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LTC4081EDD-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC4081EDD-PBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LTC4081 11 4081f OPERATIO The LTC4081 is a full-featured linear battery charger with an integrated synchronous buck converter designed pri- marily for handheld applications. The battery charger is capable of charging single-cell 4.2V Li-Ion batteries. The buck converter is powered from the BAT pin and has a programmable output voltage providing a maximum load current of 300mA. The converter and the battery charger can run simultaneously or independently of each other. BATTERY CHARGER OPERATION Featuring an internal P-channel power MOSFET, MP1, the battery charger uses a constant-current/constant- voltage charge algorithm with programmable current. Charge current can be programmed up to 500mA with a final float voltage of 4.2V ±0.5%. The ⎯C⎯H⎯R⎯G open-drain status output indicates when C/10 has been reached. No blocking diode or external sense resistor is required; thus, the basic charger circuit requires only two external com- ponents. An internal charge termination timer adheres to battery manufacturer safety guidelines. Furthermore, the LTC4081 battery charger is capable of operating from a USB power source. A charge cycle begins when the voltage at the VCC pin rises above 3.6V and approximately 82mV above the BAT pin voltage, a 1% program resistor is connected from the PROG pin to ground, and the ⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin is pulled below the shutdown threshold (VIL). When the BAT pin approaches the final float voltage of 4.2V, the battery charger enters constant-voltage mode and the charge current begins to decrease. When the current drops to 10% of the full-scale charge current, an internal comparator turns off the N-channel MOSFET driving the ⎯C⎯H⎯R⎯G pin, and the pin becomes high impedance. An internal thermal limit reduces the programmed charge current if the die temperature attempts to rise above a preset value of approximately 115°C. This feature protects the LTC4081 from excessive temperature and allows the user to push the limits of the power handling capability of a given circuit board without the risk of damaging the LTC4081 or external components. Another benefit of the thermal limit is that charge current can be set according to typical, rather than worst-case, ambient temperatures for a given application with the assurance that the battery charger will automatically reduce the current in worst-case conditions. An internal timer sets the total charge time, tTIMER (typi- cally 4.5 hours). When this time elapses, the charge cycle terminates and the ⎯C⎯H⎯R⎯G pin assumes a high impedance state even if C/10 has not yet been reached. To restart the charge cycle, remove the input voltage and reapply it or momentarily force the ⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin above VIH. A new charge cycle will automatically restart if the BAT pin voltage falls below VRECHRG (typically 4.1V). Constant-Current / Constant-Voltage / Constant-Temperature The LTC4081 battery charger uses a unique architecture to charge a battery in a constant-current, constant-volt- age and constant-temperature fashion. Figure 1 shows a Simplified Block Diagram of the LTC4081. Three of the amplifier feedback loops shown control the constant-cur- rent, CA, constant-voltage, VA, and constant-temperature, TA modes. A fourth amplifier feedback loop, MA, is used to increase the output impedance of the current source pair, MP1 and MP3 (note that MP1 is the internal P-channel power MOSFET). It ensures that the drain current of MP1 is exactly 400 times the drain current of MP3. Amplifiers CA and VA are used in separate feedback loops to force the charger into constant-current or constant- voltage mode, respectively. Diodes D1 and D2 provide priority to either the constant-current or constant-voltage loop, whichever is trying to reduce the charge current the most. The output of the other amplifier saturates low which effectively removes its loop from the system. When in constant-current mode, CA servos the voltage at the PROG pin to be precisely 1V. VA servos its non-inverting input to 1.22V when in constant-voltage mode and the internal resistor divider made up of R1 and R2 ensures that the battery voltage is maintained at 4.2V. The PROG pin volt- age gives an indication of the charge current anytime in the charge cycle, as discussed in “Programming Charge Current” in the Applications Information section. |
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