LTC4081
11
4081fa
For more information www.linear.com/LTC4081
OPERATION
The LTC4081 is a full-featured linear battery charger with
an integrated synchronous buck converter designed pri-
marily 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 CHRG 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. An internal charge termination timer adheres
to battery manufacturer safety guidelines. Furthermore,
the LTC4081 battery charger is capable of operating from
a USB power source.
A charge cycle begins when the voltage at the V
CC
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
EN_CHRG pin is pulled
below the shutdown threshold (V
IL
).
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
CHRG 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 LTC4081 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
LTC4081 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, t
TIMER
(typi-
cally 4.5 hours). When this time elapses, the charge cycle
terminates and the
CHRG 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 EN_CHRG pin above V
IH
. A new
charge cycle will automatically restart if the BAT pin volt-
age falls below V
RECHRG
(typically 4.1V).
Constant-Current/Constant-Voltage/Constant-Temperature
The LTC4081 battery charger uses a unique architecture
to charge a battery in a constant-current, constant-voltage
and constant-temperature fashion. Three of the amplifier
feedback loops shown control the constant-current, CA,
constant-voltage, VA, and constant-temperature, TA modes
(see Block Diagram). 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 cur-
rent 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 voltage gives an indication of the charge current any-
time in the charge cycle, as discussed in “Programming
Charge Current” in the Applications Information section.
If the die temperature starts to creep up above 115°C
due to internal power dissipation, the transconductance
amplifier, TA, limits the die temperature to approximately
115°C by reducing the charge current. Diode D3 ensures
that TA does not affect the charge current when the die
