Datasheet

LTC4071
10
4071fc
applicaTions inForMaTion
adapter voltage (V
WALL
) is 12V and the maximum charge
current is calculated as:
I
MAX _CHARGE
=
V
WALL
− V
BAT _ MIN
(
)
R
IN
=
12V − 3.2V
(
)
162
Ω
= 54mA
Figure 3. 2-Cell Battery Charger
Care must be taken in selecting the input resistor. Power
dissipated in R
IN
under full charge current is given by the
following equation:
P
DISS
=
V
WALL
− V
BAT _ MIN
(
)
2
R
IN
=
12V − 3.2V
(
)
2
162Ω
= 0.48W
The charge current decreases as the battery voltage
increases. If the battery voltage is 40mV less than the
programmed float voltage the LTC4071 consumes only
550nA of current, and all of the excess input current flows
into the battery. As the battery voltage reaches the float
voltage, the LTC4071 shunts current from the wall adapter
and regulates the battery voltage to V
FLOAT
= V
CC
. The
more shunt current the LTC4071 sinks, the less charge
current the battery gets. Eventually, the LTC4071 shunts
all the current flowing through R
IN
; up to the maximum
shunt current. The maximum shunt current in this case,
with no NTC adjustment is determined by the input resistor
and is calculated as:
I
SHUNT _MAX
=
V
WALL
− V
FLOAT
(
)
R
IN
=
12V − 4.1V
(
)
162Ω
= 49mA
At this point the power dissipated in the input resistor is
388mW.
The LTC4071 can also be used to regulate series-connected
battery stacks as illustrated in Figure 3. Here two LTC4071
devices are used to charge two batteries in series. A
single resistor sets the maximum charge/shunt current.
LTC4071
BAT
R
IN
GND
Li-Ion
BATTERY
V
CC
+
1µF
4071 F03
LTC4071
BAT
GND
Li-Ion
BATTERY
V
CC
+
1µF
WALL
ADAPTER
General Charging Considerations
The LTC4071 uses a different charging methodology from
previous chargers. Most Li-Ion chargers terminate the
charging after a period of time. The LTC4071 does not have
a discrete charge termination. Extensive measurements
on Li-Ion cells show that the cell charge current drops
to very low levels with the shunt charge control circuit
effectively terminating the charge. For improved battery
lifetime choose 4.0V or 4.1V float voltage.
The battery disconnect function requires some care in se-
lecting the input supply compliance for charging a battery
while powering a load at V
CC
. The internal battery discon-
nect switch remains off while charging the battery through
the body diode of the internal switch until V
CC
exceeds
V
LBC_VCC
. If the source voltage compliance is not greater
than V
LBC_VCC
, then the battery will never re-connect to
V
CC
and the system load will not be able to run on battery
power. Users may detect that the battery is connected by
monitoring the NTCBIAS pin as it will periodically pulse
high once V
CC
has risen above V
LBC_VCC
, and stops pulsing
once V
CC
falls below V
LBD
.
The simplest application of the LTC4071 is shown in Fig-
ure 2. This application requires only an external resistor
to program the charge/shunt current. Assume the wall
Figure 2. Single-Cell Battery Charger
4071 F02
LTC4071
BAT
R
IN
= 162Ω, 0.5W
GND
Li-Ion
BATTERY
V
CC
+
1µF
WALL
ADAPTER