LTC1731-4.2 Linear Technology, LTC1731-4.2 Datasheet - Page 5

• Manufacturer Part Number: LTC1731-4.2
• Description: Lithium-Ion Linear Battery Charger Controller
• Manufacturer: Linear Technology

OPERATIO
APPLICATIONS
The LTC1731-4.2 is a linear battery charger controller. The
charge current is programmed by the combination of a
program resistor (R
and a sense resistor (R
pins. R
trimmed 800 resistor setting up a voltage drop from V
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
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
Charging begins when the potential at V
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 per cell, 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 immediately.
The charger goes into the fast charge constant-current
mode after the voltage on the BAT pin rises above 2.457V
per cell. In constant-current mode, the charge current is
set by the combination of R
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
CC
= 4.1V, the UVLO circuit has built-in hysteresis.
PROG
CC
BAT
pin is higher than 4.1V but is less than 54mV
, or the PROG pin is floating. The DRV pin will
sets a program current through an internal
CC
SENSE
and the internal resistor divider is discon-
U
U
, thus reducing the charge current.
PROG
SENSE
INFORMATION
CC
) from the PROG pin to ground
U
pin is below 4.1V, the voltage
SENSE
) between the V
and R
W
CC
SENSE
PROG
pin rises above
CC
.
and SENSE
which sets
U
CC
CC
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.
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
LTC1731-4.2 can be turned into a current source by
pulling the TIMER pin to V
current only 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.
Trickle Charge and Defective Battery Detection
At the beginning of the charging sequence, if the battery
voltage is low (below 2.457V per cell) 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.
Shutdown
The LTC1731-4.2 can be forced into shutdown by float-
ing the PROG pin and allowing the internal 2.5 A current
source to pull the pin above the 2.457V shutdown
1% (max) accuracy. For NiMH and NiCd batteries, the
CC
. When in the constant-
CC
LTC1731-4.2
to 5 A. This greatly
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