How to quickly and cost-effectively add wireless charging to space-constrained sealed devices
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How to quickly and cost-effectively add wireless charging to space-constrained sealed devices

Posted Date: 2024-01-25

Author: Stephen Evanczuk

To cope with the growing demand for tiny, sealed wireless devices, more efficient charging solutions are needed. Traditional charging methods cannot meet the needs of end users, not only bring challenges to space-constrained devices, but also cannot adapt to harsh environments. While wireless charging solves many of these problems, existing solutions still fall short of the integration, power, and efficiency requirements of these devices.

This article discusses the need for stronger charging solutions for space-constrained sealed devices. It then introduces Analog Devices' versatile wireless charging solution and shows how it helps developers easily implement suitable, safe and efficient charging.

Demand for more efficient charging solutions continues to grow

The growing demand for more compact wearable electronics such as headphones, in-ear devices and fitness equipment will continue to drive the need for charging solutions that fit the physical size constraints of these applications and ensure device sealing Integrity in different working environments. Traditional charging methods that rely on physical connectors are prone to wear and tear and are also affected by environmental factors such as dust and moisture, and cannot meet these requirements. Therefore, wireless charging technology is not just a new feature, but a basic requirement for this type of product.

Wireless Power Transfer (WPT) systems eliminate the need for an external charging port and offer a potential solution by operating on the air gap between the charging source and the enclosed device. But in reality, designing effective WPT solutions faces many technical challenges, including power transfer efficiency, fault handling, and battery and thermal management. The situation will also be further complicated by the need to fit into tight spaces.

Highly integrated device simplifies WPT design

Analog Devices' LTC4124 Wireless Li-Ion Charger and LTC4125 Wireless Power Transmitter are designed to help designers meet the high integration, power and efficiency requirements of space-constrained hermetically sealed devices.

Available in an LQFN package measuring just 2 × 2 mm and 0.74 mm high, the LTC4124 integrates all the functionality required to charge Li-ion batteries with selectable charge currents up to 100 mA (Figure 1).


Figure 1: With its comprehensive feature set, the LTC4124 wireless Li-ion charger simplifies WPT implementation. (Image source: Analog Devices)

The device's integrated charging capabilities are comprehensive and can be used as a standalone lithium-ion battery charger without the need for additional components. This charger features full-featured, pin-programmable constant-current/constant-voltage (CC/CV) linear battery charging capabilities, including safety timer termination, bad battery detection, and auto-charge capabilities.

The LTC4124's low-charge disconnect feature helps protect a battery in a very low state of charge from further discharge, which could shorten battery life. The disconnect feature causes the LTC4124 to shut down when there is no input power and the battery voltage is below the specified minimum. When shutting down, the device opens a disconnect switch (M3 in Figure 1), preventing further battery discharge. Using the shipping mode feature, the LTC4124 prevents the battery from discharging until power is applied to its ACIN or DCIN pin.

The LTC4124 can also be configured to prevent charging when the battery temperature is too high and to visually display the charging status by adding a negative temperature coefficient (NTC) thermistor and a light-emitting diode (LED) (Figure 2).


Figure 2: Using just two components (an LED and an NTC resistor) and the LTC4124 charger, developers can implement a complete temperature-qualified charger with a visual charge status indicator. (Image source: Analog Devices)

Developers can easily extend this basic design to create the receiver side of a WPT system by connecting an external parallel inductor-capacitor (LC) resonant circuit to the ACIN pin of the LTC4124. Pairing this approach with Analog Devices' LTC4125 device provides a complete 100 mA WPT solution (Figure 3).


Figure 3: The LTC4125 transmitter and LTC4124 charger enable a compact 100 mA WPT solution. (Image source: Analog Devices)

Like the LTC4124, the LTC4125 is a highly integrated device designed specifically for WPT applications. The device is available in a QFN package measuring 5 × 4 × 0.75 mm and delivers over 5 W of output power from a 3 V to 5 V supply (Figure 4).


