Guess what's on your health monitor?

Click on the picture to watch "ADI Medical Life Science Instrument Solutions"

Patient-centric healthcare applications can provide users with information such as when to take medication, the number of calories consumed, and the degree of dehydration. This is usually monitored through mobile phones. For example, Apple said that the average iPhone user unlocks their phone 80 times a day, making the phone an ideal tool for receiving and viewing biometric data. This brings a large target customer base for healthcare application developers and manufacturers of portable health monitoring product hardware platforms.

With the continuous development of healthcare information technology systems, doctors are now able to access patient data with the patient's consent. Users can use their mobile phones to obtain routine examination reports and easily view radiology and pathology reports. This will undoubtedly greatly promote the popularization of patient-centric healthcare applications. Figure 1 shows an example of a monitoring system.

Figure 1. Example of patient home treatment monitoring

Medical DevicesADI has provided a variety of technologies to the medical market for decades. We have seen the rapid development of portable and wireless medical devices driven by low power precision components. Such medical products are subject to high standards of reliability, operating time, and durability, and a large part of these requirements are for power management systems and their components. Medical monitoring products must operate normally and switch seamlessly between various power sources such as batteries, AC outlets, supercapacitors, and even ambient energy harvesting sources.

Power system architects must design systems that are protected from and sometimes tolerate fault conditions, maximizing uptime (if powered by batteries), while also ensuring that the system operates reliably and properly as long as valid power is available.

Another factor driving the continued development of portable and wireless medical devices is the change in the way patients are cared for. Specifically, it can be attributed to the increasing use of remote monitoring systems by patients who are treated at home. The main reason for this trend is economic considerations: the high cost of hospitalization for patients and medical institutions is prohibitive. This has led to the development of portable electronic monitoring systems, which have built-in radio frequency (RF) transmitters that can send collected patient data directly to monitoring systems in hospitals or doctors' offices for later review and analysis.

It can be inferred that providing patients with appropriate medical devices for home treatment is much less expensive than hospital observation and treatment. However, because these devices are not used under professional supervision, it is critical to ensure that the devices used by patients are safe and reliable. Manufacturers and designers of these products must ensure that the devices can operate seamlessly from multiple power sources and can wirelessly transmit collected patient data with high reliability. Therefore, the power management and conversion architecture needs to be reliable, flexible, compact, efficient, and maintain low noise over a wide bandwidth.

Although switching regulators generate more noise than linear regulators, they are much more efficient. It turns out that if the behavior of the switching regulator is predictable, the noise and EMI levels in many noise-sensitive applications can be managed. If the switching regulator switches at a constant frequency in normal mode, and the switching edges are clean, predictable, and free of overshoot or high-frequency ringing, the EMI will be minimal. The small package size and high operating frequency can provide a small and compact layout, thereby minimizing EMI radiation. And if the regulator is used with low-ESR ceramic capacitors, the input and output voltage ripple can be minimized. However, not all system architects have a comprehensive background in switch-mode power conversion to solve these noise interference problems.

Many of these systems require multiple low-voltage rails to power low-power sensors, memory, microcontroller cores, I/O, and logic circuits; these systems are further complicated by thermal design limitations due to poor cooling due to insufficient air flow or heat dissipation within the system.

The ADI Power product team works with power experts to develop power solutions using industry-leading products. For example, in medical electronic systems, many applications require continuous power even when the main power is interrupted, so a reliable backup power supply is needed. The backup power operation time of these systems depends on the time required to properly shut down the system after the main power is interrupted. This time period can be a few minutes or a few hours, depending on the end application.

Supercapacitors are a great choice for systems that require high power, short-term backup power. Any IC supporting such an application will typically need to be able to support a 2.9 V to 5.5 V rail during a main power interruption. Supercapacitors have high peak power, making them ideal for system applications that require high peak power backup power for a short period of time.

