LTC1732-4.2 Ver la hoja de datos (PDF) - Linear Technology
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LTC1732-4.2 Datasheet PDF : 12 Pages
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LTC1732-4/LTC1732-4.2
APPLICATIONS INFORMATION
Charger Conditions
The charger is off when any of the following conditions exist:
the VCC pin voltage is less than 4.1V, the dropout voltage
(VCC – VBAT) is less than 54mV, or if the program resistor
is floating. The DRV pin is pulled up to VCC thus keeping the
MOSFET off, and the internal resistor divider is disconnected
to reduce the drain on the battery.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until
VCC rises above 4.1V. To prevent oscillation around
VCC = 4.1V, the UVLO circuit has 200mV of hysteresis.
Trickle Charge and Defective Battery Detection
At the beginning of the charging sequence, if the battery
voltage is below 2.457V, the charger goes into trickle
mode. The charge current is dropped to 10% of the full-
scale current. If the low cell voltage persists for one
quarter of the total charging time, the battery is considered
defective, the charging will be terminated and the CHRG
pin output is forced to a high impedance state.
Shutdown
The LTC1732 can be forced into shutdown by floating the
PROG pin and allowing the internal 2.5µA current source
to pull the pin above the 2.457V shutdown threshold
voltage. The DRV pin will then be pulled up to VCC and turn
off the external P-channel MOSFET. The internal timer is
reset in the shutdown mode.
Programming Charge Current
The formula for the battery charge current (see Block
Diagram) is:
IBAT
=
2.457V
RPROG
• 800Ω
RSENSE
where RPROG is the total resistance from the PROG pin to
ground.
For example, if 0.5A charge current is needed, select a
value for RSENSE that will drop 100mV at the maximum
charge current. RSENSE = 0.1V/0.5A = 0.2Ω, then calculate:
RPROG = (2.457V/500mA)(800Ω/0.2Ω) = 19.656k
For best stability over temperature and time, 1% resistors
are recommended. The closest 1% resistor value is 19.6k.
Programming the Timer
The programmable timer is used to terminate the charge
cycle. The length of the timer is programmed by an
external capacitor at the TIMER pin. The total charge time
is:
Time = (3 Hours)(CTIMER/0.1µF)
The timer starts when an input voltage greater than 4.1V
is applied and the program resistor is connected to ground.
After a time-out occurs, the CHRG output will turn into a
high impedance state to indicate that the charging has
stopped. Connecting the TIMER pin to VCC disables the
timer and also puts the charger into a constant-current
mode. To disable only the timer function, short the TIMER
pin to GND.
Battery Detection and Recharge
LTC1732-4: replacing the battery before the timer has ex-
pired will reset the timer, thus starting a new charge cycle,
provided the cell voltage of the new battery is less than
3.8V. If the new battery is greater than 3.8V, the timer will
not be reset and charging will continue for the remaining
portion of the time period. Replacing the battery after the
timer has expired will start a new charge cycle, regardless
of the battery voltage, provided the previous battery volt-
age exceeded 3.9V before the timer expired. After a com-
plete charge cycle has occurred (VBAT > 3.9V), and the
battery remains connected to the charger, a new charge
cycle will begin if the battery voltage drops below 3.8V
because of a load on the battery or self discharge.
LTC1732-4.2: replacing the battery before the timer has
expired will reset the timer, thus starting a new charge
cycle, provided the cell voltage of the new battery is less
than 4.05V. If the new battery is greater than 4.05V, the
timer will not be reset and charging will continue for the
remaining portion of the time period. Replacing the battery
after the timer has expired will start a new charge cycle,
regardless of the battery voltage, provided the previous
battery voltage exceeded 4.1V before the timer expired.
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