LTC4050 Datasheet, PDF (7/12 Pages) Linear Technology – Lithium-Ion Linear Battery Charger with Thermistor Interface

U
OPERATIO
The LTC4050 is a linear battery charger controller. The
charge current is programmed by the combination of a
program resistor (R_PROG) from the PROG pin to ground
and a sense resistor (R_SENSE) between the V_CC and SENSE
pins. R_PROG sets a program current through an internal
trimmed 800Ω resistor setting up a voltage drop from V_CC
to the input of the current amplifier (CA). The current
amplifier servos the gate of the external P-channel MOSFET
to force the same voltage drop across R_SENSE which sets
the charge current. When the voltage at the BAT pin
approaches the preset float voltage, the voltage amplifier
(VA) will start sinking current which reduces the voltage
drop across R_SENSE, thus reducing the charge current.
A charge cycle begins when the voltage at V_CC pin rises
above the UVLO level, a program resistor is connected
from the PROG pin to ground, and the NTC thermistor
temperature is between 0°C and 50°C. At the beginning of
the charge cycle, if the battery voltage is below 2.49V, the
charger goes into trickle charge mode. The trickle charge
current is 10% of the full-scale current. If the cell voltage
stays low for one quarter of the total charge time, the
charge sequence will terminate.
The charger goes into the fast charge constant-current
mode after the voltage on the BAT pin rises above 2.49V.
In constant-current mode, the charge current is set by the
combination of R_SENSE and R_PROG.
When the battery approaches the final float voltage, the
charge current will begin to decrease. When the current
drops to 10% of the full-scale charge current, an internal
comparator will turn off the pull-down N-channel MOSFET
LTC4050
at the CHRG pin and connect a weak current source to
ground, indicating that the battery is nearly fully charged
(C/10 occurs at approximately 94% charge).
An external capacitor on the TIMER pin sets the total
charge time. After a time-out occurs, the charge cycle is
terminated and the CHRG pin is forced high impedance. To
restart the charge cycle, remove the input voltage and
reapply it, or momentarily float the PROG pin.
Replacing the battery when charging will cause the timer
to be reset if the cell voltage of the new battery is below
3.88V (for 4.1V cells) or 3.98V (for 4.2V cells). If the
voltage is above 3.88V(for 4.1V cells) or 3.98V (for 4.2V
cells) the timer will continue for the remaining charge
time. In the case when a time out has occurred, a new
battery with a cell voltage of less than 3.88V or 3.98V can
be inserted and charged automatically with the full pro-
grammed charge time.
For batteries like lithium-ion that require accurate final
float voltage, the internal 2.47V reference, voltage ampli-
fier and the resistor divider provide regulation with ±1%
(max) accuracy.
The charger can be shut down by floating the PROG pin.
An internal current source will pull it high and clamp at
3.5V.
When the input voltage is not present, the charger goes
into a sleep mode, dropping I_CC to 5µA. This greatly
reduces the current drain on the battery and increases the
standby time.
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