A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

The LTC4124 is a high-performance 100 mA wireless lithium-ion charger receiver that requires few external components to form a complete small solution, suitable for space-constrained applications. LTC4124 is paired with LTC4125 (a wireless power transmitter with optimized power search and foreign body detection functions) to create a safe and efficient wireless charging environment.

Wenwei Li

The LTC4124 is a high-performance 100 mA wireless lithium-ion charger receiver that requires few external components to form a complete small solution, suitable for space-constrained applications. LTC4124 is paired with LTC4125 (a wireless power transmitter with optimized power search and foreign body detection functions) to create a safe and efficient wireless charging environment.

Highly integrated wireless charger receiver

For small-sized portable and wearable devices, wireless charging is becoming more and more popular. This is not surprising. The equipment without exposed connectors and ports will be more reliable, and the end user experience will be easier and more convenient. In order to overcome the space limitation caused by these devices (such as hearing aids), the LTC4124 integrates a wireless power manager, which converts the AC voltage from the wireless resonance circuit into a stable DC voltage. Then, the DC voltage will be fed into a fully functional linear battery charger to provide a good battery charging function. With such a high level of integration, only a receiver resonant circuit and the battery itself need to be added to realize a very compact and fully functional wireless charging device.

Efficient wireless power manager

As shown in Figure 2, if the energy received by the LTC4124 exceeds the energy required to charge the battery, the wireless power manager in the IC can make the IC’s input voltage V by shunting the receiver resonant circuit to ground.CCKeep it low. In this way, the linear charger will be very efficient because its input is always kept exactly at the battery voltage VBATTAbove. When the shunt circuit is joined, the receiver resonant frequency will be out of tune with the transmitter frequency, and the resonant circuit will therefore receive less energy.

A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

Figure 1. A complete 6 mm wireless battery charger solution.

A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

Figure 2. AC input rectification and DC rail voltage regulation.

Use the LTC4125 transmitter to form a complete wireless charging design

The LTC4125 shown in Figure 3 is a high-performance AutoResonant™ wireless transmitter with a complete protection function for wireless charging applications. The optimized power search function in the LTC4125 can adjust the transmit power according to the receiver’s load requirements. LTC4125 also includes a variety of foreign object detection methods to prevent other objects from receiving useless power from the transmitter.

When paired with the LTC4124, the LTC4125 full-bridge resonant driver can be converted to a half-bridge driver to use a finer search step, so that the low-power receiver can receive just enough power to charge the battery. When the battery is close to a fully charged state, the LTC4124 enters a constant voltage mode to reduce the regulated charging current. LTC4125 will automatically reduce its power transmission level to match the lower power requirements of the receiver. This helps to reduce the power consumption of the entire charging cycle, keeping the LTC4124 charger and battery at a lower temperature.

Figure 4 shows the temperature of the receiver circuit in full power and current-limiting constant voltage modes. At room temperature, the temperature in both modes is below 40°C.

A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

Figure 3. The LTC4124 100 mA charger receiver is paired with the LTC4125 AutoResonant transmitter running optimized power search.

A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

Figure 4. 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 drops to standby mode (as shown in Figure 5). Figure 6 shows the situation when a metal foreign object is placed on the transmitter: LTC4125 detects a high resonant frequency and enters standby mode.

A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

Figure 5. LTC4125 does not detect the operation of the receiver.

A complete and efficient 100 mA wireless charging solution with optimized power search and multiple safety features

Figure 6. LTC4125 detects foreign matter.

in conclusion

The LTC4124 integrates a wireless power manager and a full-featured lithium-ion battery charger, simplifying the design of wireless charger receivers in space-constrained applications. LTC4125 can be used as a half-bridge transmitter of the LTC4124 receiver, thus providing a complete and efficient wireless charging solution with complete protection functions.

About the Author

Wenwei Li is an application engineer for power products at Analog Devices in Chelmsford, Massachusetts. 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. Contact information:[email protected]

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