LT1769CGN Linear Technology, LT1769CGN Datasheet - Page 11

• Manufacturer Part Number: LT1769CGN
• Description: IC BATT CHARGER CONST I/V 28SSOP
• Manufacturer: Linear Technology

Specifications of LT1769CGN

• Function: Charge Management
• Battery Type: Lead Acid, Li-Ion, NiCd, NiMH
• Voltage - Supply: 8 V ~ 25 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP (0.150", 3.95mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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%. 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 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
APPLICATIONS
R3
390k
R4 V
I
Figure 3. PWM Current Programming
BAT
5V
0V
BAT
= (DC)(2A)
BAT
2.465
PWM
could float up to V
U
2.465
INFORMATION
LT1769
U
300
R
4.7k
Q1
VN2222
PROG
PROG
162k 8.4 2.465
IN
. An alternative method
W
CC
BAT
2.465
even though the
BAT
C
1 F
1769 F03
PROG
= 8.4V, R4 =
regulated to
U
is to use a Zener diode with a breakdown voltage two or three
volts higher than battery voltage to clamp the V
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
an open collector or drain. Some external means must be
used to detect the need for additional charging or the
V
BAT
Figure 5. Current Comparator for Initiating Float Time Out
BAT
R1*
1.6k
R2
560k
R3
430k
LT1769
Figure 4. Disconnecting Voltage Divider
0.1 F
0.05
C1
OVP
1N4148
R
S1
D2
I
BAT
VN2222
3
2
200
200
R
R
Q3
–
+
S3
S2
LT1011
ADAPTER
OUTPUT
R4
4.99k
0.25%
R3
12k
0.25%
1
8
D1
1N4148
220k
4
R5
* TRIP CURRENT =
SENSE
BAT
LT1769
=
3.3V OR 5V
7
(560k + 430k)(0.05 )
V
(1.6k)(8.4V)
IN
R4
470k
+
NEGATIVE EDGE
TO TIMER
C
1769 F04
LT1769
8.4V
(R2 + R3)(R
pin low with
BAT
R1(V
V
BAT
voltage.
11
BAT
270mA
1769 F04
)
S1
1769fa
)