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參數(shù)資料
| 型號： | LM4929MM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | Stereo 40mW Low Noise Headphone Amplifier with OCL Output |
| 中文描述： | 0.08 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO8 |
| 封裝： | MSOP-8 |
| 文件頁數(shù)： | 11/13頁 |
| 文件大?。?/td> | 899K |
| 代理商： | LM4929MM |
Application Information
(Continued)
The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a
system with a microprocessor or microcontroller, use a digi-
tal output to apply the control voltage to the SHUTDOWN
pin. Driving the SHUTDOWN pin with active circuitry elimi-
nates the pull-up resistor.
Shutdown enable/disable times are controlled by a combina-
tion of C
and V
. Larger values of C
results in longer turn
on/off times from Shutdown. Smaller V
values also in-
crease turn on/off time for a given value of C
. Longer
shutdown times also improve the LM4929’s resistance to
click and pop upon entering or returning from shutdown. For
a 2.4V supply and C
= 4.7μF, the LM4929 requires about 2
seconds to enter or return from shutdown. This longer shut-
down time enables the LM4929 to have virtually zero pop
and click transients upon entering or release from shutdown.
Smaller values of C
will decrease turn-on time, but at the
cost of increased pop and click and reduced PSRR. Since
shutdown enable/disable times increase dramatically as
supply voltage gets below 2.2V, this reduced turn-on time
may be desirable if extreme low supply voltage levels are
used as this would offset increases in turn-on time caused by
the lower supply voltage. This technique is not recom-
mended for OCL mode since shutdown enable/disable times
are very fast (0.5s) independent of supply voltage.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4929 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4929 is unity-gain stable which gives the designer
maximum system flexibility. The LM4929 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1V
are available
from sources such as audio codecs. Very large values
should not be used for the gain-setting resistors. Values for
R
and R
should be less than 1M
. Please refer to the
section,
Audio Power Amplifier Design
, for a more com-
plete explanation of proper gain selection
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in
Figure 2
. The input coupling capacitor, C
, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response and turn-on time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor, C
i
. A high value capacitor can
be expensive and may compromise space efficiency in por-
table designs. In many cases, however, the headphones
used in portable systems have little ability to reproduce
signals below 60Hz.Applications using headphones with this
limited frequency response reap little improvement by using
a high value input capacitor.
In addition to system cost and size, turn on time is affected
by the size of the input coupling capacitor C
. A larger input
coupling capacitor requires more charge to reach its quies-
cent DC voltage. This charge comes from the output via the
feedback Thus, by minimizing the capacitor size based on
necessary low frequency response, turn-on time can be
minimized. A small value of C
i
(in the range of 0.1μF to
0.39μF), is recommended.
AUDIO POWER AMPLIFIER DESIGN
A 25mW/32
AUDIO AMPLIFIER
Given:
Power Output
Load Impedance
Input Level
Input Impedance
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the
Typical Per-
formance Characteristics
section, the supply rail can be
easily found.
3V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4929 to reproduce peak in excess of 25mW
without producing audible distortion. At this time, the de-
signer must make sure that the power supply choice along
with the output impedance does not violate the conditions
explained in the
Power Dissipation
section.
Once the power dissipation equations have been addressed,
the required gain can be determined from Equation 2.
25mWrms
32
1Vrms
20k
(4)
From Equation 4, the minimum A
is 0.89; use A
= 1. Since
the desired input impedance is 20k
, and with a A
gain of
1, a ratio of 1:1 results from Equation 1 for R
f
to R
. The
values are chosen with R
= 20k
and R
= 20k
. The final
design step is to address the bandwidth requirements which
must be stated as a pair of -3dB frequency points. Five times
away from a -3dB point is 0.17dB down from passband
response which is better than the required
±
0.25dB speci-
fied.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz * 5 = 100kHz
As stated in the
External Components
section, R
i
in con-
junction with C
i
creates a
C
i
≥
1 / (2
π
* 20k
* 20Hz) = 0.397μF; use 0.39μF.
The high frequency pole is determined by the product of the
desired frequency pole, f
, and the differential gain, A
. With
an A
= 1 and f
= 100kHz, the resulting GBWP = 100kHz
which is much smaller than the LM4929 GBWP of 10MHz.
This figure displays that is a designer has a need to design
an amplifier with higher differential gain, the LM4929 can still
be used without running into bandwidth limitations.
Figure 3 shows an optional resistor connected between the
amplifier output that drives the headphone jack sleeve and
ground. This resistor provides a ground path that supressed
power supply hum. Thishum may occur in applications such
as notebook computers in a shutdown condition and con-
nected to an external powered speaker. The resistor’s 100
value is a suggested starting point. Its final value must be
L
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4929MM/NOPB | 功能描述:音頻放大器 Stereo 40mW Lo Noise Headphone Amp 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: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4929MMBD | 功能描述:BOARD EVALUATION LM4929MM RoHS:否 類別:編程器，開發(fā)系統(tǒng) >> 評估板 - 音頻放大器 系列:Boomer® 產(chǎn)品培訓模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標準包裝:1 系列:- |
| LM4929MMX/NOPB | 功能描述:音頻放大器 Stereo 40mW Lo Noise Headphone Amp 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: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM492-S | 功能描述:基本/快動開關(guān) Foot Switch RoHS:否 制造商:Omron Electronics 觸點形式:SPDT 執(zhí)行器:Lever 電流額定值:5 A 電壓額定值 AC:250 V 電壓額定值 DC:30 V 功率額定值: 工作力:120 g IP 等級:IP 67 NEMA 額定值: 端接類型:Wire 安裝:Panel |
| LM492-SCE | 功能描述:基本/快動開關(guān) Foot Switch RoHS:否 制造商:Omron Electronics 觸點形式:SPDT 執(zhí)行器:Lever 電流額定值:5 A 電壓額定值 AC:250 V 電壓額定值 DC:30 V 功率額定值: 工作力:120 g IP 等級:IP 67 NEMA 額定值: 端接類型:Wire 安裝:Panel |
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