New Industry Products

10mA Wireless Li-Ion Charger with 1.2V Step-Down DC-DC

August 05, 2019 by Paul Shepard

The LTC®4126-10 from Analog Devices is a low-power wireless single-cell Li-Ion battery charger with an integrated step-down dc-dc regulator. The step-down regulator is a low-noise multimode charge pump which is powered from the battery and provides a regulated 1.2V at the output. The switching frequency is set to either 50kHz or 75kHz depending on the mode to keep any switching noise out of the audible range.

The tiny 2mm × 2mm LQFN package and minimal external component count make the LTC4126-10 well suited for Li-Ion battery powered hearing aid applications, wireless headsets, IoT wearables, and other low power portable devices where a small solution size is required.

The LTC4126-10 charger is a full-featured constant-current constant-voltage Li-Ion battery charger with automatic recharge, automatic termination by safety timer, and battery temperature monitoring via an NTC pin. Charge current is fixed at 10mA with a 3-hour termination timer. Undervoltage protection disconnects the battery from all loads when the battery voltage is below 3.0V.

Wireless 10mA Li-Ion Battery Charger (4.2V) Tuned at 266kHz with Pushbutton Enabling (click on schematic to enlarge)

A complete wireless power transfer system consists of transmit circuitry with a transmit coil and receive circuitry with a receive coil. The LTC4126-10 resides on the receiver side, where an external parallel resonant LC tank connected to the ACIN pin allows the part to receive power wirelessly from an alternating magnetic field generated by the transmit coil. The Rectification and Input Power Control circuitry rectifies the ac input at the ACIN pin and regulates that rectified voltage at the VCC pin to less than VCC(HIGH) (typically 5.5V).

Operation without Wireless Power

The LTC4126-10 can be alternately powered by connecting a DC voltage source to the VCC pin directly instead of receiving power wirelessly through the ACIN pin. Ground the ACIN pin if a voltage supply is connected to VCC.

Battery Charger

Top and Bottom View of the IC with Complete Application Circuit

The LTC4126-10 includes a full-featured constant-current (CC)/constant-voltage (CV) linear battery charger with automatic recharge, automatic termination by safety timer, bad battery detection, and out-of-temperature range charge pausing. Charge current is internally fixed at 10mA and the final charge voltage is pin-selectable via the VSEL pin to either 4.2V or 4.1V.

As soon as the voltage at the VCC pin rises 80mV (typical) above the BAT pin voltage, the charger attempts to charge the battery and a new charge cycle is initiated. A 3-hour charge termination timer starts at the beginning of this new charge cycle. When the VCC-to-BAT differential voltage rises above 150mV (typical), the charger enters constant-current (CC) mode and charges the battery at the full rated current of 10mA.

When the BAT pin approaches the final charge voltage, the charger enters constant-voltage (CV) mode and the charge current begins to drop. The charge current continues to drop while the BAT pin voltage is maintained at the proper charge voltage. This state of CC/CV charging is indicated by a slow blinking LED (typically 1.14Hz) at the CHRG pin. After the 3-hour charge termination timer expires, charging stops completely.

Summary of features

  • Wireless Li-Ion Battery Charger Plus High Efficiency Multi-Mode Charge Pump DC-DC
  • Wideband Rx Frequency: DC to >10MHz
  • Integrated Rectifier with Overvoltage Limit
  • Pin Selectable Charge Voltage: 4.1V or 4.2V
  • Charge Current: 10mA (Fixed)
  • Low Battery Disconnect: 3.0V
  • NTC Pin for Temperature Qualified Charging
  • DC-DC Regulated Output: 1.2V
  • DC-DC Output Current: Up to 60mA
  • 50kHz/75kHz Switching, No Audible Noise
  • Pushbutton and/or Digital on/off Control for DC-DC
  • Thermally Enhanced 12-Lead 2mm × 2mm LQFN Package