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AAT3688IWP-4.2-T1 Datasheet(PDF) 11 Page - Advanced Analog Technology, Inc. |
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AAT3688IWP-4.2-T1 Datasheet(HTML) 11 Page - Advanced Analog Technology, Inc. |
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11 / 23 page  AAT3688 USB Port Lithium-Ion/Polymer Battery Charger 3688.2006.10.1.5 11 Charging Operation The AAT3688 has four basic modes for the battery charge cycle and is powered from the USB input: pre-conditioning/trickle charge; constant current/fast charge; constant voltage; and end of charge. For ref- erence, Figure 1 shows the current versus voltage profile during charging phases. Battery Preconditioning Before the start of charging, the AAT3688 checks several conditions in order to assure a safe charging environment. The input supply must be above the minimum operating voltage, or under-voltage lockout threshold (VUVLO), for the charging sequence to begin. In addition, the cell temperature, as reported by a thermistor connected to the TS pin from the bat- tery, must be within the proper window for safe charging. When these conditions have been met and a battery is connected to the BAT pin, the AAT3688 checks the state of the battery. If the cell voltage is below the Preconditioning Voltage Threshold (VMIN), the AAT3688 begins preconditioning the cell. The battery preconditioning trickle charge current is equal to the fast charge constant current divided by 10. For example, if the programmed fast charge current is 500mA, then the preconditioning mode (trickle charge) current will be 50mA. Cell precon- ditioning is a safety precaution for a deeply dis- charged battery and also aids in limiting power dis- sipation in the pass transistor when the voltage across the device is at the greatest potential. Fast Charge / Constant Current Charging Battery cell preconditioning continues until the volt- age on the BAT pin exceeds the Preconditioning Voltage Threshold (VMIN). At this point, the AAT3688 begins the constant current fast charging phase. The fast charge constant current (ICC) amplitude is determined by the selected charge mode USBH or USBL and is programmed by the user via the RSETH and RSETL resistors. The AAT3688 remains in con- stant current charge mode until the battery reaches the voltage regulation point, VBAT. Constant Voltage Charging The system transitions to a constant voltage charg- ing mode when the battery voltage reaches output charge regulation threshold (VBAT) during the con- stant current, fast charge phase. The regulation voltage level is factory programmed to 4.2V ( 1%). The charge current in the constant voltage mode drops as the battery cell under charge reaches its maximum capacity. End of Charge Cycle Termination and Recharge Sequence When the charge current drops to 7.5% of the pro- grammed fast charge current level in the constant volt- age mode, the device terminates charging and goes into a sleep state. The charger will remain in a sleep state until the battery voltage decreases to a level below the battery recharge voltage threshold (VRCH). When the input supply is disconnected, the charg- er will also automatically enter power-saving sleep mode. Only consuming an ultra-low 0.3µA in sleep mode, the AAT3688 minimizes battery drain when it is not charging. This feature is particularly useful in applications where the input supply level may fall below the battery charge or under-voltage lockout level. In such cases where the AAT3688 input volt- age drops, the device will enter the sleep mode and automatically resume charging once the input sup- ply has recovered from its fault condition. Figure 1: Current vs. Voltage Profile During Charging Phases. Constant Current Charge Phase Constant Voltage Charge Phase Preconditioning Trickle Charge Phase Charge Complete Voltage Constant Current Mode Voltage Threshold Regulated Current Trickle Charge and Termination Threshold I = CC / 10 I = Max CC |
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