High-Performance Design for Efficient Power Management in Medical Devices
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By Mark Patrick, Mouser Electronics
Published February 12, 2020
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.
Figure 1: Simplified block diagram of the MAXM86161 from Maxim Integrated.
(Source: Mouser Electronics)
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).
Figure 2: Typical application circuit of the MAX20345. (Source: Mouser
Electronics)
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).
Figure 3: The TPS6572x from Texas Instruments features a power save mode at
light current loads with up to 92% efficiency. (Source: Mouser Electronics)
Conclusion
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.
Part of Mouser's EMEA team in Europe,
Mark joined Mouser Electronics in July 2014 having previously held senior marketing roles at RS Components.
Prior to RS, Mark spent 8 years at Texas Instruments in Applications Support and Technical Sales roles and holds
a first class Honours Degree in Electronic Engineering from Coventry University.
