LTC4052-42 데이터 시트보기 (PDF) - Linear Integrated System
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LTC4052-42 Datasheet PDF : 8 Pages
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OPERATIO
When the cell voltage exceeds 2.45V, the charger goes
into fast charge mode. In this mode, the charge pump
turns on and ramps up the gate voltage of the pass
transistor turning it on. The voltage at the VIN pin then
ramps down to VBAT plus the voltage drop across the pass
transistor and RSENSE, 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
LTC4052-4.2
stay at 400ms. The voltage at the BAT pin will be slightly
higher than the final float voltage due to the ESR associ-
ated with the battery pack. This voltage level should not
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 4V, due to external loading or internal leakage, a
new charge cycle will automatically resume.
APPLICATIO S I FOR ATIO
Input Voltage (Wall Adapter)
The input voltage to the LTC4052 must have some
method of current limit capability. The current limit
level of the input power source must be lower than
the overcurrent limit (IMAX) set by the sense resistor
IMAX = 105mV/(RSENSE + 10mΩ). The 10mΩ represents
bond wire resistance internal to the IC. If a wall adapter
without current limit is used, or the current limit level is
above IMAX, the charger will turn on briefly and then
immediately turn off after the overcurrent condition is
detected. This cycle will be resumed every 640ms (CTIMER
= 0.1µF) until the total charge time has run out. If overcurrent
protection is not needed, short the SENSE pin to VIN.
Trickle Charge and Defective Battery Detection
At the beginning of the charge cycle, if the cell voltage is
low (less than 2.45V) the charger goes into a 24mA trickle
charge mode. If the low cell voltage persists for one
quarter of the total charge time, the battery is considered
defective and the charge cycle is terminated. The CHRG
pin output is then forced to a high impedance state.
Battery Charge Current
The battery charge current is determined by the current
limit of the input supply (wall adapter). However, this
current must not exceed the maximum charge current,
IMAX. If an overcurrent condition is detected, the charging
is immediately terminated, the GATE pin is pulled to
ground and the charge pump turns off. The charging will
resume after a 640ms time off (CTIMER = 0.1µF).
Programming the Timer
The programmable timer is used to terminate the charge
cycle and sets the minimum ON/OFF time and the
overcurrent time-off period. The length of the timer is
programmed by an external capacitor from the TIMER pin
to ground. The total charge time is:
Time (Hours) = (3 Hours)(CTIMER/0.1µF) or
CTIMER = 0.1µF • Time (Hours)/3 (Hours)
The timer starts when the input voltage (at least 40mV
greater than VBAT) is applied. After a time-out has oc-
curred, the charging stops and the CHRG pin becomes
high impedance.
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