Figure 4: Analog Devices' LTC4125 wireless power transmitter integrates all the functional blocks to deliver more than 5 W of power to a properly tuned receiver. (Image source: Analog Devices)

At the heart of the device is Analog Devices' proprietary auto-resonance technology that automatically detects and matches the resonant frequency of the series LC circuit connected on the switch pins (SW1 and SW2). In addition to optimizing transmit power, automatic resonance technology also plays a vital role in foreign object detection. When a foreign object is placed near the transmit coil, the effective inductance of the coil is significantly reduced and the driving frequency of the LTC4125 is increased. As discussed below, an increase in drive frequency can be used to indicate the presence of foreign matter.

Optimize WPT

During WPT, the LTC4124 receiver's integrated wireless power manager rectifies the AC voltage in the alternating magnetic field generated by the transmit coil of one half of the transmitter/receiver pair of the WPT system. The LTC4124 wireless power manager uses its integrated comparator (CP1) and switches (SW1 and SW2) to maintain the rectified voltage on the VCC pin by shunting the resonant circuit to ground when the received energy exceeds that required to charge the battery. at slightly above the battery voltage (VBATT).

However, the power dissipated by this shunt mechanism increases the thermal load on the device. The LTC4125 transmitter provides a more direct mechanism to reduce the energy reaching the receiver.

While its auto-resonance technology optimizes power output, the LTC4125 also features an excellent power search function that monitors and adjusts the transmitter output power to match the receiver load in a continuous sequence of search cycles. During each cycle, the LTC4125 steps up transmit power by increasing the pulse width voltage (VPTH), which is proportional to the width of the pulse delivered to the coil current driver bridge. A significant change in the resonant circuit feedback voltage (VFB) indicates that the transmit power is sufficient to meet or exceed the receiver load, and the search stops at this pulse width voltage, maintaining the desired transmitter output power level until the next search cycle. (Figure 5).


Figure 5: The LTC4125 transmitter's optimal power search function matches the output power to the receiver load by performing a step-by-step search to find the appropriate output level. (Image source: Analog Devices)

The LTC4125's optimal power search function executes each search cycle through a fixed process until a valid exit condition or one of several fault conditions is detected (Figure 6).


Figure 6: While executing its optimal power search algorithm, the LTC4125 transmitter continues to incrementally increase output power until it encounters a valid exit condition or one of several fault conditions. (Image source: Analog Devices)

During this process, the LTC4125 recognizes multiple pre-programmed valid exit conditions to indicate optimal transmit power. In addition, developers can specify two programmable exit conditions, including input current threshold (VITH) to limit input current and differential oscillator voltage threshold (DTH), for use involving poor coupling between transmit and receive coils. Optimize transmit power in the scene.

The LTC1425 automatically detects several fault conditions that can affect the safety and efficiency of power transfer:

・ Exceeding the coil temperature threshold determined by the NTC voltage (VNTC) measured at the NTC input pin
・Exceeding the maximum threshold of the oscillation voltage detected by the FB pin voltage: VFB > VIN
・Exceeding the over-temperature threshold of the internal chip (typically 150°C)
・Exceeding the frequency threshold indicates the presence of foreign matter because the transmitting coil inductance decreases and the driving frequency increases.
・Exceeded input current limit value (ILIM)
・Complete search ramp without finding a valid exit condition

The presence of either fault condition will cause the device to remain powered off until the next search.

For developers, features such as automatic resonant drive and optimal power search run automatically based on exit and fault conditions. While the thresholds for some of these conditions are fixed across devices, developers still have considerable control over the different aspects used to determine power settings, exit conditions, and fault conditions.

Using Analog Devices' DC2770A-A-KIT demonstration kit and 100 mA DC2770A-B-KIT demonstration kit, developers can quickly evaluate the performance of the LTC4124 receiver and LTC4125 transmitter when charging lithium-ion batteries at up to 100 mA. Each kit includes an LTC4125-based transmitter circuit board and an LTC4124-based receiver circuit board. Both are equipped with jumpers and connection points for setting device performance characteristics and monitoring results.

Conclusion

Devices are becoming increasingly compact and sealed, a trend that complicates the design of efficient charging methods on which they rely. WPT offers an effective solution, but implementing an efficient wireless charging design is challenging. Analog Devices' wireless power receivers and transmitters are designed to address these challenges, simplifying the implementation of WPT in space-constrained, sealed devices.


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