lm4838mtx National Semiconductor Corporation, lm4838mtx Datasheet - Page 16

• Manufacturer Part Number: lm4838mtx
• Description: Stereo 2w Audio Power Amplifiers With Dc Volume Control And Selectable Gain
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4838’s exposed-DAP (die attach paddle) packages
(MTE, 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 surrounding
PCB copper traces, ground plane and, finally, surrounding
air. The result is a low voltage audio power amplifier that
produces 2.1W 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 LM4838’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MTE and LQ packages must have their exposed DAPs
soldered to a grounded copper pad on the PCB. The DAP’s
PCB copper pad is connected to a large grounded plane of
continuous unbroken copper. This plane forms a thermal
mass 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 thermal 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 LM4838 MTE and
LQ packages should be 5in
voltage and load resistance. The last two area recommen-
dations apply for 25˚C ambient temperature. Increase the
area to compensate for ambient temperatures above 25˚C.
In systems using cooling fans, the LM4838MTE can take
advantage of forced air cooling. With an air flow rate of 450
linear-feet per minute and a 2.5in
inner layer copper plane heatsink, the LM4838MTE can
continuously drive a 3Ω load to full power. The LM4838LQ
achieves the same output power level without forced air
cooling. In all circumstances and conditions, the junction
temperature must be held below 150˚C to prevent activating
the LM4838’s thermal shutdown protection. The LM4838’s
power de-rating curve in the Typical Performance Charac-
teristics shows the maximum power dissipation versus tem-
perature. Example PCB layouts for the exposed-DAP
TSSOP and LQ packages are shown in the Demonstration
Board Layout section. Further detailed and specific infor-
mation concerning PCB layout, fabrication, and mounting an
LQ (LLP) package is available in National Semiconductor’s
AN1187.
The micro SMD and GR packages (LM4838ITL and
LM4838GR) thermals work in a similar way to the LQ and
MTE packages in that a thermal plane increases the heat
transfer from the die. The thermal plane can be any electrical
potential but needs to be below the package to aid in the
spreading the heat from the die out to surrounding PCB
areas to reduce the thermal resistance of the micro SMD
package. The thermal plane is most effective when placed
on the top or first internal PCB layers. The traces connecting
the bumps also contribute to spreading heat away from the
die. The same recommendations for the size of the thermal
2
(min) for the same supply
2
exposed copper or 5.0in
2
(min) area is
2
16
plane as given above apply for the ITL and GR packages,
namely 2.5in
minimum for internal or bottom layers.
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.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4838 output stage consists of
two pairs of operational amplifiers, forming a two-channel
(channel A and channel B) stereo amplifier. (Though the
following discusses channel A, it applies equally to channel
B.)
Figure 2 shows that the first amplifier’s negative (-) output
serves as the second amplifier’s input. This results in both
amplifiers producing signals identical in magnitude, but 180˚
out of phase. Taking advantage of this phase difference, a
load is placed between −OUTA and +OUTA and driven dif-
ferentially (commonly referred to as “bridge mode”). This
results in a differential gain of
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage
swing across the load. This produces four times the output
power when compared to a single-ended amplifier under the
same conditions. This increase in attainable output power
assumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single-supply
2
minimum for top layer thermal plane and 5in
A
VD
= 2 * (R
f
/R
i
)
(1)
2