Battery Life of  
Wireless Medical Devices  
Figure 3: Power consumption of the three wireless standards vs. sleep interval  
BLE in a Medical Device  
A typical wireless medical device comprises a low power 32-bit MCU interfacing to biometric sensors and  
a RF front-end SoC (system on chip) as shown in Figure 4.  
Figure 4: Block diagram of a wireless medical device  
SiTime Corporation  
Extending Battery Life of  
Wireless Medical Devices  
The low power MCU typically serving as sensor data aggregator sends vitals to the BLE RF front-end via  
an I2C or UART interface, and runs off the following clock sources:  
12 MHz crystal  
Frequency tolerance: +/- 30 ppm for 0 to 70°C  
Used for clocking ARM Cortex-M3 core and peripherals  
32.768 kHz crystal  
Frequency tolerance: -200 ppm for 0 to 70°C  
Used for real-time clock(RTC) and watch-dog timer  
The BLE RF front-end implements the Bluetooth-4.0 PHY layer and BLE Link-layer including GATT  
profiles (glucose, temperature, blood pressure, etc.) and runs off of two clock sources:  
24 MHz crystal  
Frequency tolerance: +/- 20 ppm for 0 to 70°C  
Used for base-band processing and RF 2.5 GHz synthesis  
32.768 kHz crystal  
Frequency tolerance: -200 ppm for 0 to 70°C  
Used for sleep clock timing  
Empirical measurements have shown power consumption of a BLE medical device is inversely  
proportional to the time it spends in the “sleep” state, and the sleep clock accuracy (SCA) of the 32 kHz  
clock used to time this “sleep” state has a direct impact on the battery life of the device. To understand  
this, let’s briefly review how a BLE slave (patient’s medical device) and a “paired” BLE master (internet  
hub) establish a connection event. A scope capture of the dynamic IDD timing of a BLE slave is  
representative of the connection event timing profile of a BLE device as shown in Figure 5.  
SiTime Corporation