Datasheet LTC4126 (Analog Devices) - 10

link to page 9 LTC4126
OPERATION
The LTC4126 is a low power battery charger with an charger enters constant-voltage (CV) mode and the charge integrated step-down DC/DC converter designed to wire- current begins to drop. The charge current continues to drop lessly charge single-cell Li-Ion batteries and provide a while the BAT pin voltage is maintained at the proper charge 1.2V output suitable for powering a hearing-aid ASIC. voltage. This state of CC/CV charging is indicated by a slow The part has three principal circuit components: an AC blinking LED (typically 1.14Hz) at the CHRG pin. power controller, a full-featured linear battery charger, and After the 6-hour charge termination timer expires, charg- a step-down DC/DC converter. ing stops completely. Once the charge cycle terminates, the LED at the CHRG pin stops blinking and assumes a
AC POWER CONTROLLER
pull-down state. To start a new charge cycle, remove the A complete wireless power transfer system consists of power source at ACIN or VCC and reapply it. transmit circuitry with a transmit coil and receive circuitry with a receive coil. The LTC4126 resides on the receiver
Automatic Recharge
side, where an external parallel resonant LC tank con- After charging has terminated, the charger draws only 3.7µA nected to the ACIN pin allows the part to receive power (typical) from the battery. If it remains in this state long wirelessly from an alternating magnetic field generated enough, the battery will eventually discharge. To ensure by the transmit coil. The Rectification and Input Power that the battery is always topped off, a new charge cycle Control circuitry (Figure 1) rectifies the AC voltage at the automatically begins when the battery voltage falls below ACIN pin and regulates that rectified voltage at the VCC VRECHRG (typically 97.5% of the charge voltage). In the pin to less than VCC(HIGH) (typically 5.5V). event that the battery voltage falls below VRECHRG while the safety timer is still running, the timer will not reset.
Operation without Wireless Power
This prevents the timer from restarting every time the bat- The LTC4126 can be alternately powered by connecting a tery voltage dips below VRECHRG during a charging cycle. DC voltage source to the VCC pin directly instead of receiv- ing power wirelessly through the ACIN pin. Ground the
Bad Battery Fault
ACIN pin if a voltage supply is connected to VCC. If the battery fails to reach a voltage above VRECHRG by the end of a full charge cycle of 6 hours, the battery is deemed
BATTERY CHARGER
faulty and the LED at the CHRG pin indicates this bad battery fault condition by blinking fast (typically 4.58Hz). The LTC4126 includes a full-featured constant-current (CC)/constant-voltage (CV) linear battery charger with
Differential Undervoltage Lockout (DUVLO)
automatic recharge, automatic termination by safety timer, bad battery detection, and out-of-temperature-range A differential undervoltage lockout circuit monitors the charge pausing. Charge current is internally fixed at 7.5mA differential voltage between VCC and BAT and disables and the final charge voltage is pin-selectable via the V the charger if the V SEL CC voltage falls to within 27mV (typical pin to either 4.2V or 4.35V. ΔVUVLO) of the BAT voltage. This condition is indicated by a low on the ACPR pin. Charging does not resume until As soon as the voltage at the VCC pin rises 80mV (typical) this difference increases to 80mV at which time the ACPR above the BAT pin voltage, the charger attempts to charge the pin transitions back high. The DC/DC must be enabled for battery and a new charge cycle is initiated. A 6-hour charge proper ACPR indication. termination timer starts at the beginning of this new charge cycle. When the VCC-to-BAT differential voltage rises above
Differential Undervoltage Current Limit (DUVCL)
154mV (typical), the charger enters constant-current (CC) The LTC4126 charger also includes differential undervolt- mode and charges the battery at the full rated current of 7.5mA. age current limiting (DUVCL) which gradually reduces the When the BAT pin approaches the final charge voltage, the charge current from the full 7.5mA towards zero as the Rev. 0 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts