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Design Articles

Wireless Trends and Solutions in Portable Medical Designs

A longer lasting medical handheld device that is easy to interface with a PC or other handheld is key to the end user experience. As wireless technology eases this pain, the data must eventually make it to the doctor or hospital to have the largest impact on healthcare.

By Edmund Suckow, Medical Segment FAE, Fairchild Semiconductor

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The medical segment is hungry for features, interoperability, and, of course, wireless interfaces. Below is a discussion on wireless trends and its challenges, to include: standards and frequencies being designed in right now and power considerations when adopting wireless modules. Single cell rechargeable battery applications will be the general end application for units such as blood glucose and pulse oximeter. In conclusion, we will discuss how these trends effect healthcare.

While consumer applications can seek out the tech-savvy user and inundate them with feature sets and accessories, while the typical medical end user can vary from the at home/patient use to a nurse taking vital signs in a hospital.  Both have specific needs and desired features but in the end, the medical device must be reliable and easy to use.  Technology is already available and, more importantly, proven to facilitate this need and current design trends in progress right now.  Cell phone technology is available and easily leveraged if the designer knows where to look and how to apply it.

Medical portables often serve a specific purpose such as a pulse oximeter or blood sugar measurement and do this core competency very well.  As these portable units morph into multi-sensor routines and data trend recording, it is not effective to keep this information on a small handset with a limited display size.  Two common methods to pull this data from or interface to the handset is either wirelessly or via USB.  Removable memory cards were used just a short time ago but even this simple task of removing a card and inserting it into a PC was too cumbersome. 

Wireless Trends and Solutions

Designs are favoring two standards in medical handsets now: low power Bluetooth and Zigbee.  There are also those based on proprietary solutions in Wireless Medical Telemetry Service (WMTS) and Industrial Science and Medical (ISM) bands ranging from 400Mhz to 2.5Ghz, and, of course, cellular bands.  The band of choice will depend on the application needs and proximity to the receiver.  The size of the unit and power available will also dictate the band.  Regardless of the frequency of choice, power is a common dilemma for all wireless applications but here is where we can most heavily leverage semiconductor technology from the mobile handset industry.  It is important to choose a single cell Li-ion battery as a rechargeable power source to best apply this technology.  While AA and coin cell batteries are available, their low voltages often require more costly buck/boost topologies and are less prevalent for second source options.

The core difficulty with wireless is its dynamic power needs.  A wireless transceiver does not operate in a continuous broadcast or receive pattern, but rather pulses data relevant on the application’s needs.  It is this pulsing that is the hardest on the battery power rail (Vbatt).  Yes, tying the bias of the wireless transceiver directly to the battery with a bank of capacitors is one solution but rechargeable batteries simply have too large of an operating range, often 2.5V to 4.2V.  A DC/DC step-down converter should be used for this core rail.  This allows Vbatt to remain steady and not trigger under voltage events to sensors downstream that are also eventually connected to Vbatt, which would affect data accuracy.

Figure 1
Figure 1: FAN4603 DC/DC converter for 3.7V Li-ion cells with
fixed output from 1.0V to 1.8V at up to 600mA.

Medical design teams may not have a mobile power expert on board but still are in the race to release new and differentiating products to gain market share in the quick paced industry.  Eliminating the need for a detailed DC/DC design and modeling, the FAN4603 is an completely integrated power module targeted at wireless transceivers operating off a single cell rechargeable Li-ion.  Everything from the input capacitor, inductor, and output capacitor are concealed within the 2.5mm x 4.0mm surface mount package. Its transient response feature is key to handling the dynamic power needs of wireless. Input range is in alignment with rechargeable batteries and output is fixed from 1.0V to 1.8V depending on part number suffix, with up to 600mA available.  The 6Mhz switching frequency allows the small integrated inductor and transient response qualities desired by portable wireless.  Figure 1 shows the internals of the FAN4603.

In addition to transient response performance, when selecting a DC/DC converter it is also important to view the efficiency curve and note the efficiency at the nominal operating current.  Figure 2 shows a sample curve from the FAN4603 datasheet.  The datasheet also goes into detail on transient performance with oscilloscope displays showing Vout stability while the current spikes from steady state, i.e. simulating a wireless broadcast or the enabling of a higher power sensor such as LEDs in a pulse oximeter.

Figure 2
Figure 2: Efficiency curve for FAN4603 DC/DC converter with
3.7V input and various output voltages with loads from 1mA
to 600mA.

Intelligent MOSFETs

Downstream from the DC/DC regulator, power supervisory is often needed to distribute this regulated rail to other loads be it sensors, backlight drivers, etc.  With battery life being a demanding driver in handheld specifications it is common to use intelligent load switches to isolate power drains from the battery when not in use.  To serve both these functions, the Intellimax series from Fairchild is one of the options available that is often implemented in both mobile phone and medical handheld designs.  The concept is a MOSFET with integrated drive and protection features such as: slew rate control to reduce in-rush current, over-current protection, thermal protection, and under voltage lock out.  The key feature exercised is typically the in-rush current control and logic level enable.

Figure 3 shows the FPF1003 Intellimax device implemented downstream from the DC/DC to provide a logic enabled 1.8V rail.   This device is a 30mohm P-channel MOSFET available in a wafer level chip scale package (WL-CSP) with sub 1uA quiescent current (the current used to offer the drive and protection features).  A variety of other configurations and packages are available in this product family and it is important to select the device with the best aligning features.  In-rush current control can vary widely between part numbers as well as over current trip points.  The end result is a more power conscious design transparent to the system architecture.

Figure 3
Figure 3: Intelligent load switch used to protect and enable
power to downstream modules in its simplest form. Current
limit set points, fault pins, and load discharge are available
on other devices.

Effect on Healthcare

Semiconductor technology directly affects the feature sets and time to market for end medical units.  Leveraging IC technology and power architecture from the mobile segment allows shorter design cycles, proven solutions, and multiple sources.  The FDA approval process is a time consuming process in releasing a medical device so getting to that stage quickly with a proven technology is key.  Integrated power modules like the FAN4603 are just one of many ICs that compliment this strategy.  Note, however, that had a 3.7V rechargeable single cell not be chosen, the designer would be limited to a hand full of DC/DC regulators.  Making the correct decisions early in the architecture phase is key to applying mobile technology. 

A longer lasting medical handheld device that is easy to interface with a PC or other handheld is key to the end user experience.  As wireless technology eases this pain, the data must eventually make it to the doctor or hospital to have the largest impact on healthcare.  Wireless is key to this end goal but the data privacy and FDA hurdles must be overcome with standards defined and implemented.  Further collaboration between the standards committees, semiconductor vendors, and portable medical designers is key to a reliable and interoperable solution.



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