Medical electronics for healthcare technology

Small battery technology has remained relatively unchanged over the past five years. Advances in power design techniques and IC technology have directly extended the life of these sensors. These advances, enabled by mobile phone standards, include USB charging, efficient DC/DC regulators, adoption of I/O standards (I2C, SPI, SDIO, etc.), and improvements in display technology.

In the healthcare industry, one of the most important questions is when a product will be available to patients. Project design cycles are typically very long, often spanning several years. While not all of that time is spent in the electrical design phase, many times the electrical design team is often the smaller team within a company, while the chemical, legal, and testing teams are much larger. A key trend in electronics is to reduce time to market while reducing risk. All portable products require some form of power supply, typically either primary or rechargeable. The power from the battery must be regulated to meet the needs of downstream complex sensors, a design that can extend the life of the battery and enable independent use of feature sets.

In the past, such power conditioning modules consumed resources and actual operating power, taking up board space. Today, for DC/DC applications, gone are the days of placing multiple MOSFETs around a simple switching PWM, no longer requiring numerous passive components to add minimal protection. We are now talking about integrated DC/DC step-down ICs that have been gaining adoption over the past 5 years. The electronics industry is currently undergoing rapid development, including the active development of single-chip power modules. Unlike the higher-power DC/DC modules in the telecommunications industry, these devices are true molded packaged ICs available through a large number of sales channels with dedicated part numbers.

The development of portable medical electronics has reduced the cost of healthcare. Battery-powered non-invasive sensors are mobile and, with on-board memory, can capture complete data patterns for a disease. Due to the continuous advancement of IC technology, these devices are becoming smaller and smaller, and their continuous working time is getting longer, making them easier to adopt in this field.

After patients leave the hospital, their lives often return to normal. They should follow their doctors' advice to take care of their bodies, such as relaxing, increasing heart rate, and strengthening nutrition. But no matter what the situation, people always reach their limits. A simple patient monitoring device can be a win-win solution for patients and healthcare providers, which can not only reduce the number of visits to the hospital, but also greatly increase the value of each visit.

Just a few years ago, sensors were too large to be used for continuous monitoring around the clock, but advances in electronics have reduced their size and extended the operating time of a single battery. Glucose monitors and syringes were among the first successful applications, and more recent advances are closed-loop testing and insulin management devices. These devices are now used all day, every day by people with Type I diabetes, but the concept should not be limited to people with diabetes.

DC/DC module integration improves efficiency and reduces risk
Fairchild Semiconductor's FAN4603 uModule is such an IC. Its principle is shown in Figure 1.

Figure 1. Fairchild Semiconductor’s FAN4603 uModule

The basic controller with integrated FETs is packaged together with the input/output capacitors and the switching inductor required for the DC/DC buck topology in a single module. The upfront advantages are obvious, such as reduced size, only one inventory component, and a correspondingly shorter design cycle. In addition, there are some technical advantages.

Because all active components are so close together in a single module, high current and high frequency paths are shortened, which reduces EMI, a critical specification for the medical device industry that requires complex sensors and human body interfaces. Increasing the switching frequency of the module's buck topology to 6MHz allows the integration of stacked inductors. Due to the increase in switching frequency, the inductor size is reduced. Because these passive components such as inductors and capacitors are selected by actual PWM and FET designers, the indicators are fine-tuned for ideal interoperability within the recommended load range.

The disadvantage of this regulation is that the output voltage of the module is fixed, while the variable output voltage will lead to unbalanced matching of passive components. Therefore, different modules with certain output voltages can be provided to solve the problem. The first modules launched have an output voltage of 1.8V and Vin of 2.3~5.5V. This voltage range is ideal for single rechargeable batteries, AA and AAA dual battery boxes, and single 3V lithium-ion batteries.

Another important advancement in the electronics industry driven by the design needs of portable medical devices is the improvement of the efficiency of power modules at different loads. Medical application equipment is often in a dormant state, consuming very little power when the sensor is not biased and not collecting actual data. During this time, the system Icc can drop below 10mA. In these light and medium load conditions, PFM technology can be used to minimize the losses inside the module. Figure 2 shows a typical efficiency curve for the FAN4603

Figure 2 Typical efficiency curve of FAN4603

Note that the rated efficiency ranges from 70% to 85%, depending on the input voltage, at just 1mA on the logarithmic X-axis. This curve is critical to battery life when the application ranges from 10mA to 200mA, such as during data acquisition or RF communication with a base station.

Downstream Smart FET Technology Simplifies Power Distribution
To better distribute the energy from the power module to the downstream sensors, processors and LCDs, the use of point-of-load power switches is becoming popular in the industry. The use of simple P-channel FETs to transfer power is not new in itself. The FETs are surrounded by numerous diodes and transistors to add functions such as load discharge, inrush current limiting and reverse current blocking (RCB) to name a few. The obvious trend is to move to true smart FETs that integrate these functions in a single IC. The IntellimaxTM series from Fairchild Semiconductor is one of the smart FET families that designers can choose from, and its outstanding feature is the integration of all the following functions, overvoltage protection (OVP), overcurrent protection (OCP), RCB, slew rate control, and error flags to notify the processor of a fault. Figure 3 shows the internal schematic of a typical IntelliMAX device and the package used.

Figure 3. Typical IntelliMAX device internal schematic and package

A big advantage in medical applications is the ability to limit power to external sensors and connectors when not in use. When the power to the sensor or connector exceeds the recommended level, the smart FET can isolate the power supply and send an error flag to the processor. This increases the reliability of the end application, reduces its frequency of field failures, and ultimately reduces overall system costs.

Advances in signal path technology enable further energy savings and interoperability
In addition to the dramatic improvements in DC/DC efficiency discussed previously, advances in general small signal technology are bringing more value to medical device applications. Analog switches are very common IC products that have historically had limited uses, primarily for multiplexing or isolating low-speed data lines. New features now integrated into switches allow for better on-resistance and flatness, power-off protection, higher data rates, and much lower power consumption. Understanding these features and their different benefits is key to fully leveraging medical equipment.

In the past, on-resistance was a static value, but the newer Ron flatness parameter ensures that Ron values ​​are within a certain range under given conditions. This can be used for calibration and sensor multiplexing in medical equipment that requires Ron levels below 400mΩ, a significant improvement from the previous 8Ω level. Supplementary sensor functions include the recently introduced power-off protection function, which will show the input signal when Vcc=0V, which is very beneficial for hot-swap applications of connectors and sensors. The actual operating current is now very low, even if the control voltage is lower than Vcc, the data is measured in μA (microamperes). Input-to-output leakage is as small as nA (nanoamperes), which can further extend battery life. When upgrading current medical equipment to a newer feature set, it is usually sufficient to add interconnection using these analog switches and inherent added functions without replacing the DSP and performing extensive recalibration.