Wearable technology has rapidly become a must-have in both daily life and professional environments, offering users a variety of functionalities from health monitoring to time management. These devices are evolving at an unprecedented pace, with market research firm ABI Research forecasting a compound annual growth rate of 56.1% and predicting that global shipments will reach 487 million units by 2018. For aging baby boomers, wearable devices couldn't have arrived at a better time, as precise medical monitoring can help address serious health issues. As users' expectations grow, system designers must focus on creating smaller, more efficient, and cost-effective solutions to make these devices accessible to a broader audience.
Products like Samsung Gear and Apple Watch showcase advanced features such as network connectivity, high-resolution displays, and multifunctionality. Health-focused wearables, including Fitbit Flex and Jawbone UP4, emphasize data collection and fitness tracking. However, battery life remains a top concern for consumers, significantly influencing their purchasing decisions. Addressing this issue, this article delves into the system module design of a typical wearable device and explores how a boost-buck regulator can enhance power efficiency, thereby extending battery life.
Wearable Device Architecture
The typical architecture of a wearable device includes components such as microprocessors, memory, displays, sensors, communication ICs, and battery charging blocks. Depending on the specific application, at least three DC-DC converters and three to five low dropout (LDO) regulators are required. Figure 1 illustrates a standard power system for a basic wearable device.
**Figure 1: Block diagram of a typical power solution for a wearable device**
- **Battery Charger:** Manages charging for lithium-ion batteries.
- **Backlight Boost Regulator:** Powers the display backlight.
- **Buck Regulator:** Reduces voltage for various components.
- **GPS Module:** Provides location data.
- **Memory:** Stores operational data.
- **Bluetooth Dual Mode:** Facilitates wireless communication.
- **Touchscreen Controller:** Handles user input.
- **Sensors:** Monitors physical parameters such as acceleration and altitude.
- **Heart Rate Sensor:** Tracks cardiovascular activity.
Boost-buck regulators play a crucial role in wearable systems by efficiently utilizing a wide range of battery voltages, typically ranging from 4.375V to 2.5V. They operate in pure boost mode when the battery voltage (Vbat) is between 2.5V and 3V, switch to boost-buck mode when the input voltage is between 3V and 3.9V, and function in pure buck mode when Vbat is between 3.9V and 4.5V.
**Using Step-Up Converters as Pre-Regulators**
Applications like WiFi and display modules often rely on LDOs, which suffer from significant power loss when powered directly from the battery. Utilizing a step-up converter as a pre-regulator improves system efficiency by maintaining a constant input voltage for the LDO, reducing thermal dissipation. Additionally, as wearable devices incorporate more features, they demand higher processing speeds, necessitating more efficient power management. Boost-buck converters serve as pre-regulators for power-hungry components like LCDs and LDO-powered devices, preventing voltage drops during high current spikes.
**Extending Battery Life**
The ISL9120 step-up regulator demonstrates exceptional efficiency across varying load conditions. Its adaptive pulse frequency modulation (PFM) mode enables efficiencies of up to 98% at high loads and over 86% at low loads. This results in reduced power consumption and heat generation, ultimately extending battery life and minimizing the need for external heatsinks. The ISL9120 employs a multi-level current limiting scheme, dividing the current limit into 32 levels ranging from 350mA to 2A for optimal efficiency.
The transition between current limit levels depends on the number of pulses in each PFM burst. As output current increases, the number of pulses rises until it reaches the upper threshold, prompting a transition to the next higher current limit. Conversely, if the pulse count falls below the lower threshold, the current limit shifts to a lower level. At the maximum current limit, the regulator enters forced bypass mode, reducing quiescent current consumption to less than 0.5μA.
**Boost-Buck Application Example**
In wearable devices, heart rate monitoring systems typically require around 3.3V input voltage. Designers often recommend using two to three LEDs for accurate readings, as this setup is less sensitive to the device's positioning. However, this configuration demands substantial current. Employing the ISL9120 as a pre-regulator offers several advantages, including higher system efficiency, resilience against input disturbances, and minimal output ripple. When the heart rate monitor is inactive, the ISL9120 can be set to forced bypass mode, consuming only 0.5μA until activation.
Small LCDs, commonly found in wearables, often use white LEDs for backlighting. While traditional solutions rely on 5V boost converters, the ISL9120 allows powering these displays within the 3V to 3.6V range, making it an ideal choice for more efficient designs. Additionally, wearable devices increasingly integrate WiFi, requiring a 3.3V supply with low ripple. Given the space constraints of wearables, compact designs are essential, and the ISL9120 serves as an excellent pre-regulator for these applications.
In conclusion, as wearables continue to shrink and expand their functionality, efficient power management becomes paramount. A new step-up and step-down regulator with adaptive current-limited PFM not only meets growing demands but also extends battery life, enabling future generations of wearable devices to operate longer and withstand higher temperatures.
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