LM4755TS National Semiconductor, LM4755TS Datasheet - Page 17

• Manufacturer Part Number: LM4755TS
• Description: IC,Audio Amplifier,DUAL,BIPOLAR,SIP,9PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4755TS

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 11x2@4OhmW
• Audio Amplifier Function: Speaker
• Total Harmonic Distortion: 0.08@8Ohm@2W%
• Single Supply Voltage (typ): 12/15/18/24/28V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Power Dissipation: 62.5W
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 50dB
• Single Supply Voltage (min): 9V
• Single Supply Voltage (max): 32V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 9 +Tab
• Package Type: TO-263
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

The above values for θ
portions (i.e. variations in width and length will vary θ
For audio applications, where peak power levels are short in
duration, this part will perform satisfactory with less heatsink-
ing/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
following equation:
(single channel)
P
The above equation is for a single channel class-AB power
amplifier. For dual amplifiers such as the LM4755, the equa-
tion for calculating the total maximum power dissipated is:
(dual channel)
P
or
V
(Bridged Outputs)
P
HEATSINK DESIGN EXAMPLE
Determine the system parameters:
Device parameters from the datasheet:
Calculations:
2 • P
14.6W
θ
- 2°C/W–0.2°C/W = 4.3°C/W
Conclusion: Choose a heatsink with θ
TO-263 HEATSINK DESIGN EXAMPLES
Example 1:
Given:
P
SA
DMAX
DMAX
S
DMAX
DMAX
V
R
T
T
θ
2
JC
S
A
J
L
/ (
≤
= 150°C
= 24V
= 4Ω
= 55°C
DMAX
= 2°C/W
π
[(T
(W) = [V
(W) = 2 • [V
(W) = 4[V
from P
2
J
• R
-T
S
= 2 • [V
, and the load resistance, R
A
L
) / P
)
D
S
vs P
2
S
DMAX
2
/ (2 •
S
S
(Stereo Single-Ended Output)
T
T
R
V
θ
/ (2
Operating Supply Voltage
Minimum Load Impedance
Worst Case Ambient Temperature
Maximum Junction Temperature
Junction-to-Case Thermal Resistance
2
2
JC
O
A
J
S
L
/ 2 •
=150°C
=30°C
/ (2 •
=4Ω
=12V
Graph:
=2°C/W
] - θ
π
SA
P
π
2
DMAX
2
vary widely due to dimensional pro-
π
• R
JC
• R
π
2
–θ
2
L
• R
L
)]
• R
≈
CS
)]
L
3.7W
L
)] = (24V)
= [ (150°C - 55°C) / 14.6W]
)]
SA
L
≤
and is given by the
2
4.3°C/W.
/ (2 •
π
2
SA
• 4Ω) =
).
17
Calculating P
Calculating Heatsink Thermal Resistance:
Therefore the recommendation is to use 1.5 × 1.5 square inch
of single-sided copper clad.
Example 2:
Given:
P
Calculating P
Calculating Heatsink Thermal Resistance:
Therefore the recommendation is to use 2.0 × 2.0 square inch
of single-sided copper clad.
Example 3:
Given:
Calculating P
Calculating Heatsink Thermal Resistance:
Therefore the recommendation is to use 2.0 × 2.0 square inch
of single-sided copper clad.
LAYOUT AND GROUND RETURNS
Proper PC board layout is essential for good circuit perfor-
mance. When laying out a PC board for an audio power
amplifier, particular attention must be paid to the routing of the
output signal ground returns relative to the input signal and
bias capacitor grounds. To prevent any ground loops, the
ground returns for the output signals should be routed sepa-
rately and brought together at the supply ground. The input
signal grounds and the bias capacitor ground line should also
be routed separately. The 0.1 µF high frequency supply by-
pass capacitor should be placed as close as possible to the
IC.
DMAX
P
θ
θ
θ
DMAX
SA
SA
SA
P
P
from P
< 100°C / 3.7W − 2.0°C/W − 0.2°C/W = 24.8°C/W
DMAX
DMAX
< 120°C/3.7W − 2.0°C/W − 0.2°C/W = 30.2°C/W
< 100°C/3.7W − 2.0°C/W − 0.2°C/W = 24.8°C/W
= 4[V
θ
θ
= V
= V
DMAX
DMAX
SA
DMAX
SA
θ
D
SA
CC
vs P
CC
< [(T
< [(T
CC
2
< T
:
:
:
/(2
(Stereo Single-Ended Output)
T
T
R
V
θ
2
2
/(
JC
O
/(
A
J
S
L
=150°C
J
π
J
π
J
=50°C
=4Ω
=12V
π
Graph:
=2°C/W
− T
− T
− T
2
2
2
P
R
R
R
DMAX
L
L
L
A
A
A
)= (12V)
(Bridged Output)
T
T
R
V
θ
)] = 4(12V)
) = (12V)
) / P
) / P
/ P
JC
A
J
S
L
=150°C
=50°C
=8Ω
=12V
=2°C/W
≈
DMAX
DMAX
DMAX
3.7W
2
2
− θ
] − θ
] − θ
/(
/
π
2
π
/(2
JC
2
2
(4Ω)) = 3.65W
(4Ω)) = 3.65W
JC
JC
π
− θ
2
− θ
− θ
(8Ω)) = 3.65W
CS
CS
CS
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