Datasheet

0 2 4 6 8 10 12 14 16
COPPER HEAT SINK AREA (in
2
)
20
30
40
50
60
70
THERMAL IMPEDANCE (T
JA
)
1 3 5 7 9 11 13 15
LME49600
SNAS422E –JANUARY 2008–REVISED APRIL 2013 www.ti.com
Figure 30. Thermal Resistance for 5-lead TO–263 Package Mounted on 1oz. Copper
A copper plane may be placed directly beneath the tab. Additionally, a matching plane can be placed on the
opposite side. If a plane is placed on the side opposite of the LME49600, connect it to the plane to which the
buffer’s metal tab is soldered with a matrix of thermal vias per JEDEC Standard JESD51-5.
Determining Copper Area
Find the required copper heat sink area using the following guidelines:
1. Determine the value of the circuit’s power dissipation, P
D
.
2. Specify a maximum operating ambient temperature, T
A(MAX).
(Note that the die temperature, T
J
, will be higher
than T
A
by an amount that is dependent on the thermal resistance from junction to ambient, θ
JA
). Therefore, T
A
must be specified such that T
J
does not exceed the absolute maximum die temperature of 150°C.
3. Specify a maximum allowable junction temperature, T
J(MAX)
, This is the LME49600’s die temperature when the
buffer is drawing maximum current (quiescent and load). It is prudent to design for a maximum continuous
junction temperature of 100°C to 130°C. Ensure, however, that the junction temperature never exceeds the
150°C absolute maximum rating for the part.
4. Calculate the value of junction to ambient thermal resistance, θ
JA
5. θ
JA
as a function of copper area in square inches is shown in Figure 30. Choose a copper area that will ensure
the specified T
J(MAX)
for the calculated θ
JA
. The maximum value of junction to ambient thermal resistance, θ
JA
, is
defined as:
θ
JA
= (T
J(MAX)
- T
A(MAX)
)/ P
D(MAX)
(°C/W)
where
• T
J(MAX)
= the maximum recommended junction temperature
• T
A(MAX)
= the maximum ambient temperature in the LME49600’s environment
• P
D(MAX)
= the maximum recommended power dissipation (1)
NOTE
The allowable thermal resistance is determined by the maximum allowable temperature
increase:
T
RISE
= T
J(MAX)
- T
A(MAX)
Thus, if ambient temperature extremes force T
RISE
to exceed the design maximum, the part must be de-rated by
either decreasing P
D
to a safe level, reducing θ
JA
further or, if available, using a larger copper area.
Procedure
1. First determine the maximum power dissipated by the LME49600, P
D(MAX)
. For the simple case of the buffer
driving a resistive load, and assuming equal supplies, P
D(MAX)
is given by:
P
DMAX(AC)
= (I
S
x V
S
) + (V
S
)
2
/ (2π
2
R
L
) (Watts) (2)
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