MEDICAL ELECTRONICS FEATURE
IMPROVING BATTERY RUN TIME IN
PORTABLE MEDICAL DEVICES
Tony Armstrong, director of marketing for Power by Linear Group, Analog Devices
Inc.investigates how the latest switching regulators are proving suitable for wide adoption in battery backed-up medical systems
A
s with many 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 back-up and even harvested ambient energy sources. Furthermore, great lengths must be taken to protect against and tolerate faults, maximise operating time when powered from batteries and ensure that normal system operation is reliable whenever a valid power source is present.
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. 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 minimised. A small package size and high
operating frequency can provide a small tight layout, which minimises EMI emissions. Furthermore, if the regulator can be used with low ESR ceramic capacitors, both input and output voltage ripple can be minimised, 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 optimised 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 utilised. This increases operating margin and extends battery run time as more of the battery’s life is usable, especially as it nears the lower end of its discharge profile. A new power IC that meets these requirements is the LTC3119 from the Power by Linear Group at Analog Devices. It is a synchronous current mode monolithic buck-boost converter that delivers up to 5A of continuous output current in buck mode from a wide variety of input sources, including single- or multiple-cell batteries, unregulated wall
/ ELECTRONICS
Figure 1: LTC3119 schematic showing a high level of integration & performance
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 analogue 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. The LTC3119 includes four internal low N-channel MOSFETs to deliver
RDSON
efficiencies of up to 95%. Burst Mode operation can be disabled, offering low noise continuous switching. The device’s combination of tiny externals, wide operating voltage range, compact packaging, plus 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 12V conversion systems.
Analog Devices (UK) Ltd
www.analog.com e:
uksales@linear.com
ELECTRONICS | FEBRUARY 2019 19
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