LTC1731-4.1 Linear Technology, LTC1731-4.1 Datasheet - Page 5

• Manufacturer Part Number: LTC1731-4.1
• Description: Lithium-Ion Linear Battery Charger Controller
• Manufacturer: Linear Technology

OPERATIO
APPLICATIONS
Charge Termination
The charger is off when any of the following conditions
exist: the voltage at the V
at the V
above V
be pulled to V
nected to reduce the current drain on the battery.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until V
rises above 4.1V. To prevent oscillation around
V
Trickle Charge and Defective Battery Detection
At the beginning of the charging sequence, if the battery
voltage is low (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.
When the battery approaches the final float voltage, the
charge current will begin to decrease. When the current
drops to 10% of the full-scale charge current, an internal
comparator will turn off the pull-down N-channel MOSFET
at the CHRG pin and connect a weak current source to
ground to indicate an end-of-charge (C/10) condition.
An external capacitor on the TIMER pin sets the total
charge time. After a time-out occurs, the charging will be
terminated immediately and the CHRG pin is forced to a
high impedance state. To restart the charge cycle, simply
remove the input voltage and reapply it, or float the PROG
pin momentarily.
CC
= 4.1V, the UVLO circuit has built-in hysteresis.
CC
BAT
pin is higher than 4.1V but is less than 54mV
, or the PROG pin is floating. The DRV pin will
CC
and the internal resistor divider is discon-
U
U
INFORMATION
CC
U
pin is below 4.1V, the voltage
W
U
CC
For batteries like lithium-ion that require accurate final
float potential, the internal 2.457V reference, voltage
amplifier and the resistor divider provide regulation with
LTC1731 can be turned into a current source by pulling
the TIMER pin to V
mode, the voltage amplifier, timer and the trickle charge
function are all disabled.
When the input voltage is not present, the charger goes
into a sleep mode, dropping I
reduces the current drain on the battery and increases the
standby time. The charger can be shut down by floating the
PROG pin. An internal current source will pull it high and
clamp at 3.5V.
Shutdown
The LTC1731 can be forced into shutdown by floating the
PROG pin thus allowing the internal 2.5 A current source
to pull the pin above the 2.457V shutdown threshold
voltage. The DRV pin will then be pulled up to V
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:
where R
ground.
For example, if 0.5A charge current is needed, select a
value for R
charge current. R
For best stability over temperature and time, 1% resistors
are recommended. The closest 1% resistor value is 19.6k.
1% (max) accuracy. For NiMH and NiCd batteries, the
I
R
BAT
PROG
= (I
= (2.457V/R
PROG
= (2.457V/500mA)(800 /0.2 ) = 19.656k
PROG
SENSE
is the total resistance from the PROG pin to
)(800 /R
SENSE
that will drop 100mV at the maximum
CC
PROG
. When in the constant-current only
= 0.1V/0.5A = 0.2 , then calculate:
)(800 /R
SENSE
)
CC
LTC1731-4.1
SENSE
to 7 A. This greatly
)
CC
and
5