An very simple lithium Ion charger circuit can be constructed using the LM3632 charger controller , manufactured by National Semiconductor .
LM3622 provides a constant voltage or constant current (CVCC) configuration that changes, as necessary, to optimally charge lithium-ion battery cells. Voltage charging versions (4.1V, 4.2V, 8.2V, and 8.4V) are available for one or two cell battery packs and for coke or graphite anode battery chemistry.
The LM3622 accepts a wide input voltages from 4.5V to 24V. LM3622’s proprietary output voltage sensing circuit drains less than 200nA from the battery when the input source is disconnected.
The LM3622 circuitry includes functions for regulating the charge voltage with a temperature compensated bandgap reference and regulating the current with an external sense resistor.
As you can see in this schematic , this lithium ion charger circuit require few external components .
The low dropout linear charger shown in this circuit diagram provides constant current and constant voltage charging of 1-cell lithium-ion battery packs. J1 and J2 are used for selecting the operation of the low-voltage detection. The LM3622 initializes the charge cycle based on the battery voltage and the enable status of the low-voltage detection.
When the low-voltage detection is disabled, the LM3622 starts the charge cycle constant current mode if the battery voltage is below the controller’s regulation level. In constant current mode, LM3622 modulates the base drive of Q2 to regulate a constant 100mV across the current sense resistor R1, thus generating charge current of I-charge = 0.1V/R1
which is equal to 0.5A in this case.
Once the battery voltage reaches the target regulation level set by the LM3622, Q2 is controlled to regulate the voltage across the battery, and the constant voltage mode of the charging cycle starts. Once the charger is in the constant voltage mode, the charger maintains a regulated voltage across the battery and the charging current is dependent on the state of the charge of the battery. As the cell approaches a fully charged condition, the charge current falls to a very low value.
When the low-voltage detection is enabled and the initial battery voltage is below the low-voltage threshold, the LM3622 turns Q2 off and forces the LV pin low to drive Q1 on to start a wake up charge phase. Q1 in conjunction with R2 provides a low current source to recondition the battery.
During the wake up charge mode, Q1 is driven into saturation and the wake up charge current is programmed by R2, I-charge (wake) = (VIN – VCE1 – VD1 – LVth)/R2 , where VIN is the input supply voltage, VCE1 is the collector-emitter on state voltage of Q1, VD1 is the diode forward voltage of D1, and LVth is the low-voltage threshold level set by switch J2.
Once the battery voltage reaches the low-voltage threshold, the LV pin transitions to a high-impedance state and the EXT pin resumes the base drive of Q2 to start the constant current mode .