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LTC4080X Datasheet(PDF) 11 Page - Linear Technology

Part # LTC4080X
Description  500mA Standalone Li-Ion Charger with Integrated 300mA Synchronous Buck in 3mm3mm DFN
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC4080X Datasheet(HTML) 11 Page - Linear Technology

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LTC4080X
11
4080Xf
OPERATIO
The LTC4080X is a full-featured linear battery charger
with an integrated synchronous buck converter designed
primarily for handheld applications. The battery charger
is capable of charging single-cell 4.2V Li-Ion batteries.
The buck converter is powered from the BAT pin and has
a programmable output voltage providing a maximum load
current of 300mA. The converter and the battery charger
can run simultaneously or independently of each other.
BATTERY CHARGER OPERATION
Featuring an internal P-channel power MOSFET, MP1, the
battery charger uses a constant-current/constant-voltage
charge algorithm with programmable current. Charge
current can be programmed up to 500mA with a final
float voltage of 4.2V ±0.5%. The
⎯C⎯H⎯R⎯G open-drain status
output indicates when C/10 has been reached. No blocking
diode or external sense resistor is required; thus, the basic
charger circuit requires only two external components. The
⎯A⎯C⎯P⎯R open-drain output indicates if the VCC input voltage,
and the difference between VCC and BAT, are sufficient for
charging. An internal charge termination timer adheres to
battery manufacturer safety guidelines. Furthermore, the
LTC4080X battery charger is capable of operating from a
USB power source.
A charge cycle begins when the voltage at the VCC pin
rises above 3.6V and approximately 82mV above the BAT
pin voltage, a 1% program resistor is connected from the
PROG pin to ground, and the
⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin is pulled below
the shutdown threshold (VIL).
When the BAT pin approaches the final float voltage of
4.2V, the battery charger enters constant-voltage mode
and the charge current begins to decrease. When the
current drops to 10% of the full-scale charge current, an
internal comparator turns off the N-channel MOSFET driving
the
⎯C⎯H⎯R⎯G pin, and the pin becomes high impedance.
An internal thermal limit reduces the programmed charge
current if the die temperature attempts to rise above a
preset value of approximately 115°C. This feature protects
the LTC4080X from excessive temperature and allows the
user to push the limits of the power handling capability
of a given circuit board without the risk of damaging the
LTC4080X or external components. Another benefit of the
thermal limit is that charge current can be set according
to typical, rather than worst-case, ambient temperatures
for a given application with the assurance that the battery
charger will automatically reduce the current in worst-case
conditions.
An internal timer sets the total charge time, tTIMER (typi-
cally 4.5 hours). When this time elapses, the charge cycle
terminates and the
⎯C⎯H⎯R⎯G pin assumes a high impedance
state even if C/10 has not yet been reached. To restart
the charge cycle, remove the input voltage and reapply
it or momentarily force the
⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin above VIH. A
new charge cycle will automatically restart if the BAT pin
voltage falls below VRECHRG (typically 4.1V).
Constant-Current / Constant-Voltage /
Constant-Temperature
The LTC4080X battery charger uses a unique architecture
to charge a battery in a constant-current, constant-volt-
age and constant-temperature fashion. Figure 1 shows a
Simplified Block Diagram of the LTC4080X. Three of the
amplifier feedback loops shown control the constant-cur-
rent, CA, constant-voltage, VA, and constant-temperature,
TA modes. A fourth amplifier feedback loop, MA, is used to
increase the output impedance of the current source pair,
MP1 and MP3 (note that MP1 is the internal P-channel
power MOSFET). It ensures that the drain current of MP1
is exactly 400 times the drain current of MP3.
Amplifiers CA and VA are used in separate feedback loops
to force the charger into constant-current or constant-
voltage mode, respectively. Diodes D1 and D2 provide
priority to either the constant-current or constant-voltage
loop, whichever is trying to reduce the charge current the
most. The output of the other amplifier saturates low which
effectively removes its loop from the system. When in
constant-current mode, CA servos the voltage at the PROG
pin to be precisely 1V. VA servos its non-inverting input
to 1.22V when in constant-voltage mode and the internal
resistor divider made up of R1 and R2 ensures that the
battery voltage is maintained at 4.2V. The PROG pin volt-
age gives an indication of the charge current anytime in
the charge cycle, as discussed in “Programming Charge
Current” in the Applications Information section.


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