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參數(shù)資料
| 型號： | LM4912 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Stereo 40mW Low Noise Headphone Amplifier |
| 中文描述： | 立體聲耳機(jī)40mW的低噪聲放大器 |
| 文件頁數(shù)： | 13/14頁 |
| 文件大小： | 699K |
| 代理商： | LM4912 |
Application Information
(Continued)
MUTE
When in C-CUPL mode, the LM4912 also features a mute
function that is independent of load impedance and enables
extremely fast turn-on/turn-off with a minimum of output pop
and click. The mute function leaves the outputs at their bias
level, thus resulting in higher power consumption than shut-
down mode, but also provides much faster turn on/off times.
Mute mode is enabled by providing a logic high signal on the
MUTE pin in the opposite manner as the shutdown function
described above. Threshold voltages and activation tech-
niques match those given for the shutdown function as well.
Additionally, Mute should not be enabled during shutdown or
while entering or returning from shutdown. This is not a valid
operation condition and may result in much higher pop and
click values.
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 LM4912 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4912 is unity-gain stable which gives the designer
maximum system flexibility. The LM4912 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
rms
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 Figures 2 and 3. The input coupling capacitor, C
i
,
forms a first order high pass filter which limits low frequency
response. 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
. 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
i
. 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 LM4912 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
(3)
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 LM4912 GBWP of 10MHz.
This figure displays that is a designer has a need to design
an amplifier with higher differential gain, the LM4912 can still
be used without running into bandwidth limitations.
L
www.national.com
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM49120 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Audio Sub-System with Mono Class AB Loudspeaker Amplifier and Stereo OCL/SE Headphone Amplifier |
| LM49120TL | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Audio Sub-System with Mono Class AB Loudspeaker Amplifier and Stereo OCL/SE Headphone Amplifier |
| LM49120TL/NOPB | 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 |
| LM49120TLX | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Audio Sub-System with Mono Class AB Loudspeaker Amplifier and Stereo OCL/SE Headphone Amplifier |
| LM49120TLX/NOPB | 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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