In 2012, Google Glass made its stunning debut, bringing the concept of smart wearable devices into people’s field of vision. In recent years, more and more new health tracking devices represented by smart watches have appeared, making these small portable hardware devices greatly enrich our daily life through ubiquitous cloud data interaction. According to the latest report released by IDC in March 2020, China’s wearable device market shipments reached 99.24 million units in 2019, a year-on-year increase of 37.1%. It is expected that by 2023, the wearable device market shipments will reach 2 billion units.

In 2012, Google Glass made its stunning debut, bringing the concept of smart wearable devices into people’s field of vision. In recent years, more and more new health tracking devices represented by smart watches have appeared, making these small portable hardware devices greatly enrich our daily life through ubiquitous cloud data interaction. According to the latest report released by IDC in March 2020, China’s wearable device market shipments reached 99.24 million units in 2019, a year-on-year increase of 37.1%. It is expected that by 2023, the wearable device market shipments will reach 2 billion units.

Highly-integrated wearable devices raise the challenge of charging technology upgrades

How to win in the huge market competition poses a severe challenge to companies that use many conventional technologies in the industry, and the choice of charging method bears the brunt. Because they need to be worn for a long time, they must not look too conspicuous, and must be small, thin, and not bulky, so there is little space for batteries and related circuits. In contrast, the functions of such devices are constantly increasing. Take smartwatches as an example. This most common wearable smart device not only provides fitness tracking functions, but also usually has a full-color touch screen, making calls, sending messages, etc. Powerful features, even the most efficient lithium-ion (Li-Ion) and lithium-polymer (Li-Pol) battery technologies today cannot save their battery life. Judging from product reviews, most smartwatches have to be charged within a day or two (some may not even last a day). In the case that the problem of frequent charging has caused great trouble to users, it is necessary to adopt a simpler charging method than plug-in charging – wireless charging.

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

Typical wearable device – smart watch

In the wireless charging scenario, the charging process of the smart device becomes very simple, usually just put it on the charging base. At the same time, wireless charging also enables wearables to be completely sealed or waterproof, an attractive protective feature for body-worn electronics. However, wireless charging has additional energy conversion losses compared to charging via USB or a charging head, resulting in less efficient charging and longer charging times, which can affect the user experience and can compromise battery performance. Power management vendors typically reduce the impact by adjusting the digital control loop between the wireless charging transmitter and receiver, but digital control adds complexity to the overall design and increases the size of the device. In response to this situation, high-performance analog chip maker ADI, based on its extensive power management portfolio design experience, has introduced a closed-loop control wireless charger prototype that can be built without increasing the number of components (and valuable overall size) on the receiver circuit board. s solution.

How Wireless Power Systems Power Batteries

The wireless power system consists of two parts separated by an air gap: the transmit (Tx) circuit (including the transmit coil) and the receive (Rx) circuit (including the receive coil).

When designing a wireless power battery charging system, the amount of power that can actually increase the battery’s energy is a key parameter. Received power depends on many factors, including:

・The power value of the transmission

・The distance and alignment between the transmitting coil and the receiving coil are usually expressed by the coupling coefficient between the coils

・Tolerance of transmit and receive components

The primary goal of any wireless power transmitter design is to enable the transmit circuit to generate a strong field that ensures the required received power is provided under worst-case power transfer conditions. However, it is equally important to avoid overheating and electrical overloading of the receiver in the best-case scenario. This is especially important when output power requirements are low and coupling performance is excellent. For example, the battery uses a battery charger when the receiver coil close to the transmitter coil is fully charged.

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

Wireless Power Transfer System

Build a complete wireless power transfer power supply solution with high efficiency, safety and small size

The LTC4124 is a highly integrated 100 mA wireless Li-Ion charger chip designed for space-constrained applications. It contains an efficient wireless power manager, a pin-programmable full-featured linear battery charger, and an ideal diode PowerPath controller. In operation, it first converts the AC voltage from the wireless resonant circuit to a regulated DC voltage, which is then fed into a fully functional linear battery charger to provide good battery charging. With such a high level of integration, only the addition of a receiver resonant circuit and the battery itself enables a very small and fully functional wireless charging unit.

As shown in the figure below, if the LTC4124 receives more energy than is required to charge the battery, the wireless power manager in the IC can keep the input voltage VCC of the IC low by shunting the receiver resonant circuit to ground. In this way, the linear charger will be very efficient because its input is always kept just above the battery voltage VBATT. When the shunt circuit is engaged, the receiver resonant frequency will be out of tune with the transmitter frequency and the resonant circuit will therefore receive less energy.

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

Complete 6 mm Wireless Battery Charger Solution

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

AC input rectification and DC rail voltage regulation

The LTC4125 is a single-chip full-bridge AutoResonant wireless power transmitter designed to maximize the power available to the receiver, improve overall efficiency, and provide comprehensive protection for wireless charging systems. It has complete protection for wireless charging applications. The optimized power search function in the LTC4125 adjusts the transmit power based on receiver load requirements. The LTC4125 also includes several foreign object detection methods to prevent other objects from receiving unwanted power from the transmitter.

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

LTC4124 100 mA charger receiver paired with LTC4125 AutoResonant transmitter running optimized power search

When paired with the LTC4124, the LTC4125 full-bridge resonant driver can be converted to a half-bridge driver to take advantage of a finer search step size, allowing the low-power receiver to receive just enough power to charge the battery. When the battery is nearing a fully charged state, the LTC4124 enters constant voltage mode, reducing the regulated charge current. The LTC4125 will automatically reduce its power delivery level to match the receiver’s lower power requirements. This helps reduce power consumption throughout the charge cycle, keeping the LTC4124 charger and battery cooler.

The following graphs show the temperature of the receiver circuit in full power and current-limited CV mode. At room temperature, both modes are below 40°C.

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

Thermal performance comparison: (a) 100 mA charging current at 4.1 V output, (b) 10 mA charging current at 4.2 V output

When the charger receiver is removed from the transmitter, the LTC4125 cannot find an active load and its power will drop to standby mode. The figure below shows what happens when a metal foreign object is placed on the transmitter: The LTC4125 detects a high resonant frequency and goes into standby mode.

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

Operation of the LTC4125 when no receiver is detected

Simplifying the control circuit, this wireless charging solution keeps wearable devices away from charging troubles

LTC4125 detected foreign object

Summarize

Wireless charging is becoming more and more popular. Devices without exposed connectors and ports will be more reliable and end-user experience easier. By using the LTC4125 & LTC4124 wireless charging solutions, the end user can allow greater deviation when placing the receiver above the transmitter without worrying about whether the receiver is able to draw the required power. In addition, this closed-loop approach allows the transmitter output power to always match the receiver’s power requirements, improving overall efficiency and making the entire charging cycle safer and more reliable.

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