參數(shù)資料
型號(hào)： LM4843MHX/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻控制
英文描述： 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, PDSO20
封裝： TSSOP-20
文件頁(yè)數(shù)： 3/19頁(yè)
文件大?。?/td> 597K
代理商： LM4843MHX/NOPB
Application Information
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4843’s exposed-DAP (die attach paddle) package
(MH) provides 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 4 load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4843’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The MH package must have its 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)
(MH)
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
2 (min) area is
necessary for 5V operation with a 4
load. Heatsink areas
not placed on the same PCB layer as the LM4843 MH
package should be 5in
2 (min) for the same supply voltage
and load resistance. The last two area recommendations
apply for 25C ambient temperature. Increase the area to
compensate for ambient temperatures above 25C. In sys-
tems using cooling fans, the LM4843MH can take advantage
of forced air cooling. With an air flow rate of 450 linear-feet
per minute and a 2.5in
2 exposed copper or 5.0in2 inner layer
copper plane heatsink, the LM4843MH can continuously
drive a 3
load to full power. The junction temperature must
be held below 150C to prevent activating the LM4843’s
thermal shutdown protection. The LM4843’s power de-rating
curve in the Typical Performance Characteristics shows
the maximum power dissipation versus temperature. Ex-
ample PCB layouts for the exposed-DAP TSSOP package
are shown in the Demonstration Board Layout section.
Further detailed and specific information concerning PCB
layout and fabrication is available in National Semiconduc-
tor’s AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3
AND 4 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, 0.1
trace resistance reduces
the output power dissipated by a 4
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 LM4843 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
A
VD =2 * (Rf/R i)
(1)
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
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load.
P
DMAX =(VDD)
2/(2
π2R
L)
Single-Ended
(2)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
LM4843
www.national.com
11
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