Datasheet

13
LT1511
APPLICATIONS INFORMATION
WUU
U
Figure 7. Lower V
BOOST
P
AVV
V
V
W
DRIVER
=
()()()
+






()
=
384331
33
30
55 15
011
..
.
.
The average I
VX
required is:
P
V
W
V
mA
DRIVER
X
==
011
33
34
.
.
Fused-lead packages conduct most of their heat out the
leads. This makes it very important to provide as much PC
board copper around the leads as is practical. Total
thermal resistance of the package-board combination is
dominated by the characteristics of the board in the
immediate area of the package. This means both lateral
thermal resistance across the board and vertical thermal
resistance through the board to other copper layers. Each
layer acts as a thermal heat spreader that increases the
heat sinking effectiveness of extended areas of the board.
Total board area becomes an important factor when the
area of the board drops below about 20 square inches. The
graph in Figure 8 shows thermal resistance vs board area
for 2-layer and 4-layer boards with continuous copper
planes. Note that 4-layer boards have significantly lower
thermal resistance, but both types show a rapid increase
for reduced board areas. Figure 9 shows actual measured
lead temperatures 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
For example, V
X
= 3.3V then:
P 3.5mA V 1.5mA V
V
V
7.5mA 0.012 I
P
IV
55 V
P
IRV
V
tVI f
BIAS IN BAT
BAT
2
IN
BAT
DRIVER
BAT BAT
2
IN
SW
BAT
2
SW BAT
IN
OL IN BAT
=
()()
+
()
+
()
+
()()
[]
=
()( )
+






()
=
()( )( )
+
()()( )()
1
30
V
BAT
R
SW
= Switch ON resistance ≈ 0.16Ω
t
OL
= Effective switch overlap time ≈ 10ns
f = 200kHz
Example: V
IN
= 15V, V
BAT
= 8.4V, I
BAT
= 3A;
P 3.5mA 15 1.5mA 8.4
8.4
15
7.5mA 0.012 3 0.27W
P
3 8.4
5515
0.33W
P
3 0.16 8.4
15
10 15 3 200kHz
0.81 0.09 0.9W
BIAS
2
DRIVER
2
SW
2
9
=
()()
+
()
+
()
+
()()
[]
=
=
()( )
+






()
=
=
()( )( )
+
()()( )
=+=
−
1
84
30
.
Total Power in the IC is: 0.27 + 0.33 + 0.9 = 1.5W
Temperature rise will be (1.5W)(30°C/W) = 45°C. This
assumes that the LT1511 is properly heat sunk by con-
necting the seven fused ground pins to 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 Figure 1) to a lower system voltage (lower
than V
BAT
) instead of V
BAT
.
Then P
DRIVER
=
()( )()
+






()
IVV
V
V
BAT BAT X
X
IN
1
30
55
SW
BOOST
SPIN
1511 • F07
LT1511
V
X
I
VX
C2
D2
10µF
L1
+