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參數(shù)資料
| 型號： | LM4888SQ/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 3 W, 2 CHANNEL, AUDIO AMPLIFIER, QCC24 |
| 封裝： | LLP-24 |
| 文件頁數(shù)： | 6/23頁 |
| 文件大?。?/td> | 1181K |
| 代理商： | LM4888SQ/NOPB |
Application Information
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4888’s SQ exposed-DAP (die attach paddle) pack-
age 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 LM4888’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The SQ package must have its DAP 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 6 (3x2) SQ 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
2 (min) area is
necessary for 5V operation with a 4
load. Heatsink areas
not placed on the same PCB layer as the LM4888 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 all circumstances and condi-
tions, the junction temperature must be held below 150C to
prevent activating the LM4888’s thermal shutdown protec-
tion. The LM4888’s power de-rating curve in the Typical
Performance Characteristics shows the maximum power
dissipation versus temperature. Example PCB layouts for
the exposed-Dap SQ package is shown in the Demonstra-
tion Board Layout section. Further detailed and specific
information concerning PCB layout, fabrication, and mount-
ing an SQ package is available from 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 1, the LM4888 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. External feedback resistors R2
(or R3, R4) and R8 (or R6, R7) and input resistors R1 and
R9 set the closed-loop gain of Amp A (-out) and Amp B (-out)
whereas two internal 20k
resistors set Amp A’s (+out) and
Amp B’s (+out) gain at 1. The LM4888 drives a load, such as
a speaker, connected between the two amplifier outputs,
OUTA and +OUTA.
Figure 1 shows that Amp A’s (-out) output serves as Amp A’s
(+out) input. This results in both amplifiers producing signals
identical in magnitude, but 180 out of phase. Taking advan-
tage of this phase difference, a load is placed between
OUTA and +OUTA and driven differentially (commonly re-
ferred to as “bridge mode”). This results in a differential gain
of
A
VD =2*(Rf/R i)
(1)
or
A
VD =2*(R2/R1)
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 as-
sumes 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.
The LM4888 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and a
4
load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
P
DMAX =4*(VDD)
2/(2
π2R
L)
Bridge Mode
(3)
LM4888
www.national.com
14
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