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LTC4001 Datasheet(PDF) 8 Page - Linear Technology |
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LTC4001 Datasheet(HTML) 8 Page - Linear Technology |
8 / 20 page 8 LTC4001 4001f OPERATIO The LTC4001 is a constant current, constant voltage Li-Ion battery charger based on a synchronous buck architecture. Low power dissipation makes continuous high rate (2A) battery charging practical. The battery DC charge current is programmed by a resistor RPROG (or a DAC output current) at the PROG pin. The final battery float voltage is internally set to 4.2V. Charging begins when the VIN voltage rises above the UVLO level (approximately 2.75V), VIN is 250mV greater than the battery voltage and EN is low. At the beginning of the charge cycle, if the battery voltage is less than the trickle charge threshold, 3V, the charger goes into trickle charge mode and delivers approximately 50mA to the battery using a linear charger. If the battery voltage stays low for more than one quarter of the charge time, the battery is considered faulty, the charge cycle is terminated and the FAULT pin produces a logic high output. When the battery voltage exceeds the trickle charge thresh- old, the low rate linear charger is turned off and the high rate PWM charger ramps up (based on the SS pin capaci- tance) reaching its full-scale constant current (set via the PROG pin). When the battery approaches the float voltage, the charge current will start to decrease. When the charge current drops below the charge rate detection threshold (set via the IDET pin) for more than 5ms, an internal comparator turns off the internal pull-down N-channel MOSFET at the CHRG pin, and connects a weak current source (30 µA typical) to ground to indicate a near end-of- charge condition. Total charge time is set by an external capacitor connected to the timer pin. After timeout occurs, the charge cycle is terminated and the CHRG pin is forced to a high imped- ance state. To restart the charge cycle, remove and reapply the input voltage, or momentarily shut the charger down via the EN pin. Also, a new charge cycle will begin if the battery voltage drops below the recharge threshold volt- age (100mV below the float voltage). A recharge cycle lasts only one-half of the normal charge time. A negative temperature coefficient (NTC) thermistor lo- cated close to the battery pack can be used to monitor battery temperature and suspend charging when battery temperature is outside the 0 °C to 50°C window. A tem- perature fault drives the FAULT pin high and makes the CHRG pin blink. When the input voltage (VIN) is present, the charger can be shut down by pulling the EN pin up. IDET Blanking The IDET comparator provides an end-of-charge indica- tion by sensing when battery charge current is less than the IDET threshold. To prevent a false end-of-charge indication from occurring during soft-start, this compara- tor is blanked until the battery voltage approaches the float voltage. Automatic Battery Recharge After the charge cycle is completed and if both the battery and the input power supply (wall adapter) are still con- nected, a new charge cycle will begin if the battery voltage drops below 4.1V due to self-discharge or external load- ing. This will keep the battery near maximum capacity at all times without manually restarting the charge cycle. In some applications such as battery charging in GPRS cellphones, large load current transients may cause bat- tery voltage to momentarily drop below the recharge threshold. To prevent these transients from initiating a recharge cycle when it is not needed, the output of the recharge comparator is digitally qualified. Only if the battery voltage stays below the recharge threshold for at least 4ms will battery recharging occur. (GPRS qualifica- tion is available even if timeout is disabled.) Undervoltage Lockout and Automatic Shutdown Internal undervoltage lockout circuits monitor VIN and keep the charger circuits shut down until VIN rises above the undervoltage lockout threshold (3V). The UVLO has a built-in hysteresis of 100mV. Furthermore, to protect against reverse current, the charger also shuts down if VIN is less than VBAT. If automatic shutdown is tripped, VIN must increase to more than 250mV above VBAT to allow charging. |
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