-By Mark Patrick, Mouser Electronics
The conflicting requirements for portable medical hardware represent a constant challenge to design engineering teams. These always-on devices must be able to manage battery life with maximum effectiveness while keeping within dimensions that suit being situated upon the human body (to ensure patients’ comfort—especially when worn 24 hours a day). Systems need to deliver elevated levels of performance and have robust construction but also be cost-effective. Constituent power management integrated circuits (PMICs) need to utilize ultra-low power architectures in order to optimize the sensitivity of measurements in fitness tracking and medical wearable applications so that signal-to-noise ratio (SNR) figures are kept high.
The growing popularity of mobile networks has been one of the most important factors in the development of wearable technology—from both a consumer and a healthcare perspective. Initially, designed for sports and wellness, wearable devices are now finding increasing uptake in the medical market. New generations of medical wearable devices have incorporated a range of micro-electromechanical system (MEMS) sensors—such as accelerometers, gyroscopes, and heartrate monitors. Over time, other sensors have started to be added, for determining parameters such as pulse variability and skin conductance, but these have SNR issues associated with them. In order to deal with the dynamics that define the medical sector, designers need to look for new energy-saving solutions and also marry these with better noise reduction techniques.
Reducing Noise in Optical Measurements
Various biological factors influence the accuracy of optical detection, and design engineers have sought to maximize sensitivity by offering significantly better SNR over a wide range of use cases. Low quiescent voltage regulator ICs are generally accompanied by elements that will degrade SNR, such as a high amplitude ripple, low frequency ripple, and long settling times.
An essential measurement parameter in the medical field is, of course, that of heart rate. Going beyond simply the number of beats per minute, a significant amount of additional information regarding the behavior of the heart can be gleaned for monitoring this (in terms of how frequency is effected by activity, etc.). An optical measurement method, known as photo-plethysmography (PPG), measures the change in blood volume through the distension of arteries and arterioles in the subcutaneous tissue. It can also be used to determine the saturation of oxygen in the blood (SPO2). In the medical field, this technology is usually implemented in a clip worn on the finger. The device emits a beam of light through the skin (from an LED placed on one side) and measures the variations in the transmission of light inside the finger (through a photodiode placed on the other side of the device). Designers face several problems ensuring a continuous and reliable heart-rate measurement. Operational effectiveness depends on several factors—such as the effect of ambient light, interference between the LED and the photodiode, movement of the wearable device on the epidermis and suchlike.
Maxim’s MAXM86161 (Figure 1) is a complete Single-Channel Optical Data Acquisition System packaged in an ultra-low-power module. The MAXM86161 sensor module is designed for in-ear medical and mobile applications and optimized for reflective Heart Rate (HR), Oxygen Saturation (SPO2), and continuous monitoring for Heart Rate Variability (HRV). The transmitter side of the MAXM86161 has three programmable high-current LED drivers. The receiver side of the MAXM86161 consists of a high-efficiency PIN photo-diode and an optical readout channel. The optical readout has a low-noise signal conditioning Analog Front-End (AFE), including 19-bit ADC (Analog-to-Digital Converter), a high-performance Ambient Light Cancellation (ALC) circuit, and a picket fence detect-and-replace algorithm.
The impetus to optimize energy efficiency is a constraint on optical measurement mechanisms. Novel switching configurations are used (instead of standard LDO regulators) to improve efficiency, with various inductors employed to deliver the correct supply bus. The voltage-regulation element must provide a low high frequency ripple so that it does not directly interfere with heart-rate measurements. LEDs must operate at a different voltage range from what is supplied by Li-ion batteries. The addition of new buck-boost converter technologies can save board space and also curb energy consumption. The single-inductor multiple-output (SIMO) buck-boost architecture means that the number of inductors and ICs necessary to generate the output voltage is far fewer.
Efficient Power Management via Next Generation PMICs
With the increasing success of personal and remote monitoring devices, reduction in size, accurate measurement of parameters, and extension of battery life have all become essential. The energy optimization strategy for wearable devices must be based firmly on the management of downtime, (i.e. the device must be placed in standby mode whenever it is possible to do so). PMICs for wearables accept a very low input voltage and have architectures employing high energy density accumulators.
The MAX20310 from Maxim Integrated is a compact power-management integrated circuit (PMIC) for space-constrained, battery-powered applications where size and efficiency are crucial. The device includes a Single-Inductor Multiple-Output (SIMO) buck-boost switching regulator that provides two programmable voltage rails using a single inductor, minimizing the device’s footprint. The MAX20310 operates with battery voltages down to 0.7V for use with Zinc Air, Silver Oxide, or Alkaline batteries. The architecture allows for output voltages above or below the battery voltage. The MAX20310 also includes a programmable power controller allowing the device configurations for use in applications that require a true off state or for always-on applications such as portable medical devices and wearables (see Figure 2).
Members of the TPS6572x series of PMICs, developed by Texas Instruments, each integrate a battery charger and a highly efficient step-down converter. They allow the use of relatively small inductors and capacitors to achieve solutions of reduced dimensions. The TPS65720 provides an output current of up to 200mA, while TPS657201, TPS657202, and TPS65721 all provide up to 400mA. Every TPS6572x PMIC also integrates a 200mA LDO with an input voltage range of 1.8V to 5.6V. This enables them to be powered from the output of the step-down converter or directly by the system voltage (see Figure 3).
Advanced portable technology that can monitor physical activity, collect data, and provide real-time responses is set to be the future of personalized care, enabling greater convenience for patients and boosting the efficiency of healthcare staff, too. Body-worn health-monitoring devices must exhibit high degrees of accuracy, have strong reliability, and be simple to manage, with lengthy working lifespans. By having PMICs that can simultaneously address power budget and SNR needs, the desired monitoring hardware can be produced, and society will benefit.
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Source: Mouser Electronics