LTC4001 Datasheet, PDF (8/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001
U
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 R_PROG (or a
DAC output current) at the PROG pin. The final battery float
voltage is internally set to 4.2V.
Charging begins when the V_IN voltage rises above the
UVLO level (approximately 2.75V), V_IN 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 (30µA 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 (V_IN) 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 V_IN and
keep the charger circuits shut down until V_IN 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 V_IN
is less than V_BAT. If automatic shutdown is tripped, V_IN
must increase to more than 250mV above V_BAT to allow
charging.
4001f
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