Datasheet

11
LT1769
1769fa
APPLICATIONS INFORMATION
WUU
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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, V
BAT
= 8.4V, 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%. 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.
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 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 method
Figure 4. Disconnecting Voltage Divider
PWM
R
PROG
4.7k
300Ω
PROG
C
PROG
1µF
Q1
VN2222
5V
0V
LT1769
1769 F03
I
BAT
= (DC)(2A)
Figure 3. PWM Current Programming
R3
12k
0.25%
R4
4.99k
0.25%
OVP
V
IN
+
8.4V
V
BAT
Q3
VN2222
LT1769
1769 F04
R5
220k
is to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the V
BAT
voltage.
Battery manufacturers recommend terminating the con-
stant-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
C
pin low with
an open collector or drain. Some external means must be
used to detect the need for additional charging or the
Figure 5. Current Comparator for Initiating Float Time Out
NEGATIVE EDGE
TO TIMER
1769 F04
3.3V OR 5V
ADAPTER
OUTPUT
3
8
7
1
4
2
D1
1N4148
C1
0.1µF
BAT
SENSE
R1*
1.6k
R
S1
0.05Ω
R4
470k
R3
430k
R2
560k
LT1011
D2
1N4148
* TRIP CURRENT =
= ≈ 270mA
R1(V
BAT
)
(R2 + R3)(R
S1
)
(1.6k)(8.4V)
(560k + 430k)(0.05Ω)
+
–
V
BAT
BAT
R
S3
200Ω
R
S2
200Ω
LT1769
I
BAT