LMH6321 Datasheet, PDF (17/21 Pages) National Semiconductor (TI) – 300 mA High Speed Buffer with Adjustable Current Limit

Application Hints (Continued)
THERMAL MANAGEMENT
Heatsinking
For some applications, a heat sink may be required with the
LMH6321. This depends on the maximum power dissipation
and maximum ambient temperature of the application. To
accomplish heat sinking, the tabs on TO-263 and PSOP
package may be soldered to the copper plane of a PCB for
heatsinking (note that these tabs are electrically connected
to the most negative point in the circuit, i. e.,V^−).
Heat escapes from the device in all directions, mainly
through the mechanisms of convection to the air above it and
conduction to the circuit board below it and then from the
board to the air. Natural convection depends on the amount
of surface area that is in contact with the air. If a conductive
plate serving as a heatsink is thick enough to ensure perfect
thermal conduction (heat spreading) into the far recesses of
the plate, the temperature rise would be simply inversely
proportional to the total exposed area. PCB copper planes
are, in that sense, an aid to convection, the difference being
that they are not thick enough to ensure perfect conduction.
Therefore, eventually we will reach a point of diminishing
returns (as seen in Figure 5). Very large increases in the
copper area will produce smaller and smaller improvement
in thermal resistance. This occurs, roughly, for a 1 inch
square of 1 oz copper board. Some improvement continues
until about 3 square inches, especially for 2 oz boards and
better, but beyond that, external heatsinks are required.
Ultimately, a reasonable practical value attainable for the
junction to ambient thermal resistance is about 30 ˚C/W
under zero air flow.
A copper plane of appropriate size may be placed directly
beneath the tab or on the other side of the board. If the
conductive plane is placed on the back side of the PCB, it is
recommended that thermal vias be used per JEDEC Stan-
dard JESD51-5.
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 tempera-
ture rise due to power dissipation, the die temperature,
T_J, will be higher than T_A by an amount that is depen-
dent 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 resis-
tance, θ_JA
5. Choose a copper area that will guarantee the specified
T_J(MAX) for the calculated θ_JA. θ_JA as a function of cop-
per area in square inches is shown in Figure 4.
The maximum value of thermal resistance, junction to ambi-
ent θ_JA, is defined as:
θ_JA = (T_J(MAX) - T_A(MAX) )/ P_D(MAX)
(6)
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
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
(7)
where: I_S = quiescent supply current
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 4.
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 (7)
P_D(MAX) = I_S (2V⁺) + V⁺²/4R_L = (15 mA)(30V) +
225V²/200Ω = 1.58W
2. From (8)
T_R(MAX) = 125˚C - 85˚C = 40˚C
3. From (9)
θ_JA = 40˚C/1.58W = 25.3˚C/W
Examining the plot of Copper Area vs. θ_JA, we see that we
cannot attain this low of a thermal resistance for one layer of
1 oz copper. It will be necessary to derate the part by
decreasing either the ambient temperature or the power
dissipation. Other solutions are to use two layers of 1 oz foil,
or use 2 oz copper (see Table 1), or to provide forced air flow.
One should allow about an extra 15% heat sinking capability
for safety margin.
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