LT1769 Linear Technology, LT1769 Datasheet - Page 11

• Manufacturer Part Number: LT1769
• Description: Constant-Current/ Constant-Voltage 2A Battery Charger with Input Current Limiting
• Manufacturer: Linear Technology

Lithium-Ion Charging
The 2A Lithium-Ion Battery Charger (Figure 1) charges at
a constant 2A until battery voltage reaches a limit set by R3
and R4. The charger will then automatically go into a
constant-voltage mode with current decreasing to near
zero over time as the battery reaches full charge. This is the
normal regimen for lithium-ion charging, 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 15 A. The input current
to the OVP pin is 3nA and the error can be neglected.
With divider current set at 15 A, V
2.465/15 A = 162k and,
Li-Ion batteries typically require float voltage accuracy of
1% to 2%. Accuracy of the LT1769 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
LT1769 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
R3
390k
R4 V
I
Figure 3. PWM Current Programming
BAT
5V
0V
= (DC)(2A)
BAT
2.465
PWM
U
2.465
INFORMATION
LT1769
U
300
R
4.7k
Q1
VN2222
PROG
PROG
162k 8.4 2.465
W
BAT
2.465
C
1 F
1769 F03
PROG
= 8.4V, R4 =
U
When power is on, there is about 200 A 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 200 A, V
loop has turned switching off. To keep V
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
is to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the V
Some battery manufacturers recommend terminating the
constant-voltage float mode after charge current has
dropped below a specified level (typically around 10% of
the full current) and a further time out period of 30 to 90
minutes has elapsed. This may extend battery life, so
check with the manufacturer for details. The circuit in
Figure 5 will detect when charge current has dropped
below 270mA. This logic signal is used to initiate a timeout
period, after which the LT1769 can be shut down by
pulling the V
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
hysteresis in the trip level to avoid multiple comparator
transitions.
LT1769
Figure 4. Disconnecting Voltage Divider
C
BAT
OVP
pin low with an open collector or drain.
could float up to V
VN2222
Q3
R4
4.99k
0.25%
R3
12k
0.25%
220k
R5
IN
. An alternative method
CC
V
IN
S1
even though the
BAT
). D2 generates
+
1769 F04
LT1769
8.4V
regulated to
BAT
V
BAT
voltage.
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