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output power of the LTC4125 transmitter, which is indicated by the peak transmit tank voltage VTX_PEAK, responds smoothly to the change of the coupling coefficient between the two coils to keep the charge current constant. During a charge current rise transient, the LTC4124 shunting event stops, allowing the LTC4125 to charge up its PTH1 pin internally. As a result, the LTC4125 increases its half-bridge driver duty cycle to boost the transmit power. Once the transmit power is large enough for the LTC4124 to regulate its charge current, the shunt- ing event resumes and the duty cycle remains at an optimum level. During a charge current fall transient, the LTC4124 shunts much more frequently. The LTC4125 quickly discharges the capacitor on its PTH1 pin to lower the duty cycle and reduce the LTC4125 transmit power. As the transmit power always matches the demand of the receiver, the overall efficiency is greatly improved compared to typical configura- tions of a LTC4124- and LTC4125-based wireless charger without closed-loop control. The effi- ciency curve is smoother without the internal DAC steps in the LTC4125 optimum power search operation. As power loss is greatly reduced, the LTC4124 charger and the battery remains near room temperature throughout the charging period. CONCLUSION The LTC4125 can be configured as a power adjustable transmitter with a control input. The shunting event of the LTC4124 wireless charger receiver can be used to provide a feedback signal to the transmitter. This feedback signal can then be demodulated using a half-wave rectifier, a voltage divider, a low-pass filter, and a compara- tor. The processed signal can then be fed into the power adjustable LTC4125-based transmitter to close the control loop. A prototype has been built to prove this concept. This prototype responds quickly and smoothly to the change of the cou- pling coefficient and the charge current. This method allows end-users to place the receiver on top of the transmitter with a higher degree of misalignment, without worrying that the receiver will get the power it needs. Furthermore, this closed-loop method also improves the overall efficiency by always matching the transmitter output power with the power demand of the receiver, keeping the entire charging period much safer and more reliable.. ABOUT THE AUTHOR Wenwei Li is a power products applica- tions engineer at Analog Devices in North Chelmsford, MA. He received his B.E. de- gree from Hunan University in Changsha, China, in 2014, and his M.S. degree from the Ohio State University in Columbus, OH, in 2016. Figure 12. Efficiency of various configurations of the LTC4125- and LTC4124-based wireless charger at 3.5 mm air gap. 32 NOVEMBER 2022 | www.eetimes.eu Technical Articles