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參數(shù)資料
| 型號(hào)： | LM4923 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select |
| 中文描述： | 1.1瓦特全差分音頻功率放大器，具有關(guān)斷選擇 |
| 文件頁數(shù)： | 11/12頁 |
| 文件大?。?/td> | 433K |
| 代理商： | LM4923 |
Application Information
(Continued)
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor. This scheme guarantees that the
shutdown pin will not float, thus preventing unwanted state
changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical when optimizing
device and system performance. Although the LM4923 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4923 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4923 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1Vrms are available
from sources such as audio codecs. Please refer to the
Audio Power Amplifier Design section for a more complete
explanation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LM4923
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V
DD
. Exact
allowable input common-mode voltage levels are actually a
function of V
DD
, R
i
, and R
f
and may be determined by
Equation 5:
V
CMi
<
(V
DD
-1.2)*((R
f
+(R
i
)/(R
f
)-V
DD
*(R
i
/ 2R
f
)
(5)
-R
F
/ R
I
= A
VD
(6)
Special care must be taken to match the values of the
feedback resistors (R
and R
F2
) to each other as well as
matching the input resistors (R
and R
) to each other (see
Figure 1) more in front. Because of the balanced nature of
differential amplifiers, resistor matching differences can re-
sult in net DC currents across the load. This DC current can
increase power consumption, internal IC power dissipation,
reduce PSRR, and possibly damaging the loudspeaker. The
chart below demonstrates this problem by showing the ef-
fects of differing values between the feedback resistors while
assuming that the input resistors are perfectly matched. The
results below apply to the application circuit shown in Figure
1, and assumes that V
= 5V, R
L
= 8
, and the system has
DC coupled inputs tied to ground.
Tolerance R
F1
20%
10%
5%
1%
0%
R
F2
1.2R
1.1R
V
02
- V
01
-0.500V
-0.250V
I
LOAD
62.5mA
31.25mA
15.63mA
3.125mA
0
0.8R
0.9R
0.95R 1.05R -0.125V
0.99R 1.01R -0.025V
R
R
0
Similar results would occur if the input resistors were not
carefully matched. Adding input coupling capacitors in be-
tween the signal source and the input resistors will eliminate
this problem, however, to achieve best performance with
minimum component count it is highly recommended that
both the feedback and input resistors matched to 1% toler-
ance or better.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8
Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
1Wrms
8
1Vrms
20k
100Hz–20kHz
±
0.25dB
A designer must first determine the minimum supply rail to
obtain the specified output power. The supply rail can easily
be found by extrapolating from the Output Power vs Supply
Voltage graphs in the
Typical Performance Characteris-
tics
section.Asecond way to determine the minimum supply
rail is to calculate the required V
OPEAK
using Equation 7 and
add the dropout voltages. Using this method, the minimum
supply voltage is (Vopeak + (V
DO TOP
+ (V
DO BOT
)), where
V
and V
are extrapolated from the Dropout
Voltage vs Supply Voltage curve in the
Typical Perfor-
mance Characteristics
section.
(7)
Using the Output Power vs Supply Voltage graph for an 8
load, the minimum supply rail just about 5V. Extra supply
voltage creates headroom that allows the LM4923 to repro-
duce peaks in excess of 1W without producing audible dis-
tortion. At this time, the designer must make sure that the
power supply choice along with the output impedance does
not violate the conditions explained in the
Power Dissipa-
tion
section. Once the power dissipation equations have
been addressed, the required differential gain can be deter-
mined from Equation 8.
(8)
R
f
/ R
i
= A
VD
From Equation 7, the minimum A
is 2.83. Since the de-
sired input impedance was 20k
, a ratio of 2.83:1 of R
to R
i
results in an allocation of R
= 20k
for both input resistors
and R
= 60k
for both feedback resistors. The final design
step is to address the bandwidth requirement which must be
stated as a single -3dB frequency point. Five times away
from a -3dB point is 0.17dB down from passband response
which is better than the required
±
0.25dB specified.
f
H
= 20kHz * 5 = 100kHz
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
.
With a A
= 2.83 and f
= 100kHz, the resulting GBWP =
150kHz which is much smaller than the LM4923 GBWP of
10MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4923 can still be used without running into bandwidth
limitations.
L
www.national.com
11
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| LM4923_07 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:1.1 Watt Fully Differential Audio Power Amplifier With Shutdown Select |
| LM4923LQ | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
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| LM4923LQBD/NOPB | 制造商:Texas Instruments 功能描述:Evaluation Kit For 1.1 Watt Fully Differential Audio Power Amplifier With Shut-Down Select |
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