Datasheet LTC4001-1 (Analog Devices) - 8

LTC4001-1
OPERATION
The LTC4001-1 is a constant current, constant voltage A negative temperature coeffi cient (NTC) thermistor located Li-Ion battery charger based on a synchronous buck close to the battery pack can be used to monitor battery architecture. Low power dissipation makes continuous temperature and suspend charging when battery tempera- high rate (2A) battery charging practical. The battery DC ture is outside the 0°C to 50°C window. A temperature fault charge current is programmed by a resistor RPROG (or a drives the FAULT pin high and makes the CHRG pin blink. DAC output current) at the PROG pin. The fi nal battery When the input voltage (VIN) is present, the charger can fl oat voltage is internally set to 4.1V. be shut down by pulling the EN pin up. Charging begins when the VIN voltage rises above the
IDET Blanking
UVLO level (approximately 2.75V), VIN is 250mV greater than the battery voltage and EN is low. At the beginning The IDET comparator provides an end-of-charge indication of the charge cycle, if the battery voltage is less than the by sensing when battery charge current is less than the trickle charge threshold, 3V, the charger goes into trickle IDET threshold. To prevent a false end-of-charge indication charge mode and delivers approximately 50mA to the bat- from occurring during soft-start, this comparator is blanked tery using a linear charger. If the battery voltage stays low until the battery voltage approaches the fl oat voltage. for more than one quarter of the charge time, the battery is considered faulty, the charge cycle is terminated and
Automatic Battery Recharge
the FAULT pin produces a logic high output. After the charge cycle is completed and if both the battery When the battery voltage exceeds the trickle charge and the input power supply (wall adapter) are still con- threshold, the low rate linear charger is turned off and the nected, a new charge cycle will begin if the battery voltage high rate PWM charger ramps up (based on the SS pin drops below 4V due to self-discharge or external loading. capacitance) reaching its full-scale constant current (set This will keep the battery near maximum capacity at all via the PROG pin). When the battery approaches the fl oat times without manually restarting the charge cycle. voltage, the charge current will start to decrease. When In some applications such as battery charging in GPRS the charge current drops below the charge rate detec- cellphones, large load current transients may cause battery tion threshold (set via the IDET pin) for more than 5ms, voltage to momentarily drop below the recharge threshold. an internal comparator turns off the internal pull-down To prevent these transients from initiating a recharge cycle N-channel MOSFET at the CHRG pin, and connects a weak when it is not needed, the output of the recharge compara- current source (30μA typical) to ground to indicate a near tor is digitally qualifi ed. Only if the battery voltage stays end-of-charge condition. below the recharge threshold for at least 4ms will battery Total charge time is set by an external capacitor connected recharging occur. (GPRS qualifi cation is available even if to the timer pin. After timeout occurs, the charge cycle is timeout is disabled.) terminated and the CHRG pin is forced to a high impedance
Undervoltage Lockout and Automatic Shutdown
state. To restart the charge cycle, remove and reapply the input voltage, or momentarily shut the charger down via Internal undervoltage lockout circuits monitor VIN and the EN pin. Also, a new charge cycle will begin if the bat- keep the charger circuits shut down until VIN rises above tery voltage drops below the recharge threshold voltage the undervoltage lockout threshold (3V). The UVLO has (100mV below the fl oat voltage). A recharge cycle lasts a built-in hysteresis of 100mV. Furthermore, to protect only one-half of the normal charge time. 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. 40011fa 8