Wireless medical instrument design, have you taken care of the power supply?

Medical products in patient care situations typically have much higher standards for reliability, uptime, and ruggedness, ensuring that they can operate seamlessly from multiple power sources and provide high reliability in wireless transmission of data collected from patients. New power ICs can benefit wireless medical instrumentation.

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Like many other applications, low-power, high-precision components have enabled the rapid growth of portable and wireless medical instruments. However, unlike many other applications, such medical products usually have much higher standards for reliability, operating time, and ruggedness. Much of this burden falls on the power system and its components. Medical products must operate correctly and switch seamlessly between multiple power sources (such as AC outlets, backup batteries, and even harvested ambient energy sources). In addition, every effort must be made to provide protection and tolerance against a variety of different fault conditions, maximize operating time when powered by batteries, and ensure that normal system operation is reliable whenever a valid power source is connected.

Patient care, a key factor driving the growth of portable and wireless medical instruments

One of the major trends driving the growth of portable and wireless medical instrumentation is patient care. Specifically, this is the increasing use of remote monitoring systems in patients' own homes. The reason for this development is essentially purely economic. The cost of keeping patients in the hospital for observation and treatment is simply prohibitive. Therefore, many of these portable electronic monitoring systems must be combined with RF transmitters to be able to send any data collected from the patient directly back to the monitoring system in the hospital for later review and analysis by the attending physician.

Four requirements for power management architecture

In this context, it is reasonable to assume that the cost of providing patients with appropriate home medical instrumentation is far less than the cost of keeping them in the hospital for observation. However, it is of paramount importance that the devices that are placed in the hands of patients are reliable and easy to use. Therefore, manufacturers and designers of these products must ensure that they can operate seamlessly from a variety of power sources and provide high reliability in the wireless transmission of data collected from patients. This requires designers to ensure that the power management architecture is rugged, flexible, compact, and efficient.

Power IC Solutions

Many applications in medical electronics systems require continuous power even when the AC power is interrupted; a key requirement is low quiescent current to extend battery life. As a result, since 2010, power IC manufacturers have been producing switching regulators with a standby quiescent current of less than 30μA. In fact, some of our recent products have reduced this figure to only 2.5μA. As a result, these products are fully qualified for use in medical systems where battery backup power is provided.

Although switching regulators generate more noise than linear regulators, their efficiency figures are much better than the latter. It has been proven that in many sensitive applications, noise and EMI levels are controllable as long as the switching power supply operates in a predictable manner. EMI can be minimized if the switching regulator switches at a constant frequency in normal mode, and the switching pulse edges are clean and predictable without overshoot or high-frequency ringing. Small package size and high operating frequency can provide a compact layout, thereby minimizing EMI radiation. In addition, if the regulator can be used with low-ESR ceramic capacitors, the input and output voltage ripples, which are additional noise sources in the system, can be minimized.

The number of power rails in many feature-rich patient monitoring medical devices has increased, while operating voltages have continued to decrease. Even so, many of these systems still require a wide voltage range of 1.xV to 8.xV to power motors, low-power sensors, memory, microcontroller cores, I/O, and logic circuits.

Traditionally, these voltage rails have been provided by step-down switching regulators or low-dropout regulators. However, these ICs are not optimized for configurations that also incorporate a backup battery in the system to handle main power failure situations. Therefore, when a buck-boost converter (which can either step up or step down the voltage) is used, it enables the full operating range of the battery to be utilized. This increases operating margin and extends battery run time because more of the battery life is available, especially when it is near the lower end of its discharge curve.

Special attributes of power IC solutions

Accordingly, a DC/DC converter solution used in a portable medical instrument, which may also have a primary battery, should have the following characteristics:

• A buck-boost DC/DC architecture with a wide input voltage range to regulate VOUT with a variety of battery-powered sources and their associated voltage ranges

• Ultra-low quiescent current in both active and shutdown modes to extend battery run time

• Capable of efficiently powering system voltage rails

• Current limiting function to reduce inrush current and protect the battery

• Solution has a small footprint, is lightweight and has a low profile

• Advanced packaging available to improve thermal performance and space efficiency

New Power Supply: LTC3119

A new power IC that meets these requirements is the LTC3119, a synchronous current mode monolithic buck-boost converter that delivers up to 5A of continuous output current in buck mode from a variety of input sources, including single or multi-cell batteries, unregulated AC adapters, as well as solar panels and supercapacitors. Once started, the device's 2.5V to 18V input voltage range extends to 250mV. The output voltage is regulated when the input is above, below or equal to the output, and the output voltage is programmable from 0.8V to 18V. User-selectable Burst Mode® operation reduces quiescent current to only 31μA, improving light load efficiency while extending battery run time.

The proprietary 4-switch PWM buck-boost topology used by the LTC3119 provides low noise, jitter-free switching in all operating modes, making it ideal for RF applications and precision analog applications that are sensitive to power supply noise. In addition, the device includes a programmable maximum power point control (MPPC) function to ensure maximum power is delivered from power sources with high output impedance such as photovoltaic cells. See Figure 1 for a simplified schematic of the device.

Figure 1: Schematic diagram of the highly integrated and high performance LTC3119

The TC3119 contains 4 internal low RDSON N-channel MOSFETs, providing up to 95% efficiency. Burst Mode operation can be disabled to provide low noise continuous switching. External frequency programming or synchronization with the internal PLL allows operation over a wide switching frequency range of 400kHz to 2MHz, which allows a trade-off between conversion efficiency and solution size. Other features include short-circuit protection, thermal overload protection, shutdown current of less than 3μA and a power good indicator. The device uses tiny external components, has a wide operating voltage range, has a compact package, and has low quiescent current, making it ideal for RF power supplies, high current pulse load applications, system backup power, and even lead-acid batteries connected to 12V conversion systems.

Many portable medical systems need to be powered from multiple input sources including single or multi-cell battery configurations, AC adapters, and supercapacitor banks.

in conclusion

Considerable opportunities have been presented in designing numerous medical systems that are battery powered and/or battery backed up. At the same time, system designers face difficult challenges in selecting the right power conversion solution to meet key design goals, including meeting input-to-output voltage coverage limits, providing the right power level, and ease of design without compromising efficiency, run time, and meeting radiation regulations and solution size.

Designing a solution that meets system goals without compromising performance is a daunting task. Fortunately, there is a growing range of buck-boost converter solutions that simplify design, deliver best-in-class performance, and maximize run time between battery charge cycles by operating efficiently across a wide load range.