For example, the LTC4041 uses an on-chip bidirectional synchronous converter to provide high-efficiency step-down supercapacitor charging, as well as high-current, high-efficiency step-up backup power. When external power is available, the device acts as a step-down charger for one or two supercapacitor cells while giving priority to the system load. When the input supply drops below the adjustable power-fail indication (PFI) threshold, the LTC4041 switches to a boost mode of operation, delivering up to 2.5 A from the supercapacitor to the system load. During a power failure, the device's PowerPath™ control function provides reverse blocking and seamless switching from input power to backup power. Typical applications for the LTC4041 include power outage emergency power supplies commonly found in medical equipment, electric meters, industrial alarms, and solid-state drives. Figure 1 shows a typical LTC4041 application schematic.

Figure 2. LTC4041 single supercapacitor backup power supply for 3.3 V systems.

If two supercapacitors are used, an internal supercapacitor balancing circuit maintains the same voltage on each supercapacitor and limits the maximum voltage of each supercapacitor to a predetermined value. Its adjustable input current limit function supports powering from a current-limited source while the system load current takes precedence over the battery charge current. An external disconnect switch isolates the main input power supply from the system during backup power supply. The device also features input current monitoring, an input power fail indicator, and a system power fail indicator. The LTC4041 also offers an optional OVP function that protects the IC from input voltages above 60 V using an external MOSFET.

For designers of power supplies for thermally and space-constrained medical systems, it is important to have a compact and efficient 5 V step-down converter whose high conversion efficiency minimizes thermal constraints and enables a very small solution size.

The LTC3309A is designed specifically for space constraints and thermal limitations. The LTC3309A is a very small, low noise, monolithic step-down DC-DC converter capable of delivering up to 6 A output current from a 2.25V to 5.5V input supply. The device uses a Silent Switcher® architecture with external hot loop bypass capacitors to achieve low EMI and high efficiency at switching frequencies up to 3 MHz.

In addition, the LTC3309A is a constant frequency current mode step-down DC-DC converter. The oscillator turns on the internal top power switch at the beginning of each clock cycle. The inductor current increases until the top current switch comparator trips and turns off the top power switch. The peak inductor current when the top switch is off is determined by the voltage at the ITH node. The error amplifier controls the ITH node by comparing the voltage on the FB pin to the internal 500 mV reference voltage. When the load current increases, the feedback voltage related to the reference voltage decreases, causing the ITH voltage of the error amplifier to increase until the average current in the inductor matches the new load current. When the top power switch turns off, the synchronous power switch turns on until the next clock cycle begins, or, in pulse skipping mode, until the inductor current decreases to zero. If an overload condition causes excessive current to flow through the bottom switch, the next clock cycle is delayed until the switch current returns to a safe level. If the EN pin is low, the LTC3309A shuts down and enters a low quiescent current state. When the EN pin is above its threshold, the switching regulator is enabled.

Figure 3. LTC3309A application schematic: 6 A at 1.2 V from 2.25 V to 5 V input voltage

Because the LTC3309A uses a constant frequency, peak current mode control architecture, it can provide fast transient response with minimum output capacitance. A 500 mV reference voltage allows low voltage outputs, while 100% duty cycle operation allows low voltage dropout. Other features include a power good signal indicating that the output is in regulation, a precision enable threshold, output overvoltage protection, thermal shutdown, temperature monitor, clock synchronization, mode selection, and output short-circuit protection. The device is available in a compact 12-pin 2 mm × 2 mm LQFN package.

It is no surprise that system designers face significant design challenges when designing portable and wireless medical monitoring devices for home-based patient health monitoring. There are many constraints that must be addressed, some of which may conflict with each other. For example, the need for heat dissipation but space-constrained enclosures, and the ability to transmit data without interference or interruption. Fortunately, the LTC3309A and LTC4041 introduced by ADI give designers a viable solution to meet the needs for small, compact, and thermally efficient solutions that are suitable for portable devices.