7
LTC1731-4.1/LTC1731-4.2
OPERATIO
U
The LTC1731 is a linear battery charger controller for
single cell lithium-ion batteries. The charge current is
programmed by the combination of a program resistor
(R
PROG
) from the PROG pin to ground and a sense resistor
(R
SENSE
) between the V
CC
and SENSE pins. R
PROG
sets a
program current through an internal trimmed 800Ω resis-
tor setting up a voltage drop from V
CC
to the input of the
current amplifier (CA). The current amplifier servos the
gate of the external P-channel MOSFET to force the same
voltage drop across R
SENSE
which sets the charge current.
When the potential at the BAT pin approaches the preset
float voltage, the voltage amplifier (VA) will start sinking
current which shrinks the voltage drop across R
SENSE
,
thus reducing the charge current.
Charging begins when the potential at V
CC
pin rises above
the UVLO level and a program resistor is connected from
the PROG pin to ground. At the beginning of the charge
cycle, if the battery voltage is below 2.457V, the charger
goes into trickle charge mode. The trickle charge current
is 10% of the full-scale current. If the cell voltage stays low
for one quarter of the total charge time, the charge
sequence will be terminated.
The charger goes into the fast charge constant-current
mode after the voltage on the BAT pin rises above 2.457V.
In constant-current mode, the charge current is set by the
combination of R
SENSE
and R
PROG
.
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 and the CHRG pin is forced to a high imped-
ance state. To restart the charge cycle, simply remove the
input voltage and reapply it, or float the PROG pin
momentarily.
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
±1% (max) accuracy. For NiMH and NiCd batteries, the
LTC1731 can be turned into a current source by pulling
the TIMER pin to V
CC
. When in the constant-current only
mode, the voltage amplifier, timer and the trickle charge
function are all disabled.
The charger can be shut down by floating the PROG pin
(I
CC
= 1mA). An internal current source will pull it high and
clamp at 3.5V. When the input voltage is not present, the
charger goes into a sleep mode, dropping I
CC
to 7µA. This
greatly reduces the current drain on the battery and
increases the standby time.
APPLICATIONS INFORMATION
WUU
U
Charge Termination
The charger is off when any of the following conditions
exist: the voltage at the V
CC
pin is below 4.1V, the voltage
at the V
CC
pin is higher than 4.1V but is less than 54mV
above V
BAT
, or the PROG pin is floating. The DRV pin will
be pulled to V
CC
and the internal resistor divider is discon-
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
CC
rises above 4.1V. To prevent oscillation around
V
CC
= 4.1V, the UVLO circuit has built-in hysteresis.
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
charge mode. The charge current is set 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, charging will be terminated and the CHRG pin
output is forced to a high impedance state.
Shutdown
The LTC1731 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
