SUPPLEMENT MEDICAL ELECTRONICS POWER MANAGEMENT ARCHITECTURE FOR MEDICAL DEVICES
Tony Armstrong, director of product marketing, Power by Linear Products,Analog Devices Inc. explores how advanced converters are enabling battery-powered medical instrumentation
L
ow power precision components have enabled the rapid growth of portable
and wireless medical instruments. However, unlike many other applications, these types of medical products typically have 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, as well as tolerate various fault conditions, maximise 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 fuelling
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 home. The primary impetus for this trend is purely economic in nature – the costs of keeping a patient in a hospital are simply becoming too prohibitive. As a consequence, many of these portable electronic monitoring systems must incorporate RF transmitters so that any data gathered from the patient monitoring systems can readily be sent directly back to a supervisory system within the hospital where it can be later reviewed and analysed by the governing physician.
S4 FEBRUARY 2018 | ELECTRONICS Since 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, switching regulators with standby quiescent current less than 10μA are usually needed. In fact, some of the new systems that are run on a combination of a battery and energy harvesting as their main power sources, require their quiescent currents to be in the single digit micro-amps range, or in some cases, even nano- amps. This is a necessary prerequisite for adoption in such “home use” patient medical electronic systems.
REDUCING NOISE AND EMI 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 radiation. 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. The number of power rails in today’s
Figure 1:
The LTC3335 buck-boost converter with integrated coulomb counter
feature-rich patient monitoring medical devices has increased while operating voltages have continued to decrease. Nevertheless, many of these systems still require 3V, 3.3V or 3.6V rails for powering low power sensors, memory, microcontroller cores, I/O and logic circuitry. Furthermore, since their operation is sometimes critical, many of them have a battery back-up system should the main power supply to the unit fail. Traditionally their voltage rails have been supplied by step-down switching regulators or low-dropout regulators. However, these types of ICs do not capitalise on the battery cell’s full operating range, thereby shortening the device’s potential battery run time. 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 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. Analog Devices’ Power by Linear Group recently introduced the nanopower LTC3335 buck-boost converter with integrated coulomb counter. This device was designed for primary battery applications that need really low quiescent current and also need to know something about remaining battery capacity. Or, where potential battery component or load leakage may be detected by the coulomb counter as a check for system faults (Figure 1). The LTC3335 is a nanopower high efficiency synchronous buck-boost converter with an onboard precision coulomb counter that delivers up to 50mA of continuous output current. With only 680nA of quiescent current and programmable peak input currents from as low as 5mA up to 250mA, this device is ideally suited for a wide variety of low power battery applications, such as those found in battery backed up portable health monitoring systems.
Analog Devices Ltd.
www.linear.com www.analog.com T: 01628 477 066
/ ELECTRONICS
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56
