Datasheet

13
LT1510/LT1510-5
APPLICATIONS INFORMATION
WUU
U
Example: V
IN
= 15V, V
BAT
= 8.4V, I
BAT
= 1.2A;
PmAmA
mA W
PW
BIAS
DRIVER
=
()()
+
()
+
()
+
()()
[]
=
=
()()
+






()
=
35 15 15 84
84
15
75 0012 12 017
12 84 1
84
30
55 15
013
2
2
...
.
....
..
.
.
P
kHz
W
PW
SW
SENSE
=
()( )()
+




()( )( )
=+=
=
()()
=
−
12 035 84
15
10 10 15 1 2 200
028 004 032
018 12 026
2
9
2
...
•.
...
.. .
Total power in the IC is:
0.17 + 0.13 + 0.32+ 0.26 = 0.88W
Temperature rise will be (0.88W)(50°C/W) = 44°C. This
assumes that the LT1510 is properly heat sunk by con-
necting the four fused ground pins to the expanded traces
and that the PC board has a backside or internal plane for
heat spreading.
The P
DRIVER
term can be reduced by connecting the boost
diode D2 (see Figures 2 and 6 circuits) to a lower system
voltage (lower than V
BAT
) instead of V
BAT
(see Figure 8).
Then,
P
IVV
V
V
DRIVER
BAT BAT X
X
IN
=
()( )()
+






()
1
30
55
For example, V
X
= 3.3V,
P
AVV
V
V
W
DRIVER
=
()()()
+






()
=
12 84 33 1
33
30
55 15
0 045
...
.
.
The average I
VX
required is:
P
V
W
V
mA
DRIVER
X
==
0 045
33
14
.
.
Total board area becomes an important factor when the
area of the board drops below about 20 square inches. The
graph in Figure 9 shows thermal resistance vs board area
for 2-layer and 4-layer boards. Note that 4-layer boards
have significantly lower thermal resistance, but both types
show a rapid increase for reduced board areas. Figure 10
shows actual measured lead temperature for chargers
operating at full current. 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
BOARD AREA (IN
2
)
0

60
55
50
45
40
35
30
25
15 25
1510 F08
510
20 30 35
THERMAL RESISTANCE (°C/W)
MEASURED FROM AIR AMBIENT
TO DIE USING COPPER LANDS
AS SHOWN ON DATA SHEET
2-LAYER BOARD
4-LAYER BOARD
Figure 9. LT1510 Thermal Resistance
BOOST
SW
SENSE
V
X
I
VX
1510 F07
LT1510
C1
L1
D2
10µF
+
Figure 8