LTC4001-1_15 Datasheet, PDF (8/20 Pages) Linear Technology – 2A Synchronous Buck Li-Ion Charger

LTC4001-1
OPERATION
The LTC4001-1 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 4.1V.
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 bat-
tery 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
threshold, the low rate linear charger is turned off and the
high rate PWM charger ramps up (based on the SS pin
capacitance) 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 detec-
tion 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 impedance
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 bat-
tery voltage drops below the recharge threshold voltage
(100mV below the float voltage). A recharge cycle lasts
only one-half of the normal charge time.
A negative temperature coefficient (NTC) thermistor located
close to the battery pack can be used to monitor battery
temperature and suspend charging when battery tempera-
ture is outside the 0°C to 50°C window. A temperature 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 indication
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 comparator 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 4V due to self-discharge or external loading.
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 battery
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 compara-
tor is digitally qualified. Only if the battery voltage stays
below the recharge threshold for at least 4ms will battery
recharging occur. (GPRS qualification 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.
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