LTC4061
9
4061fd
operaTion
The LTC4061 is designed to charge single-cell lithium-ion
batteries. Using the constant current/constant voltage
algorithm, the charger can deliver up to 1A of charge
current with a final float voltage accuracy of ±0.35%. The
LTC4061 includes an internal P-channel power MOSFET and
thermal regulation circuitry. No blocking diode or external
sense resistor is required; thus, the basic charger circuit
requires only two external components.
Normal
Operation
The charge cycle begins when the voltage at the V
CC
pin
rises above the UVLO level and a discharged battery is
connected to BAT. If the BAT pin voltage is below 2.9V,
the charger enters trickle charge mode. In this mode,
the LTC4061 supplies 1/10th of the programmed charge
current in order to bring the battery voltage up to a safe
level for full current charging.
Once the BAT pin voltage rises above 2.9V, the charger
enters constant current mode, where the programmed
charge current is supplied to the battery. When the BAT
pin approaches the final float voltage (4.2V), the LTC4061
enters constant voltage mode and the charge current
decreases as the battery becomes fully charged.
The LTC4061 offers several methods with which to ter-
minate a charge cycle. Connecting an external capacitor
to the TIMER pin activates an internal timer that stops
the charge cycle after the programmed time period has
elapsed. Grounding the TIMER pin and connecting a resis-
tor to the I
DET
pin causes the charge cycle to terminate
once the charge current falls below a set threshold when
the charger is in constant voltage mode. Connecting the
TIMER pin to V
CC
disables internal termination, allowing
external charge user termination through the EN input. See
Applications Information for more information on charge
termination methods.
Programming
Charge Current
The charge current is programmed using a single resistor
from the PROG pin to ground. When the charger is in the
constant current mode, the voltage on the PROG pin is
1V. The battery charge current is 1000 times the current
out of the PROG pin. The program resistor and the charge
current are calculated by the following equations:
R
V
I
I
V
R
PROG
CHG
CHG
PROG
= =
1000 1000
,
The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage and ap-
plying the following equation:
I
V
R
BAT
PROG
PROG
= •1000
SmartStart
When the LTC4061 is initially powered on or brought out
of shutdown mode, the charger checks the battery volt-
age. If the BAT pin is below the recharge threshold of 4.1V
(which corresponds to approximately 80-90% battery
capacity), the LTC4061 enters charge mode and begins a
full charge cycle. If the BAT pin is above 4.1V, the LTC4061
enters standby mode and does not begin charging. This
feature reduces the number of unnecessary charge cycles,
prolonging battery life.
Automatic
Recharge
When the charger is in standby mode, the LTC4061
continuously monitors the voltage on the BAT pin. When
the BAT pin voltage drops below 4.1V, the charge cycle is
automatically restarted and the internal timer is reset to
50% of the programmed charge time (if time termination
is being used). This feature eliminates the need for peri-
odic charge cycle initiations and ensures that the battery
is always fully charged. Automatic recharge is disabled
in user termination mode.
Thermal
Regulation
An internal thermal feedback loop reduces the programmed
charge current if the die temperature attempts to rise
above a preset value of approximately 105°C. This feature
protects the LTC4061 from excessive temperature and
allows the user to push the limits of the power handling
capability of a given circuit board without risk of damaging
the LTC4061. The charge current can be set according to
typical (not worst-case) ambient temperatures with the
assurance that the charger will automatically reduce the
current in worst-case conditions.
