Datasheet LTC4123 (Analog Devices) - 8

LTC4123
operaTion
The LTC4123 is a low power battery charger designed to current begins to drop. The charge current will continue wirelessly charge single-cell NiMH batteries. The charger to drop and the BAT pin voltage will be maintained at the uses a constant-current/constant-voltage charge algorithm proper charge voltage. After the charge termination timer with a charge current programmable up to 25mA. The final expires, charge current ceases and the BAT pin assumes a charge voltage is temperature compensated to reach an high impedance state. Once the charge cycle terminates, optimum state-of-charge and prevent overcharging of the the CHRG pin stops blinking and assumes a pull-down battery. The LTC4123 also guarantees the accuracy of the state. To start a new charge cycle, remove the input volt- charge voltage to ±15mV from –5°C to 70˚C (see typical age at ACIN or VCC and reapply it. performance characteristics).
Input Voltage Qualification
An internal undervoltage lockout (UVLO) circuit monitors An external resonant LC tank connected to the ACIN the input voltage at V pin allows the part to receive power wirelessly from an CC and disables the LTC4123 until V alternating magnetic field generated by a transmit coil. CC rises above VUVLO (typically 1.95V). The UVLO circuit has a built-in hysteresis of approximately 40mV. During A complete wireless power transfer system consists undervoltage conditions, maximum battery drain current of transmit circuitry, with a transmit coil, and receive is I circuitry, with a receive coil. The Rectification and Input BAT(LEAK) (100nA maximum). Power control circuitry (Figure 1) rectifies the AC voltage The LTC4123 also includes undervoltage current limiting at the ACIN pin and regulates the rectified voltage at VCC (UVCL) that prevents charging at the programmed current to less than VCC(HIGH) (typically 5V). until the input supply voltage is above VUVCL (typically 2.2V). UVCL is particularly useful in situations when the wireless An LED can be connected to the CHRG pin to indicate the power available is limited. Without UVCL if the magnetic status of the charge cycle and any fault conditions. An coupling between the receive coil and transmit coil is low, internal thermal limit will stop charging and pause the UVLO could be easily tripped if the charger tries to provide 6-hour charge timer if the die temperature rises above the full charge current. UVLO forces the charge current to 70˚C or falls below –5˚C. zero, which allows the supply voltage to rise above the In a typical charge cycle (see Figure 2), the 6-hour charge UVLO threshold and switch on the charger again. This timer will begin when the part is powered. At the beginning oscillatory behavior will result in intermittent charging. The of the charge cycle, the LTC4123 will determine if the battery UVCL circuitry prevents this undesirable behavior. is connected in reverse or if a Zinc-Air battery is connected to the BAT pin. If any of the above fault conditions is true,
Battery Fault Conditions
the BAT pin goes to a high impedance state and charging The LTC4123 detects the presence of Zinc-Air batteries at is stopped immediately. An LED connected to CHRG will the beginning of the charge cycle. Initially, the LTC4123 blink fast (typically at 6Hz). If the battery is a NiMH battery will charge the battery at full charge current and if the inserted with correct polarity, it will continue to charge at BAT pin rises above V the programmed current level in constant-current mode Zn-AIR (typically 1.65V) in TZn-AIR (typically 80 seconds) or less from the start of the charge and CHRG will blink slowly (typically at 0.8Hz). timer, the LTC4123 determines the battery connected is When the BAT pin approaches the final charge voltage, the a Zinc-Air battery and charging is disabled immediately. LTC4123 enters constant-voltage mode and the charge The charging cycle continues normally otherwise. The 4123fa 8 For more information www.linear.com/LTC4123