Datasheet

LM4755
www.ti.com SNAS010E –FEBRUARY 1999–REVISED APRIL 2013
The choice of a heatsink for a given application is dictated by several factors: the maximum power the IC needs
to dissipate, the worst-case ambient temperature of the circuit, the junction-to-case thermal resistance, and the
maximum junction temperature of the IC. The heat flow approximation equation used in determining the correct
heatsink maximum thermal resistance is given below:
T
J
–T
A
= P
DMAX
• (θ
JC
+ θ
CS
+ θ
SA
)
where
• P
DMAX
= maximum power dissipation of the IC
• T
J
(°C) = junction temperature of the IC
• T
A
(°C) = ambient temperature
• θ
JC
(°C/W) = junction-to-case thermal resistance of the IC
• θ
CS
(°C/W) = case-to-heatsink thermal resistance (typically 0.2 to 0.5 °C/W)
• θ
SA
(°C/W) = thermal resistance of heatsink (2)
When determining the proper heatsink, the above equation should be re-written as:
θ
SA
≤ [(T
J
–T
A
) / P
DMAX
] - θ
JC
–θ
CS
(3)
DDPAK HEATSINKING
Surface mount applications will be limited by the thermal dissipation properties of printed circuit board area. The
DDPAK package is not recommended for surface mount applications with V
S
> 16V due to limited printed circuit
board area. There are DDPAK package enhancements, such as clip-on heatsinks and heatsinks with adhesives,
that can be used to improve performance.
Standard FR-4 single-sided copper clad will have an approximate Thermal resistance (θ
SA
) ranging from:
1.5 × 1.5 in. sq. 20–27°C/W (T
A
=28°C, Sine wave
testing, 1 oz. Copper)
2 × 2 in. sq. 16–23°C/W
The above values for θ
SA
vary widely due to dimensional proportions (i.e. variations in width and length will vary
θ
SA
).
For audio applications, where peak power levels are short in duration, this part will perform satisfactory with less
heatsinking/copper clad area. As with any high power design proper bench testing should be undertaken to
assure the design can dissipate the required power. Proper bench testing requires attention to worst case
ambient temperature and air flow. At high power dissipation levels the part will show a tendency to increase
saturation voltages, thus limiting the undistorted power levels.
DETERMINING MAXIMUM POWER DISSIPATION
For a single-ended class AB power amplifier, the theoretical maximum power dissipation point is a function of the
supply voltage, V
S
, and the load resistance, R
L
and is given by the following equation:
(single channel)
P
DMAX
(W) = [V
S
2
/ (2 • π
2
• R
L
)]
The above equation is for a single channel class-AB power amplifier. For dual amplifiers such as the LM4755,
the equation for calculating the total maximum power dissipated is:
(dual channel)
P
DMAX
(W) = 2 • [V
S
2
/ (2 • π
2
• R
L
)]
or
V
S
2
/ (π
2
• R
L
)
(Bridged Outputs)
P
DMAX
(W) = 4[V
S
2
/ (2π
2
• R
L
)]
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