ALLPCB
Overview
The LTC4126 is a full-featured 7.5 mA wireless Li-Ion battery charger with an integrated 1.2 V inductance-free DC-DC converter. It is designed for hearing aids, wireless earbuds, and other space-constrained wearable products that require wireless charging. The LTC4126 can be paired with a ZVS single-transistor transmitter based on LTC6990 to create a complete wireless charging solution.
Wireless power reception and regulation
Wearable devices increasingly use wireless charging to avoid cables or exposed connectors, improving user convenience. The LTC4126 charger/DC-DC converter includes a wireless power controller that allows it to receive power inductively from the AC magnetic field generated by a transmitter coil. The wireless power controller rectifies the AC voltage from the receiver resonant circuit into a DC voltage at the VCC pin. This DC voltage feeds the linear charger, which regulates battery charging.
If the LTC4126 receives more energy than required, the wireless power controller adjusts the linear charger input VCC by shunting the receiver resonant circuit to ground. This keeps the linear charger running efficiently because its input is held just above the battery voltage VBAT. When the shunt is engaged, the resonant circuit also receives less power because the resonance shifts away from the transmitter frequency.
Charger safety and status reporting
The integrated constant-current/constant-voltage (CC/CV) linear Li-Ion charger incorporates a complete set of protection features to ensure a safe charging cycle, including automatic charge timing and termination, bad-battery detection, and charging suspension if temperature is out of range. The LTC4126 provides charger status and battery voltage level signals that can be read by a system microcontroller.
Integrated inductance-free DC-DC converter
The LTC4126 includes a monolithic inductance-free charge pump DC-DC converter that regulates system load outputs from the battery. The DC-DC converter can be enabled or disabled using the EN pin, allowing control from a microcontroller. The EN pin can also be used with the PBEN pin for button control without an external debounce circuit.
The charge pump DC-DC converter supports three operating modes depending on battery voltage, optimizing overall efficiency.
Compact implementation
Thanks to the LTC4126's high level of integration, a complete wireless charger receiver requires only a few external components. On a 6 mm diameter application board, the entire design can be mounted inside a hearing aid or earbud.
Single-transistor ZVS resonant transmitter
The single-transistor transmitter shown in Figure 4 uses LTC6990 as an oscillator to drive a low-power transistor in a simple resonant circuit. For ZVS operation, the transmitter resonant circuit frequency is set to 1.29 times the oscillator frequency. This significantly reduces switching losses and improves overall wireless charging efficiency. The transmitter requires only a few components and can be packaged in a small enclosure.
Conclusion
The LTC4126 is a highly integrated, compact, and well-protected wireless charger receiver suited for wearable devices. Combining the LTC4126 on the wearable side with a ZVS single-transistor transmitter based on LTC6990 on the charging station side enables a complete, low-power, low-cost wireless charging solution.
