11
LT1511
Lithium-Ion Charging
The 3A Lithium Battery Charger (Figure 1) charges lithium-
ion batteries at a constant 3A until battery voltage reaches
a limit set by R3 and R4. The charger will then automati-
cally go into a constant-voltage mode with current de-
creasing to zero over time as the battery reaches full
charge. This is the normal regimen for lithium-ion charg-
ing, with the charger holding the battery at “float” voltage
indefinitely. In this case no external sensing of full charge
is needed.
Battery Voltage Sense Resistors Selection
To minimize battery drain when the charger is off, current
through the R3/R4 divider is set at 15µA. The input current
to the OVP pin is 3nA and the error can be neglected.
With divider current set at 15µA, R4 = 2.465/15µA = 162k
and,
R3
R4 V 2.465
2.465
162k 8.4 2.465
2.465
390k
BAT
=
()
−
()
=
−
()
=
Li-Ion batteries typically require float voltage accuracy of
1% to 2%. Accuracy of the LT1511 OVP voltage is ±0.5%
at 25°C and ±1% over full temperature. This leads to the
possibility that very accurate (0.1%) resistors might be
needed for R3 and R4. Actually, the temperature of the
LT1511 will rarely exceed 50°C in float mode because
charging currents have tapered off to a low level, so 0.25%
resistors will normally provide the required level of overall
accuracy.
APPLICATIONS INFORMATION
WUU
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When power is on, there is about 200µA of current flowing
out of the BAT and Sense pins. If the battery is removed
during charging, and total load including R3 and R4 is less
than the 200µA, V
BAT
could float up to V
CC
even though the
loop has turned switching off. To keep V
BAT
regulated to
the battery voltage in this condition, R3 and R4 can be
chosen to draw 0.5mA and Q3 can be added to disconnect
them when power is off (Figure 4). R5 isolates the OVP pin
from any high frequency noise on V
IN
. An alternative way is
to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the V
BAT
voltage.
Figure 3. PWM Current Programming
Some battery manufacturers recommend termination of
constant-voltage float mode after charging current has
dropped below a specified level (typically around 10% of
the full current)
and
a further time out period of 30 minutes
to 90 minutes has elapsed. This may extend the life of the
battery, so check with manufacturers for details. The
circuit in Figure 5 will detect when charging current has
dropped below 400mA. This logic signal is used to initiate
a timeout period, after which the LT1511 can be shut down
by pulling the V
C
pin low with an open collector or drain.
Some external means must be used to detect the need for
additional charging or the charger may be turned on
periodically to complete a short float-voltage cycle.
Current trip level is determined by the battery voltage, R1
through R3 and the sense resistor (R
S1
). D2 generates
hysteresis in the trip level to avoid multiple comparator
transitions.
PWM
R
PROG
4.7k
300Ω
PROG
C
PROG
1µF
Q1
VN2222
5V
0V
LT1511
1511 • F03
I
BAT
= (DC)(3A)
R3
12k
0.25%
R4
4.99k
0.25%
OVP
V
IN
+
–
+
–
4.2V
4.2V
V
BAT
Q3
VN2222
LT1511
LT1511 • F04
R5
220k
Figure 4. Disconnecting Voltage Divider
