Designing portable medical devices offers a multitude of challenges, from space constraints to interference issues to ergonomic concerns. Arguably, however, none of these are more serious than optimizing power use within the system. This Tech Brief reviews a number of methods designers can consider when developing their next portable medical device.
The availability of portable medical equipment to diagnose a health issue inside or outside of the doctor’s office or hospital is rapidly increasing. Portable medical equipment is also used by patients at home to monitor blood pressure, lung capacity, and glucose levels, as well as to log heart events. The requirements for increased computing power, smaller size, and longer run time when operating on battery power make the power system design for portable medical devices very challenging. The power system has an effect on battery size, run time, standby time, bill of material cost, and reliability.
Portable systems must be capable of maximum processing power when connected to the power line without generating excess heat, while also providing maximum battery lifetime when in remote/portable use (Figure 1). Maximum battery life (the time that the portable device can operate from the battery before recharging or replacement is required) is determined by power system elements, such as battery capacity, power system efficiency, and power management software. Battery life can be maximized only when all of these power system elements are working together to reduce the drain on the battery.
1: Generic handheld portable system
|Portable medical devices, such as the PocketCPR shown here, operate on power supplies as low as 2.7 VDC. The PocketCPR uses Analog Devices’ ADXL322 iMEMS low g high-performance accelerometer, which typically consumes only 340 micro amps and can be power-cycled for even greater battery life.|
To achieve optimum battery life, designers need to enhance portable system hardware efficiency by optimizing the power management software. The demand for increased computational power to run the complex application-specific software requires power-hungry high-speed microprocessors. Decreasing processor speed reduces power consumption and extends battery runtime by decreasing software performance. System architects can improve system efficiency by selecting the optimum processor speed for the application. Another way to save power in portable systems is to shutdown unused subsystems such as the microprocessor, display backlighting, data port, and sensors between measurements, using the regulator’s enable input or load switches to isolate the battery.
When designing portable power systems, there is no “one size fits all” solution. There are many ways to solve the problem of long battery life, and some will work better than others. Using the techniques described in this article will improve system efficiency, lowering internal temperatures and operating cost for both portable and line-powered medical equipment.
Ken Marasco is a system applications manager, power management group, Analog Devices Inc. He is responsible for the technical support of portable power products and has 35 years of system and component design experience. He can be reached via email at firstname.lastname@example.org.