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LTC4068-4.2 Datasheet(PDF) 7 Page - Linear Technology |
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LTC4068-4.2 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LTC4068-4.2/LTC4068X-4.2 406842fa OPERATIO The LTC4068 is a single cell lithium-ion battery charger using a constant-current/constant-voltage algorithm. It can deliver up to 950mA of charge current (using a good thermal PCB layout) with a final float voltage accuracy of ±1%. The LTC4068 includes an internal P-channel power MOSFET and thermal regulation circuitry. No blocking diode or external current sense resistor is required; thus, the basic charger circuit requires only two external com- ponents. Furthermore, the LTC4068 is capable of operat- ing from a USB power source. Normal Charge Cycle A charge cycle begins when the voltage at the VCC pin rises above the UVLO threshold level and a 1% program resistor is connected from the PROG pin to ground. If the BAT pin is less than 2.9V, the charger enters trickle charge mode. In this mode, the LTC4068 supplies approximately 1/10th the programmed charge current to bring the battery volt- age up to a safe level for full current charging. (Note: The LTC4068X does not include this trickle charge feature.) When the BAT pin voltage rises above 2.9V, the charger enters constant-current mode where the programmed charge current is supplied to the battery. When the BAT pin approaches the final float voltage (4.2V), the LTC4068 enters constant-voltage mode and the charge current begins to decrease. When the charge current drops to the programmed termination threshold (set by the external resistor RTERM), the charge cycle ends. Programming Charge Current The charge current is programmed using a single resistor from the PROG pin to ground. The charge current out of the BAT pin is 1000 times the current out of the PROG pin. The program resistor and the charge current are calcu- lated using the following equations: R V I I V R PROG CHG CHG PROG == 1000 1000 , Charge current out of the BAT pin can be determined at any time by monitoring the PROG pin voltage and using the following equation: I V R BAT PROG PROG = • 1000 Programming Charge Termination The charge cycle terminates when the charge current falls below the programmed termination threshold. This threshold is set by connecting an external resistor, RTERM, from the ITERM pin to ground. The charge termination current threshold (ITERM) is set by the following equation: I V R IR R R V I TERM TERM CHG PROG TERM TERM TERM == = 100 10 100 •, The termination condition is detected by using an internal filtered comparator to monitor the ITERM pin. When the ITERM pin voltage drops below 100mV* for longer than tTERM (typically 1ms), charging is terminated. The charge current is latched off and the LTC4068 enters standby mode where the input supply current drops to 200µA. (Note: Termination is disabled in trickle charging and thermal limiting modes.) ITERMcan be set to be 1/10th of ICHG by shorting the ITERM pin to the PROG pin, thus eliminating the need for external resistor RTERM. When configured in this way, ITERM is always set to ICHG/10, and the programmed charge current is set by the equation: I V R R V I CHG PROG PROG CHG == 500 500 , ** When charging, transient loads on the BAT pin can cause the ITERM pin to fall below 100mV for short periods of time before the DC charge current has dropped to 10% of the programmed value. The 1ms filter time (tTERM) on the termination comparator ensures that transient loads of this nature do not result in premature charge cycle termi- nation. Once the average charge current drops below the programmed termination threshold, the LTC4068 termi- nates the charge cycle and ceases to provide any current out of the BAT pin. In this state, any load on the BAT pin must be supplied by the battery. The LTC4068 constantly monitors the BAT pin voltage in standby mode. If this voltage drops below the 4.1V recharge Any external sources that hold the ITERM pin above 100mV will prevent the LTC4068 from terminating a charge cycle. These equations apply only when the ITERM pin is shorted to the PROG pin. * ** |
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