The LTC4124 is a high performance 100 mA wireless Li-Ion charger receiver that requires few external components to form a complete small solution for space-constrained applications. The LTC4124 is paired with the LTC4125, a wireless power transmitter with optimized power search and foreign object detection, to create a safe and efficient wireless charging environment.
Wireless charging is becoming more and more popular for small form factor portable and wearable devices. This is not surprising. Devices without exposed connectors and ports will be more reliable and end-user experience easier. To overcome the space constraints imposed by these devices, such as hearing aids, the LTC4124 integrates a wireless power manager, which converts the AC voltage from the wireless resonant circuit to a regulated DC voltage. This DC voltage 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 Figure 2, 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.
Figure 1. A complete 6 mm wireless battery charger solution.
Figure 2. AC input rectification and DC rail voltage regulation.
The LTC4125 shown in Figure 3 is a high-performance AutoResonant wireless transmitter with 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.
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.
Figure 4 shows the temperature of the receiver circuit in full power and current-limited CV mode. At room temperature, both modes are below 40°C.
Figure 3. The LTC4124 100 mA charger receiver is paired with the LTC4125 AutoResonant transmitter running an optimized power search.
Figure 4. Thermal performance comparison: (a) 100 mA charge current at 4.1 V output, (b) 10 mA charge 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 (as shown in Figure 5). Figure 6 shows the situation when a metal foreign object is placed on the transmitter: the LTC4125 detects a high resonant frequency and enters standby mode.
Figure 5. LTC4125 operation when no receiver is detected.
Figure 6. LTC4125 detects foreign objects.
The LTC4124 integrates a wireless power manager and a full-featured Li-Ion battery charger, simplifying wireless charger receiver design in space-constrained applications. The LTC4125 can be used as a half-bridge transmitter to the LTC4124 receiver, providing a complete and efficient wireless charging solution with full protection.
About the Author
Wenwei Li is an applications engineer for power products at Analog Devices in Chelmsford, MA. He received a bachelor’s degree in engineering from Hunan University in Changsha, China in 2014, and a master’s degree from Ohio State University in Columbus, Ohio, USA in 2016.