Datasheet

LMH6321
www.ti.com SNOSAL8C –APRIL 2006–REVISED MARCH 2013
Determining Copper Area
One can determine the required copper area by following a few basic 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 when specifying this parameter, it
must be kept in mind that, because of internal temperature rise due to power dissipation, 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)
, which is the temperature of the chip at maximum
operating current. Although no strict rules exist, typically one should design for a maximum continuous
junction temperature of 100°C to 130°C, but no higher than 150°C which is the absolute maximum rating for
the part.
4. Calculate the value of junction to ambient thermal resistance, θ
JA
5. Choose a copper area that will ensure the specified T
J(MAX)
for the calculated θ
JA
. θ
JA
as a function of copper
area in square inches is shown in Figure 54.
The maximum value of thermal resistance, junction to ambient θ
JA
, is defined as:
θ
JA
= (T
J(MAX)
- T
A(MAX)
)/ P
D(MAX)
where
• T
J(MAX)
= the maximum recommended junction temperature
• T
A(MAX)
= the maximum ambient temperature in the user’s environment
• P
D(MAX)
= the maximum recommended power dissipation (6)
NOTE
The allowable thermal resistance is determined by the maximum allowable heat rise ,
T
RISE
= T
J(MAX)
- T
A(MAX)
= (θ
JA
) (P
D(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 buffer, 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
D(MAX)
= I
S
(2V
+
) + V
+2
/4R
L
where
• I
S
= quiescent supply current (7)
2. Determine the maximum allowable die temperature rise,
T
R(MAX)
= T
J(MAX)
-T
A(MAX)
= P
D(MAX)
θ
JA
(8)
3. Using the calculated value of T
R(MAX)
and P
D(MAX)
the required value for junction to ambient thermal
resistance can be found:
θ
JA
= T
R(MAX)
/P
D(MAX)
(9)
4. Finally, using this value for θ
JA
choose the minimum value of copper area from Figure 54.
Example
Assume the following conditions:
V
+
= V
−
= 15V, R
L
= 50Ω, I
S
= 15 mA T
J(MAX)
= 125°C, T
A(MAX)
= 85°C.
1. From Equation 7
– P
D(MAX)
= I
S
(2V
+
) + V
+2
/4R
L
= (15 mA)(30V) + 15V
2
/200Ω = 1.58W
2. From Equation 8
– T
R(MAX)
= 125°C - 85°C = 40°C
3. From Equation 9
– θ
JA
= 40°C/1.58W = 25.3°C/W
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