LME49610 Datasheet, PDF (14/21 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer

LME49610
SNAS435B – APRIL 2008 – REVISED APRIL 2013
www.ti.com
If a heat conductive copper plane has perfect thermal conduction (heat spreading) through the plane’s total area,
the temperature rise is inversely proportional to the total exposed area. PCB copper planes are, in that sense, an
aid to convection. These planes, however, are not thick enough to ensure perfect heat conduction. Therefore,
eventually a point of diminishing returns is reached where increasing copper area offers no additional heat
conduction to the surrounding air. This is apparent in Figure 31. 2 oz copper boards will have decrease thermal
resistance providing a better heat sink compared to 1oz. copper. Beyond 1oz or 2oz copper plane areas, external
heatsinks are required. Ultimately, the 1oz copper area attains a nominal value of 20°C/W junction to ambient
thermal resistance (θ_JA) under zero air flow.
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0 1 2 3 4 5 6 7 8 9 10 1112 131415 16
COPPER HEAT SINK AREA (in²)
Figure 31. Thermal Resistance (Typ) for 5 Lead DDPAK/TO-263 Package Mounted on 1 Ounce of 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 LME49610, 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 maximum power dissipation of the LME49610, 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 LME49610’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. alculate 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 31. 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 LME49610’s environment
• P_D(MAX) = the maximum recommended power dissipation
• The allowable thermal resistance is determined by the maximum allowable temperature increase:
T_RISE = T_J(MAX) - T_A(MAX)
(2)
(2)
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.
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