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參數(shù)資料
| 型號： | LDC08A |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 3W Audio Power Amplifier with Shutdown Mode |
| 中文描述： | 3W音頻功率放大器關(guān)斷模式 |
| 文件頁數(shù)： | 11/15頁 |
| 文件大小： | 543K |
| 代理商： | LDC08A |
Application Information
(Continued)
LD (LLP) package is available from National Semiconduc-
tor’s Package Engineering Group under application note
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-
pendant 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 2.0W to
1.95W. This problem of decreased load dissipation is exac-
erbated as load impedance decreases. Therefore, to main-
tain 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 LM4991 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable; the second amplifier is internally fixed in a unity-gain,
inverting configuration. The closed-loop gain of the first am-
plifier is set by selecting the ratio of R
to R
while the second
amplifier’s gain is fixed.
Figure 1
shows that the output of
amplifier one serves as the input to amplifier two, which
results in both amplifiers producing signals identical in mag-
nitude, but 180 out of phase. Consequently, the differential
gain for the IC is
A
VD
= 2 *(R
f
/R
i
)
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the
Audio Power Amplifier
Design
section.
Another advantage of the differential bridge output is no net
DC voltage across load. This results from biasing V
1 and
V
2 at the same DC voltage, in this case V
/2 . 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. The
current flow created by the half-supply bias voltage in-
creases internal IC power dissipation and my permanently
damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.
P
DMAX
= 4*(V
DD
)
2
/(2
π
2
R
L
)
Since the LM4991 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended ampifier. Even with this substantial
increase in power dissipation, the LM4991 does not require
heatsinking under most operating conditions and output
loading. From Equation 1, assuming a 5V power supply and
an 8
load, the maximum power dissipation point is
625 mW. The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissi-
pation that results from Equation 2:
P
DMAX
= (T
JMAX
–T
A
)/
θ
JA
For the SO package,
θ
= 140C/W. For the LD package
soldered to a DAP pad that expands to a copper area of
1.0in
2
on a PCB, the LM4991’s
θ
is 56C/W. T
=
150C for the LM4991. The
θ
can be decreased by using
some form of heat sinking. The resultant
θ
will be the
summation of the
θ
,
θ
, and
θ
.
θ
is the junction to
case of the package (or to the exposed DAP, as is the case
with the LD package),
θ
CS
is the case to heat sink thermal
resistance and
θ
SA
is the heat sink to ambient thermal
resistance. By adding additional copper area around the
LM4991, the
θ
JA
can be reduced from its free air value for
the SO package. Increasing the copper area around the LD
package from 1.0in
2
to 2.0in
2
area results in a
θ
JA
decrease
to 46C/W. Depending on the ambient temperature, T
A
, and
the
θ
JA
, Equation 2 can be used to find the maximum internal
power dissipation supported by the IC packaging. If the
result of Equation 1 is greater than that of Equation 2, then
either the supply voltage must be decreased, the load im-
pedance increased, the
θ
decreased, or the ambient tem-
perature reduced. For the typical application of a 5V power
supply, with an 8
load, and no additional heatsinking, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 61C pro-
vided that device operation is around the maximum power
dissipation point and assuming surface mount packaging.
For the LD package in a typical application of a 5V power
supply, with a 4
load, and 1.0in
2
copper area soldered to
the exposed DAP pad, the maximum ambient temperature is
approximately 77C providing device operation is around the
maximum power dissipation point. Internal power dissipation
is a function of output power. If typical operation is not
around the maximum power dissipation point, the ambient
temperature can be increased. Refer to the
Typical Perfor-
mance Characteristics
curves for power dissipation infor-
mation for different output powers and output loading.
(1)
(2)
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the LM4991 as possible. The capacitor
L
www.national.com
11
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