Accurate optical heart-rate sensing for wearables

10th January 2019
Posted By : Alex Lynn
Accurate optical heart-rate sensing for wearables

Designers of always-on wearable and Internet of Things (IoT) devices now have an easier way to extend battery runtime while shrinking form factor with the latest tiny, highly integrated power-management IC (PMIC) from Maxim Integrated Products. The ultra-low-power MAX20345 integrates a lithium charger and debuts a unique architecture that optimises the sensitivity of optical measurements for wearable fitness and health applications.

In wearables, optical-sensing accuracy is impacted by a variety of biological factors unique to the user. Designers have been striving to increase the sensitivity of optical systems, in particular the signal-to-noise ratio (SNR), to cover a broader spectrum of use cases. Traditional low-quiescent-current regulators favoured in wearable applications come with tradeoffs that degrade SNR on the wrist, such as high-amplitude ripple, low-frequency ripple and long-settling times. 

Some designers have even turned to high-quiescent-current alternatives to overcome these drawbacks, but they must deal with increased power consumption, which reduces battery runtime or requires a larger battery. The MAX20345 features a buck-boost regulator based on an innovative architecture that’s optimised for highly accurate heart-rate, blood-oxygen (SpO2) and other optical measurements. The regulator delivers the desired low-quiescent-current performance without the drawbacks that degrade SNR and, as a result, can increase performance by up to 7dB (depending on measurement conditions).

The MAX20345 is also the latest in a line of ultra-low-power PMICs for small wearables and IoT devices that help raise efficiency without sacrificing battery runtime. To meet these needs, the MAX20345 integrates a lithium-ion battery charger; six voltage regulators, each with ultra-low quiescent current; three nanoPower bucks (900nA typical) and three LDO regulators with ultra-low quiescent current (as low as 550nA typical). 

Two load switches allow disconnecting of system peripherals to minimise battery drain. Both the buck-boost and the bucks support dynamic voltage scaling (DVS), providing additional power-saving opportunities when lower voltages can be deployed under favourable conditions. The MAX20345 is available in a 56-bump, 0.4mm pitch, 3.37x3.05mm wafer-level package (WLP).

“With sales of fitness and wellness wearable electronics expected to rise to over 114 million units by 2020, there is increasing demand for better, more precise sensing technology for measuring health vitals including heart rate and blood-oxygen levels,” said Kevin Anderson, Senior Analyst for Power Semiconductors at IHS Markit.

Frank Dowling, director of business management, Industrial & Healthcare Business Unit, at Maxim Integrated, stated: “Maxim is continuing to deliver precedent-setting innovations in the wearable healthcare arena. Our new MAX20345 extends our portfolio of ultra-low-power PMICs for wearable and always-on applications, bringing to the market a solution that enables the highest sensitivity optical sensing in wrist-worn form factors for more accurate vital-sign measurements.”

The MAX20345 is available at Maxim’s website for $4.45 (1000-up, FOB USA) and is also available from authorised distributors. The MAX20345EVKIT# evaluation kit is available for $57.00.

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