LT1769
U
W U U
APPLICATIONS INFORMATION
V
BAT
LT1769
+
R3
8.4V
12k
PROG
0.25%
300Ω
Q3
VN2222
V
IN
LT1769
OVP
R5
220k
R
C
PROG
PROG
4.7k
1µF
R4
Q1
VN2222
5V
0V
4.99k
0.25%
PWM
= (DC)(2A)
1769 F04
1769 F03
I
BAT
Figure 4. Disconnecting Voltage Divider
Figure 3. PWM Current Programming
holding the battery at “float” voltage indefinitely. In this
case no external sensing of full charge is needed.
is to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the VBAT 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
haselapsed.Thismayextendbatterylife,socheckwiththe
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
theLT1769canbeshutdownbypullingtheVC pinlowwith
an open collector or drain. Some external means must be
used to detect the need for additional charging or the
Battery Voltage Sense Resistors Selection
To minimize battery drain when the charger is off, current
throughtheR3/R4dividerissetat15µA. Theinputcurrent
to the OVP pin is 3nA and the error can be neglected.
With divider current set at 15µA, VBAT = 8.4V, R4 =
2.465/15µA = 162k and,
R4 V
)(
− 2.465 162k 8.4 − 2.465
(
)
(
)
BAT
R3 =
= 390k
=
2.465
2.465
I
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%resistorswillnormallyprovidetherequiredlevelof
overall accuracy.
R
S3
200Ω
SENSE
R
S1
LT1769
R
200Ω
0.05Ω
S2
BAT
V
BAT
ADAPTER
OUTPUT
3.3V OR 5V
D1
C1
0.1µF
R1*
1.6k
BAT
1N4148
R4
3
2
8
470k
–
7
NEGATIVE EDGE
TO TIMER
Whenpowerison, thereisabout200µAofcurrentflowing
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, VBAT could float up to VCC even though the
loop has turned switching off. To keep VBAT 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 VIN. An alternative method
LT1011
+
4
1
R2
560k
D2
1N4148
R1(V
)
R3
430k
BAT
* TRIP CURRENT =
(R2 + R3)(R
)
S1
(1.6k)(8.4V)
=
≈ 270mA
(560k + 430k)(0.05Ω)
1769 F04
Figure 5. Current Comparator for Initiating Float Time Out
1769fa
11
Linear Systems [ Linear Systems ]
