The LTC4126 is a full-featured 7.5 mA wireless lithium-ion (Li-Ion) battery charger with a 1.2 V inductorless DC-DC converter designed for use in hearing aids, wireless headphones, and other space-constrained wearables that require wireless charging. The LTC4126 can be combined with the LTC6990 based ZVS single transistor transmitter for a complete wireless charging solution.

High Efficiency Wireless Input Power Controller

Wearables are increasingly moving toward wireless battery charging, eliminating the need for cables or exposed connectors, thus improving the user experience. The LTC4126 charger, DC-DC converter features a wireless power controller that enables it to receive power wirelessly from the AC magnetic field generated by the transmit coil (like the LTC6990 solution). The wireless power controller rectifies the AC voltage of the resonant circuit at the receiver side to a DC voltage at the VCC pin. This DC voltage is fed into the linear charger, which in turn regulates the charging of the battery.

If the LTC4126 receives more energy than required, the wireless power controller regulates the input VCC of the linear charger by shunting the receiver resonant circuit to ground. In this way, the linear charger will operate efficiently because its input is kept just above the battery voltage VBAT. When the shunt circuit is engaged, the resonant circuit also receives less power because the resonant frequency is out of tune with the transmitter frequency.

Full-featured linear battery charger

The integrated constant current (CC)/constant voltage (CV) linear Li-Ion battery charger in the LTC4126 ensures proper operation of the charge cycle with a complete set of protection features, including auto-charge and auto-termination of safety timers, bad battery detection And out of temperature range charging pause function. The LTC4126 includes charger status and battery voltage level signals that can be passed to the system microcontroller.

Inductorless Low Noise DC-DC Converters

The LTC4126 includes an integral inductorless charge pump DC-DC converter that regulates the system load output from the battery. The LTC4126's DC-DC converter is switchable via its EN pin, allowing control by a microprocessor. The EN pin can also be used with the LTC4126's PBEN pin for push-button control - no additional debounce circuitry required.

The charge pump DC-DC converter has three modes of operation, depending on the battery voltage, to improve overall efficiency.

Figure 1. AC input rectification and DC rail voltage regulation.

Figure 2. Maximum theoretical converter efficiency and battery voltage.

Figure 3. Complete 6 mm diameter wireless battery charger receiver with integrated DC-DC converter and charger status output.

Figure 4. Complete wireless charging solution with single transistor ZVS transmitter and LTC4126 receiver.

Figure 5. ZVS operation with fTX_TANK = 1.29 × fDRIVE.

Complete application circuit for miniature PCB board

Due to the highly integrated design of the LTC4126, only a few external components are required to create a complete wireless charger receiver solution. On a 6 mm diameter application board, the entire design can be mounted inside a hearing aid or earbud.

Single Transistor ZVS Resonant Wireless Power Transmitter

The single-transistor transmitter shown in Figure 4 is a simple resonant circuit that uses the LTC6990 as an oscillator to drive a low-power transistor. To achieve ZVS operation, the transmitter resonant tank frequency is set to 1.29 times the oscillation frequency. In this way, switching losses are drastically reduced and overall wireless charging efficiency is improved. This transmitter requires only a few components and can be installed in a small enclosure.

in conclusion
The LTC4126 is a well protected, highly integrated, extremely compact wireless charger receiver solution ideal for wearable devices. Combining the LTC4126 (wearable device side) with a ZVS single transistor transmitter based on the LTC6990 (charging station side) makes it easy to implement a complete wireless charging solution. A complete solution based on these devices has low power consumption and low cost.

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 The Ohio State University Columbus in 2016.