There is a rapid increase in portable medical devices for diagnosing health problems in and out of physician offices or hospitals. Before sending the patient to the hospital, portable healthcare equipment can help medical professionals monitor vital signs, restore heartbeat, and use ultrasound to check the body’s condition. The goal of portable medical is to provide home healthcare equipment that is easy to use, interoperable, and has diagnostic value, so that related expenses can be included in medical insurance coverage. This avoids hospital visits and reduces medical costs. Patients can also use portable medical equipment at home to monitor blood pressure, vital capacity, blood sugar levels, and record cardiac events. Many of these portable medical devices have USB or wireless data connections, allowing medical professionals to continuously monitor patient conditions in hospitals and at home. At the same time, Continua Alliance is formulating intercommunication protocols based on USB, Zigbee and Bluetooth standards, which will accelerate the adoption of the aforementioned communication interfaces. For battery-powered portable medical equipment, the requirements for increased computing power, reduced size, and extended operating time make the design of the power supply system extremely challenging. The power system has an impact on battery size, running time, standby time, material (BOM) cost and reliability.
The portable medical system covers a wide range of applications, including blood pressure monitoring, blood glucose meters, pulse oximeters, and ultrasound applications. Some applications require the hardware to work for a long time, while other applications require shorter working hours and longer standby times. Although terminal applications vary widely, most portable systems can be reduced to a series of core functions: sensors collect data, microprocessors (with dedicated software) analyze data, memory stores software and data, and data connections are used to access results. Figure 1 shows a typical handheld portable system with a keyboard and Display. When connected to the mains, the portable system must be able to exert its maximum processing power without generating too much heat; when maintaining the portable state, the battery life must be maximized. The maximum battery life, that is, the time that a portable device can work before it needs to be charged or replaced, depends on power system factors, such as battery capacity, power system efficiency, and power management software. Only by making full use of all these factors to reduce battery consumption can the battery life be maximized. Most high-performance portable systems are powered by lithium-ion rechargeable batteries with a nominal output of 3.6V.
Figure 1 Typical handheld medical system
The portable system contains multiple integrated circuits, each of which has its own optimized semiconductor process and operating voltage requirements. ICs for portable applications use a lower operating voltage than batteries, so a step-down regulator is required. The most commonly used regulators today are low dropout (LDO) and step-down switching regulators, as shown in Figure 2. The LDO consists of a reference voltage source, an error amplifier, a voltage divider and a pass transistor. The low dropout regulator only needs two external capacitors to generate a lower DC voltage with a higher DC voltage, which is very simple. However, when Vin is much higher than Vout, LDO efficiency is low, because the power not delivered to the load will be lost in the form of heat. The LDO efficiency is approximately (Vo/Vin)×100%.
Figure 2 Functional block diagram of LDO and step-down converter
The LDO cannot store a large amount of unused energy, so the power not delivered to the load is dissipated inside the LDO in the form of heat. For example, the efficiency of a 2.6V LDO connected to a 3.6V battery is 72%. In addition, when the LDO is required to save power to the greatest extent, its quiescent current and enable function must be checked. When the system is in the idle mode between the normal working mode and the sleep mode, the low quiescent current (Iq) can reduce the power consumption of the system, thereby improving the autonomy of the system. The enable input pin allows the LDO to shut down, making the power consumption in sleep mode less than 1μA, thereby extending battery life. For example, ADP150 is an excellent low quiescent current LDO.
When the power supply voltage is much higher than the operating voltage, a switching DC/DC converter is a better choice. It can achieve higher efficiency because it can temporarily store energy when converting one DC voltage to another DC voltage. In the magnetic field of the Inductor, it is then released to the load. The portable switching regulator operates at a frequency of 500kHz to 3MHz. There are many topologies for DC/DC switching converters. Synchronous step-down regulators with built-in switching elements are used where the output voltage is much lower than the input voltage, and are most commonly used in portable systems. Replacing the LDO with a buck regulator can improve system efficiency. For example, when using LDO to reduce the system voltage from 3.6V to 1.2V and supplying power to a microprocessor core with a load current of 300 mA, the LDO efficiency is about 1.2V/3.6V×100%=33%, 67% of the input Power is lost in the form of heat. In order to improve efficiency and reduce operating temperature, LDO should be replaced with a step-down converter such as ADP2108. The buck converter can store energy in the magnetic field of the inductor, so it is more efficient. Using ADIsimPowerTM, it can be known that the efficiency of ADP2108 is 80% under the same conditions, which is 47% higher than LDO. Design engineers will find that the ADP2108 is small in size, using only two decoupling capacitors and a 1μH chip inductor, which can almost directly replace the LDO. Other power-saving features that need to be considered when choosing a buck converter include: low quiescent current, enable function, and power-saving mode when the load current is small.
In order to maximize battery life, in addition to optimizing the efficiency of portable system hardware, power management software must also be optimized. Running complex dedicated software puts forward higher requirements on computing power, requiring the use of high-speed microprocessors with high power consumption. Reducing the processor speed can reduce power consumption and extend battery runtime, but software performance will decrease. System architects can improve system efficiency by choosing the processor speed most suitable for the application. Another power saving method for portable systems is to turn off unused subsystems, such as microprocessors, display backlights, data ports, and sensors in the measurement gap, and use the enable input of the regulator or load switches such as ADP190/ADP195 to isolate Battery, as shown in Figure 1.
When designing a portable power system, there is no universal solution. There are many ways to extend battery life, and one method may be better than others. The technology described in this article is applicable to both portable and mains-powered medical equipment, which can improve system efficiency and reduce internal temperature and operating costs.