LT1769 Linear Technology, LT1769 Datasheet - Page 14

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

APPLICATIONS
LT1769
Battery voltage and input voltage will affect device power
dissipation, so the data sheet power calculations must be
used to extrapolate these readings to other situations.
Vias should be used to connect board layers together.
Planes under the charger area can be cut away from the
rest of the board and connected with vias to form both a
low thermal resistance system and to act as a ground
plane for reduced EMI.
Glue-on, chip-mounted heat sinks are effective only in
moderate power applications where the PC board copper
cannot be used, or where the board size is small. They
offer very little improvement in a properly laid out multi-
layer board of reasonable size.
14
Figure 8. LT1769 Thermal Resistance
45
40
35
30
25
20
15
10
70
60
50
40
30
20
Figure 9. LT1769 Lead Temperature
0
0
NOTE: PEAK DIE TEMPERATURE
WILL BE ABOUT 15 C HIGHER AT
2A CHARGE CURRENT
V
V
V
2-LAYER BOARD
ROOM TEMP = 24 C
MEASURED FROM AIR AMBIENT
TO DIE USING COPPER LANDS
AS SHOWN ON DATA SHEET
IN
BAT
BOOST
5
= 19V
= 12.3V
= 5V
0.5
10
CHARGE CURRENT (A)
U
BOARD AREA (IN
15
INFORMATION
2-LAYER BOARD
4-LAYER BOARD
U
1
20
5 IN
25 IN
2
BOARD
2
25
)
2
1.5
BOARD
W
30
1769 F09
1769 F08
35
2
U
0.47 F
STANDARD CONNECTION
C3
D2
V
Q1 = Si4435DY
Q2 = TP0610L
IN
Higher Duty Cycle for the LT1769 Battery Charger
Maximum duty cycle for the LT1769 is typically 90%, but
this may be too low for some applications. For example, if
an 18V 3% adapter is used to charge ten NiMH cells, the
charger must put out approximaly 15V. A total of 1.6V is
lost in the input diode, switch resistance, inductor resis-
tance and parasitics, so the required duty cycle is
15/16.4 = 91.4%. The duty cycle can be extended to 93%
by restricting boost voltage to 5V instead of using V
is normally done. This lower boost voltage also reduces
power dissipation in the LT1769, so it is a win-win deci-
sion. Connect an external source of 3V to 6V at V
Figure 10 with a 10 F C
SW
BOOST
SPIN
SENSE
LT1769
R
50k
X
Q1
Figure 11. Replacing the Input Diode
Figure 10. High Duty Cycle
+
BAT
HIGH DUTY CYCLE CONNECTION
D1
Q2
3V TO 6V
V
3V TO 6V
BAT
V
X
0.47 F
X
V
0.47 F
10 F
bypass capacitor.
X
HIGH DUTY CYCLE CONNECTION
10 F
C
C2
D2
X
C
C3
D2
X
+
SW
BOOST
SPIN
SENSE
SW
BOOST
SPIN
SENSE
LT1769
LT1769
V
CC
+
BAT
+
BAT
X
node in
BAT
1769 F11
V
1769 F10
BAT
V
as
BAT