lm4863mtx National Semiconductor Corporation, lm4863mtx Datasheet - Page 11

• Manufacturer Part Number: lm4863mtx
• Description: Dual 2.2w Audio Amplifier Plus Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

External Components Description
Application Information
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4863’s exposed-DAP (die attach paddle) packages
(MTE and LQ) provide a low thermal resistance between the
die and the PCB to which the part is mounted and soldered.
This allows rapid heat transfer from the die to the surround-
ing PCB copper traces, ground plane and, finally, surround-
ing air. The result is a low voltage audio power amplifier that
produces 2.2W at ≤ 1% THD with a 4Ω load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4863’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MTE and LQ packages must have their DAPs soldered
to a copper pad on the PCB. The DAP’s PCB copper pad is
connected to a large plane of continuous unbroken copper.
This plane forms a thermal mass and heat sink and radiation
area. Place the heat sink area on either outside plane in the
case of a two-sided PCB, or on an inner layer of a board with
more than two layers. Connect the DAP copper pad to the
inner layer or backside copper heat sink area with 32(4x8)
(MTE) or 6(3x2) (LQ) vias. The via diameter should be
0.012in - 0.013in with a 1.27mm pitch. Ensure efficient ther-
mal conductivity by plating-through and solder-filling the
vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
necessary for 5V operation with a 4Ω load. Heatsink areas
not placed on the same PCB layer as the LM4863 should be
5in
The last two area recommendations apply for 25˚c ambient
temperature. Increase the area to compensate for ambient
temperatures above 25˚c. In systems using cooling fans, the
LM4863MTE can take advantage of forced air cooling. With
an air flow rate of 450 linear-feet per minute and a 2.5in
exposed copper or 5.0in
the LM4863MTE can continuously drive a 3Ω load to full
power. The LM4863LQ achieves the same output power
Components
(Refer to Figure 1.)
1.
2.
3.
4.
5.
2
(min) for the same supply voltage and load resistance.
C
C
R
R
C
B
s
i
i
f
The Inverting input resistance, along with R
filter with f
The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. C
highpass filter with f
COMPONENTS, for an explanation of determining the value of C
The feedback resistance, along with R
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about
properly placing, and selecting the value of, this capacitor.
The capacitor, C
PROPER EXTERNAL COMPONENTS section for information concerning proper placement and selecting
C
B
’s value.
2
inner layer copper plane heatsink,
c
= 1/(2πR
B
, filters the half-supply voltage present on the BYPASS pin. Refer to the SELECTING
c
i
C
= 1/(2πR
i
).
2
(min) area is
i
C
i
). Refer to the section, SELECTING PROPER EXTERNAL
i
, set the closed-loop gain.
2
Functional Description
11
f
, set the closed-loop gain. R
level without forced air cooling. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4863’s thermal shutdown
protection. The LM4863’s power de-rating curve in the Typi-
cal Performance Characteristics shows the maximum
power dissipation versus temperature. Example PCB layouts
for the exposed-DAP TSSOP and LLP packages are shown
in the Demonstration Board Layout section. Further de-
tailed and specific information concerning PCB layout, fabri-
cation, and mounting an LLP package is available from
National Semiconductor’s package Engineering Group.
When contacting them, ask for "Preliminary Application Note
for the Assembly of the LLP Package on a Printed Circuit
Board, Revision A dated 7/14/00."
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
i
.
i
, along with C
i
, along with R
i
, form a high pass
www.national.com
i
, create a