Low 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 is 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.
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 costs of keeping them in the hospital for these same purposes. Nevertheless, it is of paramount importance that the equipment used by the patient be not only reliable but patient-proof! As a result, the manufactures and designers of these products must ensure that they can run seamlessly from multiple power sources (including backup sources) and have high reliability of the data collected from the patient, as well as 99.999% integrity of the wireless data transmission. This requires the system designer to ensure that the power management architecture to be used is not only robust and flexible, but also compact and efficient. In this manner, the needs of the hospital and those of the patient are copacetic.
Fortunately, there are a number of analogue companies which focus on bringing solutions to these problems with product innovation and expertise. 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 10mA 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 case, even nano-amps. This is a necessary prerequisite for adoption in such “home use” patient medical electronic 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 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 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.
It is clear that any DC/DC converter solution that solves the primary cell system application requirements, as well as the associated issues already discussed, 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
- Capably count coulombs accurately without significantly affecting IC quiescent current (battery consumption), to determine remaining battery state of charge
- 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
It was for these reasons that Analog Devices introduced the nanopower LTC3335 buck-boost converter with integrated coulomb counter. The 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. See Figure 1.
Figure 1. LTC3335 Buck-Boost Converter with Integrated Coulomb Counter
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, the 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. Its 1.8V to 5.5V input range and 8 user-selectable outputs between 1.8V and 5V provide a regulated output supply with an input voltage above, below or equal to the output. In addition, the device’s integrated precision (±5% battery discharge measurement accuracy) coulomb counter provides accurate monitoring of accumulated battery discharge in long-life non-rechargeable battery-powered applications which in many cases have extremely flat battery discharge curves. The LTC3335 includes four internal low RDSON MOSFETs and can deliver efficiencies of up to 90%. Other features include a programmable discharge alarm threshold, an I2C interface for accessing coulomb count and device programming, a Power Good output, and 8 selectable peak input currents from 5mA up to 250mA to accommodate a wide range of battery types and sizes.