Datasheet LTC4001 (Analog Devices) - 8

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