The "ideal companion" of wearable devices - wireless lithium-ion charger solution

Wearable devices are increasingly using wireless battery charging, which eliminates 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 wirelessly receive power from the AC magnetic field generated by the transmitter coil (like the LTC6990 solution). The wireless power controller rectifies the AC voltage of the resonant circuit at the receiver end into a DC voltage at the VCC pin. This DC voltage is fed into the linear charger, which in turn regulates the charge to the battery.

If the LTC4126 receives more energy than required, the wireless power controller regulates the linear charger's input VCC by shunting the receiver resonant tank to ground. This way, the linear charger operates efficiently because its input remains just above the battery voltage VBAT. When the shunt circuit is engaged, the resonant tank also receives less power because the resonant frequency is detuned from the transmitter frequency.

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

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

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 can be switched on and off via its EN pin, allowing control via a microprocessor. The EN pin can also be used in conjunction with the LTC4126’s PBEN pin to implement push-button control—no additional debounce circuitry is required.

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

Figure 2. Maximum theoretical converter efficiency and battery voltage.

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

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.

The single transistor transmitter shown in Figure 4 is a simple resonant circuit using 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 greatly reduced and the overall wireless charging efficiency is improved. This transmitter requires only a few components and can be mounted in a small enclosure.

Figure 5. ZVS operation when ZfTX_TANK = 1.29 × fDRIVE.

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 based on these devices with low power consumption and low cost.

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