Medical products must operate properly and switch seamlessly between a variety of power sources such as an AC mains outlet, battery backup, and even harvested ambient energy sources

BY TONY ARMSTRONG, Director of Product Marketing Power Products
Linear Technology Corporation, now part of Analog Devices
www.analog.com

As with many other applications, low-power precision components have enabled rapid growth of portable and wireless medical instruments. However, unlike many other applications, this type of medical product typically has much higher standards for reliability, run time, and robustness. Much of this burden falls on the power system and its components. Medical products must operate properly and switch seamlessly between a variety of power sources such as an AC mains outlet, battery backup, and even harvested ambient energy sources. Furthermore, great lengths must be taken to protect against and tolerate faults, maximize operating time when powered from batteries, and ensure that normal system operation is reliable whenever a valid power source is present.

One of the current key trends fueling growth in the portable and wireless medical instrumentation is patient care. Specifically, this is the increased use of remote monitoring systems within the patient’s own home. The reason for this trend is purely economic in nature — the costs associated with keeping a patient in a hospital are simply too prohibitive. As a result, many of these portable electronic monitoring systems must incorporate RF transmitters so that any data gathered from the patient can be sent directly back to a supervisory system within the hospital for later review and analysis by the governing physician.

Given the above scenario, it is reasonable to assume that the cost of supplying the appropriate medical instrumentation to the patient for home use is more than offset by the cost of keeping them in the hospital for observation purposes. Nevertheless, it is of paramount importance that the equipment used by the patient be reliable and foolproof. As a result, the manufacturers and designers of these products must ensure that they can run seamlessly from multiple power sources and have high reliability in their wireless data transmission of the data collected from the patient. This requires the designer to ensure that power management architecture is robust, flexible, compact, and efficient.

Power IC solutions
There are many applications in medical electronic systems that require continuous power even when the mains supply is interrupted; a key requirement is low quiescent current to extend battery life. Accordingly, power IC makers have been producing switching regulators with standby quiescent current less than 30 µA since 2010. In fact, some of Linear Technology’s recent product introductions have taken this figure down to a mere 2.5 µA. As a result, these products are well-positioned for adoption in battery-backed-up medical systems.

Although switching regulators generate more noise than linear regulators, their efficiency is far superior. Noise and electromagnetic interference (EMI) levels have proven to be manageable in many sensitive applications so long as the switcher behaves predictably. If a switching regulator switches at a constant frequency in normal mode, and the switching edges are clean and predictable with no overshoot or high-frequency ringing, then EMI is minimized. A small package size and high operating frequency can provide a small tight layout, which minimizes EMI emissions. Furthermore, if the regulator can be used with low-equivalent-series-resistance (ESR) ceramic capacitors, both input and output voltage ripple can be minimized, which are additional sources of noise in the system.

As the number of power rails in many feature-rich patient monitoring medical devices has increased, operating voltages have decreased. Nevertheless, many of these systems still require a broad range of voltages from 1.x V to 8.x V for powering motors, low power sensors, memory, microcontroller cores, I/O, and logic circuitry.

Traditionally, these voltage rails have been supplied by step-down switching regulators or low-dropout regulators. However, these types of ICs are not optimized for configurations that also incorporate a backup battery in the system should the main supply fail. Therefore, when a buck-boost converter is used (it can step voltages up or step them down), it will allow the battery’s full operating range to be utilized. This increases the operating margin and extends the battery run time as more of the battery’s life is usable, especially as it nears the lower end of its discharge profile.

Correspondingly, DC/DC converter solutions utilized in portable medical instrumentation, which may also incorporate a primary battery cell as well, should have the following attributes:

  • A buck-boost DC/DC architecture with wide input voltage range to regulate VOUT through a variety of battery-powered sources and their associated voltage ranges
  • Ultra-low quiescent current, both in operating mode and shutdown, to increase battery run time
  • The ability to efficiently power system rails
  • Current limiting for attenuating inrush currents, thus protecting the cells
  • Small, lightweight, and low-profile solution footprints
  • Advanced packaging for improved thermal performance and space efficiency

Accordingly, any monolithic buck-boost power IC that meets these requirements must have characteristics that include synchronous operation for high-efficiency conversion and be able to deliver as much as 5 A of continuous output current in buck mode from a wide variety of input sources, including single- or multiple-cell batteries, unregulated wall adapters, and solar panels and supercapacitors. Furthermore, it must have an input voltage range of 2.5 V to 18 V to handle this array of input sources.

From an output voltage perspective, a buck-boost converter must be capable of regulating inputs above, below, or equal to the output, as well as be programmable from 0.8 V to 18 V. Burst Mode operation is another feature that is needed because it improves light load efficiency while extending battery run time. A converter with a four-switch PWM buck-boost topology normally provides low-noise, jitter-free switching through all operating modes, making them suitable for RF and precision analog applications that are sensitive to power supply noise. The device should also include programmable maximum power point control (MPPC) capability, ensuring maximum power delivery from power sources with higher output impedance, including photovoltaic cells.

A good facsimile of these product attributes can be found in Analog Devices’ LTC3119. This monolithic buck-boost converter includes four internal low RDSON N-channel MOSFETs to deliver efficiencies of up to 95%. Burst Mode operation can be disabled, offering low-noise continuous switching. External frequency programming or synchronization using an internal PLL enables operation over a wide switching frequency range of 400 kHz to 2 MHz, which allows for the tradeoff between conversion efficiency and solution size.

Other features include short-circuit protection, thermal overload protection, less than 3 μA shutdown current, and a power-good indicator. The device’s combination of tiny external components, wide operating voltage range, compact packaging, and low quiescent current makes it well-suited for RF power supplies, high-current pulsed-load applications, system backup power supplies, and even lead-acid battery to 12-V conversion systems.

Fig. 1: The LTC3119 schematic shows a high level of integration and
performance.

Conclusion
A large opportunity has presented itself for designing a wide range of battery-powered and/or battery-backed-up medical systems. At the same time, system designers have faced difficult challenges in selecting the right power conversion solution that meets the key design objectives, which include input-to-output voltage constraints, power levels, and ease of design, without compromising efficiency and run time while meeting emissions specifications and solution size.

Designing a solution that meets the system goals without performance compromises can be a daunting task. Fortunately, there are a growing number of buck-boost converter solutions from numerous power IC suppliers that simplify the design effort, offer best-in-class features, and have the ability to maximize run times in between battery recharging cycles due to their high efficiency operation.