LTC4052-4.2_15 Datasheet, PDF (6/12 Pages) Linear Technology – Lithium-Ion Battery Pulse Charger with Overcurrent Protection

LTC4052-4.2
U
OPERATIO
The LTC4052 is a complete lithium-ion battery pulse
charger with an internal 0.35Ω N-MOSFET switch driven
by an internal charge pump. The charge current is set by
the current limit of the input supply (wall adapter). An
external R_SENSE sets the maximum allowable charge
overcurrent and prevents a wall adapter with a high
current limit from damaging the battery. If the current limit
of the input supply is above I_MAX, the charger will imme-
diately turn off the N-MOSFET switch and will retry after
640ms, thus reducing the average charge current to
approximately 1% of the overcurrent value (C_TIMER =
0.1µF). If the battery is disconnected while in full charge
mode, the charge pump turns off when the voltage at the
BAT pin rises above 4.7V and turns back on when the
voltage drops below the float voltage.
A charge cycle begins when the voltage at the V_IN pin rises
above the BAT pin by 45mV. The charger will go into trickle
charge mode if the battery voltage is below 2.45V. The
trickle charge current is preset to 24mA and is provided by
an internal current source. In trickle mode, both the pass
transistor and charge pump are off.
When the cell voltage exceeds 2.45V, the charger goes
into full charge mode. In this mode, the charge pump turns
on and ramps up the gate voltage of the pass transistor
turning it on. The current limited voltage at the V_IN pin will
ramp down to V_BAT plus the voltage drop across the pass
transistor and R_SENSE, thus reducing the power dissipa-
tion in the pass transistor. The charge current is deter-
mined by the current limit of the input supply.
When the battery voltage reaches the final float voltage,
the pass transistor turns off for 100ms (minimum off-
time). It remains off as long as the battery voltage stays
above the float voltage after the 100ms off-time. After the
minimum off-time, if the battery voltage drops below the
float voltage, the pass transistor turns back on for at least
400ms (minimum on-time). As the battery approaches full
charge, the off-time will get longer and the on-time will
stay at 400ms. When the N-MOSFET switch is on, the
voltage at the BAT pin will be slightly higher than the final
float voltage due to the ESR associated with the battery
pack. This voltage level should not be high enough to turn
on the overvoltage protection circuitry often located in the
battery pack. When the duty cycle at the GATE pin drops
below 10%, a comparator turns off the N-FET at the CHRG
pin and connects a weak current source (40µA) to ground
to indicate a near end-of-charge C/10 condition. The pulse
charging will continue until the timer stops.
An external capacitor at the TIMER pin sets the total charge
time, the minimum on- and off-time and the overcurrent
retry period. After a time-out has occurred, the charge
cycle terminates and the CHRG pin is forced high imped-
ance. After the charging stops, if the battery voltage drops
below 4.05V due to external loading or internal battery
leakage current, a new charge cycle will automatically
resume.
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