Datasheet

LTC4070
8
4070fc
operaTion
For example, if the NTC thermistor requires the float voltage
to be dropped by 100mV (ADJ = V
CC
and 0.29• V
NTCBIAS
< V
NTC
< 0.36 • V
NTCBIAS
) then the HBO rising threshold
is detected when V
CC
rises past V
FLOAT
– DV
FLOAT(NTC)
–
V
HBTH
= 4.2V – 100mV – 40mV = 4.06V. The HBO falling
threshold in this case is detected when V
CC
falls below
V
FLOAT
– DV
FLOAT(NTC)
– V
HBTH
– V
HBHY
= 4.2V – 100mV
– 40mV – 100mV = 3.96V.
Low Battery Status Output: LBO
When the battery voltage drops below 3.2V, the LBO pin
pulls high. Otherwise, the LBO pin pulls low when the
battery voltage exceeds about 3.5V.
While the low battery condition persists, NTC and ADJ pins
are no longer sampled—the functions are disabled—and
total supply consumption for the LTC4070 drops to less
than 300nA (typ).
General Charging Considerations
The LTC4070 uses a different charging methodology from
previous chargers. Most Li-Ion chargers terminate the
charging after a period of time. The LTC4070 does not have
a discrete charge termination. Extensive measurements
on Li-Ion cells show that the cell charge current drops to
nanoamps with the shunt charge control circuit effectively
terminating the charge. For long cell life, operate the charger
at 100mV lower charge voltage normally used.
The simplest application of the LTC4070 is shown in
Figure 1. This application requires only an external resis-
tor to program the charge/shunt current. Assume the wall
adapter voltage (V
WALL
) is 12V and the minimum battery
voltage (V
BAT_MIN
) is 3V, then the maximum charge cur-
rent is calculated as:
I
MAX _ CHARGE
=
V
WALL
− V
BAT _ MIN
(
)
R
IN
=
12V − 3V
(
)
162
Ω
= 55.5mA
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 − 3V
(
)
2
162Ω
= 0.5W
The charge current decreases as the battery voltage
increases. If the rising battery voltage is 40mV less than
the programmed float voltage, the LTC4070 consumes
only 450nA of current, and all of the input current flows
into the battery. As the battery voltage reaches the
float voltage, the LTC4070 shunts current from the wall
adapter and regulates the battery voltage to V
FLOAT
. The
more shunt current the LTC4070 sinks, the less charge
current the battery gets. Eventually, the LTC4070 shunts
all the current from the battery; 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.
Figure 1. Single-Cell Battery Charger
4070 F01
LTC4070
ADJ
R
IN
162Ω
0.5W
GND
12V WALL
ADAPTER
Li-Ion
BATTERY
NTCBIAS
FLOAT
IF NOT NEEDED
FLOAT
NTC
V
CC
+
applicaTions inForMaTion