Wireless Medical Instrumentation Benefits from New Power ICs


Tony Armstrong, Director of Product Marketing Power Products at Linear Technology Corporation

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.

Background

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 back-up 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 potable 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 is 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 of observation purposes. Nevertheless, it is of paramount importance that the equipment used by the patient be reliable and fool-proof. As a result, the manufactures 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A since 2010. In fact, some of Linear Technology’s recent product introductions have taken this figure down to a mere 2.5A. 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 EMI levels have proven to be manageable in many sensitive applications as 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 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.xV to 8.xV 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 back-up 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
  • Ultralow 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

A new power IC that meets these requirements is the LTC3119 from Linear Technology. It is a synchronous current mode monolithic buck-boost converter that delivers up to5A of continuous output current in buck mode from a wide variety of input sources, including single- or multiple-cell batteries, unregulated wall adapters, as well as solar panels and supercapacitors. The device’s 2.5V to 18V input voltage range extends down to 250mV once started. The output voltage is regulated with inputs above, below or equal to the output and is programmable from 0.8V to 18V. User-selectable Burst Mode® operation lowers quiescent current to only 31μA, improving light load efficiency while extending battery run time. Its proprietary 4-switch PWM buck-boost topology provides low noise, jitter-free switching through all operating modes, making it ideal for RF and precision analog applications that are sensitive to power supply noise. The device also includes programmable maximum power point control (MPPC) capability, ensuring maximum power delivery from power sources with higher output impedance including photovoltaic cells. See Figure 1 for its simplified schematic.

The LTC3119 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 400kHz to 2MHz, which allows for the tradeoff between conversion efficiency and solution size. Other features include short-circuit protection, thermal overload protection, less than 3μA shutdown current, and a power good indicator. The device’s combination of tiny externals, wide operating voltage range, compact packaging, plus low quiescent current makes the LTC3119 well suited for RF power supplies, high current pulsed load applications, system backup power supplies and even lead-acid battery to 12V conversion systems.

Figure 1: LTC3119 Schematic Showing a High Level of Integration & Performance

Many portable medical systems require being powered from multiple input sources including single or multi-cell battery configurations, wall adapters and supercapacitor stacks. 

Figure 2:  Wide Input Range 5V Regulator and Efficiency

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, including spanning the input-to-output voltage constraints, power levels and ease of design, without compromising efficiency, run time, 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 Linear Technology, which simplify the design, offer best-in-class features and have the ability to maximize run times in-between battery recharging cycles due to their high efficiency operation across a wide range of loads.
 

More information: Linear Technology    Source: Bodo's Power Systems, January 2018

 

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