NiMH charger-receiver for hearing aid applications

December 18, 2015 // By Graham Prophet
LTC4123 is a wireless battery charging IC that combines a 30-mW wireless receiver with a constant-current/constant-voltage linear charger for NiMH batteries; AN external resonant LC tank connected to the LTC4123 enables the IC to receive power wirelessly from an alternating magnetic field generated by a transmit coil.

For use with batteries such as Varta’s power one ACCU plus series, integrated power management circuitry converts the coupled AC current into the DC current for charging. Wireless charging with the LTC4123 allows for a completely sealed product and eliminates the need to constantly replace primary batteries. Zn-Air (Zinc-Air) detection allows applications to work interchangeably with both rechargeable NiMH batteries and primary Zn-Air batteries with the same application circuit. Both battery types can directly power a hearing aid ASIC without the need for additional voltage conversion. By contrast, a 3.7V Li-ion battery requires a step-down regulator in addition to the LTC4123’s functionality to power the ASIC.

The LTC4123 rectifies AC power from the receive coil, and can also accept a 2.2V to 5V input to power a full-featured constant-current/constant-voltage battery charger. Features of the charger include programmable charge current up to 25 mA, a single-cell 1.5V battery charge voltage with ±0.5% accuracy, charge status indication and an onboard safety charge termination timer. A temperature-compensated charge voltage protects the NiMH battery and prevents overcharging. The LTC4123 prevents charging when batteries are inserted with reverse polarity and pauses charging if the temperature becomes too hot or too cold.

The LTC4123 is housed in a low profile (0.75 mm) 6-pin 2 x 2 mm DFN package with backside metal pad for thermal performance. The device is guaranteed for operation from -20°C to 85°C in E grade, and pricing starts at $2.65 (1000).

Linear Technology;