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LME49610 Datasheet, PDF (13/18 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer
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area attains a nominal value of 20°C/W junction to ambient
thermal resistance (θJA) under zero air flow.
TA(MAX) = the maximum ambient temperature in the
LME49610’s environment
PD(MAX) = the maximum recommended power dissipation
Note: The allowable thermal resistance is determined by the
maximum allowable temperature increase:
TRISE = TJ(MAX) - TA(MAX)
Thus, if ambient temperature extremes force TRISE to exceed
the design maximum, the part must be de-rated by either de-
creasing PD to a safe level, reducing θJA further, or, if avail-
able, using a larger copper area.
Procedure
1. First determine the maximum power dissipated by the
LME49610, PD(MAX). For the simple case of the buffer driving
a resistive load, and assuming equal supplies, PD(MAX) is giv-
en by:
30042562
FIGURE 6. Thermal Resistance (typ) for 5 lead TO-263
Package Mounted on 1oz. copper
A copper plane may be placed directly beneath the tab. Ad-
ditionally, 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 Stan-
dard 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, PD.
2. Specify a maximum operating ambient temperature, TA
(MAX). Note that the die temperature, TJ, will be higher than
TA by an amount that is dependent on the thermal resistance
from junction to ambient, θJA. Therefore, TA must be specified
such that TJ does not exceed the absolute maximum die tem-
perature of 150°C.
3. Specify a maximum allowable junction temperature, TJ
(MAX), This is the LME49610’s die temperature when the buffer
is drawing maximum current (quiescent and load). It is pru-
dent to design for a maximum continuous junction tempera-
ture 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 resis-
tance, θJA.
5. θJA as a function of copper area in square inches is shown
in Figure 6. Choose a copper area that will guarantee the
specified TJ(MAX) for the calculated θJA. The maximum value
of junction to ambient thermal resistance, θJA, is defined as:
θJA = (TJ(MAX) - TA(MAX) ) / PD(MAX) (°C/W)
(1)
where:
TJ(MAX) = the maximum recommended junction temperature
PDMAX(AC) = (IS x VS) + (VS)2 / (2π2RL) (Watts)
(2)
PDMAX(DC) = (IS x VS) + (VS)2 / RL (Watts)
(3)
where:
VS = |VEE| + VCC (V)
IS = quiescent supply current (A)
Equation (2) is for sinusoidal output voltages and (3) is for DC
output voltages
2. Determine the maximum allowable die temperature rise,
TRISE(MAX) = TJ(MAX) - TA(MAX) °C
(4)
3. Using the calculated value of TRISE(MAX) and PD(MAX), find
the required value of junction to ambient thermal resistance
combining equation 1 and equation 4 to derive equation 5:
θJA = TRISE(MAX) / PD(MAX) (°C/W)
(5)
4. Finally, choose the minimum value of copper area from
Figure 6 based on the value for θJA.
Example
Assume the following conditions: VS = |VEE| + VCC = 30V,
RL = 32Ω, IS = 15mA, sinusoidal output voltage, TJ(MAX) = 125°
C, TA(MAX) = 85°C.
Applying Equation (2):
PDMAX = (IS x VS) + (VS)2 / 2π2RL
= (15mA)(30V) + 900V2 / 632Ω
= 1.87W
Applying Equation (4):
TRISE(MAX) = 125°C – 85°C
= 40°C
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