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LTC4056ETS8-4.2 Datasheet(PDF) 7 Page - Linear Technology |
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LTC4056ETS8-4.2 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page LTC4056-4.2 7 405642f As the battery accepts charge, its voltage rises. When it reaches the preset float voltage of 4.2V, a precisely divided down version of this voltage (1.2V) is compared to the 1.2V internal reference voltage by amplifier VA. If the battery voltage attempts to exceed 4.2V (1.2V at the input of amplifier VA), the amplifier will divert current away from the output driver thus limiting charge current to maintain 4.2V on the battery. This is the constant voltage mode. An external capacitor on the TIMER/SHDN pin and the resistance between the PROG pin and ground set the total charge time. When this time elapses, the charge cycle terminates and the CHRG pin transitions from a strong pull-down to a weak 12 µA pull-down. To restart the charge cycle, simply remove the input voltage and reapply it or momentarily force the TIMER/SHDN pin to ground. The charge cycle will also restart if the BAT pin voltage falls below the recharge threshold (VRECHRG isnominally4.05V). When VCC is applied, pulling the TIMER/SHDN pin to ground will manually shut down the charger and reset the timer. When this pin is released an internal 7 µA current source pulls the TIMER/SHDN pin above the 0.82V shut- down threshold to resume charging. Fault conditions such as overheating of the die or exces- sive DRIVE pin or PROG pin current are monitored and limited. When input power is removed or manual shutdown is entered, the charger will drain only tiny leakage currents (<1 µA) from the battery, thus maximizing battery standby time. With VCC removed the external PNP base is con- nected to the battery by the charger. In manual shutdown the base is connected to VCC by the charger. OPERATIO The LTC4056 is a linear battery charger controller with a programmable charge termination timer. Operation can be understood by referring to the Block Diagram. A charge cycle begins when VCCrises above the UVLO (undervoltage lockout) threshold VUVLOI (nominally 4.4V), an external current programming resistor is connected between the PROG pin and ground and the TIMER/SHDN pin is allowed to rise above the shutdown threshold VMSDT (nominally 0.82V). If the battery voltage is below VTRIKL (2.8V) at the begin- ning of the charge cycle, the charger goes into trickle charge mode to bring the cell voltage up to a safe level for charging at full current. In this mode, an internal current source provides approximately 2% of the programmed charge current to the BAT pin. The charger goes into the full charge constant current mode once the voltage on the BAT pin rises above VTRIKL + ∆VTRIKL (2.9V). During full current charging, the collector of the external PNP provides the charge current. The PNP emitter current flows through the ISENSE pin and through the internal 110m Ω current sense resistor. This current is close in magnitude, but slightly more than the collector current since it includes base current. Amplifier A1 forces 1V on the PROG pin. Therefore, a current equal to 1V/RPROG will flow through the internal 100 Ω resistor. Amplifier CA will force the same voltage that appears across the 100 Ω resistor to appear across the internal 110m Ω resistor. This amplifier ensures that the current flowing out of the ISENSE pin is equal to 915 times the current flowing out of the PROG pin. Therefore, neglecting base current, the charge current will be 915V/RPROG. This region of opera- tion is referred to as constant current mode. |
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