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參數(shù)資料
| 型號： | LM4860MWA |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 1.15 W, 1 CHANNEL, AUDIO AMPLIFIER, UUC |
| 封裝： | WAFER |
| 文件頁數(shù)： | 11/12頁 |
| 文件大?。?/td> | 319K |
| 代理商： | LM4860MWA |
Application Information (Continued)
HIGHER GAIN AUDIO AMPLIFIER
The LM4860 is unity-gain stable and requires no external
components besides gain-setting resistors, an input coupling
capacitor, and proper supply bypassing in the typical appli-
cation. However if a closed-loop differential gain of greater
than 10 is required, then a feedback capacitor is needed, as
shown in
Figure 2, to bandwidth limit the amplifier. The feed-
back capacitor creates a low pass filter that eliminates un-
wanted high frequency oscillations. Care should be taken
when calculating the 3 dB frequency in that an incorrect
combination of R
f and Cf will cause rolloff before 20 kHz. A
typical combination of feedback resistor and capacitor that
will not produce audio band high frequency rolloff is R
f =
100 k
and C
f = 5 pF. These components result in a 3 dB
point of approximately 320 kHz. Once the differential gain of
the amplifier has been calculated, a choice of R
f will result,
and C
f can then be calculated from the formula stated in the
External Components Description section.
VOICE-BAND AUDIO AMPLIFIER
Many applications, such as telephony, only require a
voice-band frequency response. Such an application usually
requires a flat frequency response from 300 Hz to 3.5 kHz.
By adjusting the component values of
Figure 2, this common
application requirement can be implemented. The combina-
tion of R
i and Ci form a highpass filter while Rf and Cf form a
lowpass filter. Using the typical voice-band frequency range,
with a passband differential gain of approximately 100, the
following values of R
i,Ci,Rf, and Cf follow from the equa-
tions stated in the External Components Description sec-
tion.
R
i =10 k,Rf = 510k, Ci = 0.22 F, and Cf =15 pF
Five times away from a 3 dB point is 0.17 dB down from the
flatband response. With this selection of components, the re-
sulting 3 dB points, f
L and fH, are 72 Hz and 20 kHz, re-
spectively, resulting in a flatband frequency response of bet-
ter than ±0.25 dB with a rolloff of 6 dB/octave outside of the
passband. If a steeper rolloff is required, other common
bandpass filtering techniques can be used to achieve higher
order filters.
SINGLE-ENDED AUDIO AMPLIFIER
Although the typical application for the LM4860 is a bridged
monoaural amp, it can also be used to drive a load
single-endedly in applications, such as PC cards, which re-
quire that one side of the load is tied to ground.
Figure 3
shows a common single-ended application, where V
O1 is
used to drive the speaker. This output is coupled through a
470 F capacitor, which blocks the half-supply DC bias that
exists in all single-supply amplifier configurations. This ca-
pacitor, designated C
O in Figure 3, in conjunction with RL,
forms a highpass filter. The 3 dB point of this highpass filter
is 1/(2
πR
LCO), so care should be taken to make sure that the
product of R
L and CO is large enough to pass low frequen-
cies to the load. When driving an 8
load, and if a full audio
spectrum reproduction is required, C
O should be at least
470 F. V
O2,
the output that is not used, is connected
through a 0.1 F capacitor to a 2 k
load to prevent instabil-
ity. While such an instability will not affect the waveform of
V
O1, it is good design practice to load the second output.
AUDIO POWER AMPLIFIER DESIGN
Design a 500 mW/8
Audio Amplifier
Given:
Power Output:
500 mWrms
Load Impedance:
8
Input Level:
1 Vrms(max)
Input Impedance:
20 k
Bandwidth:
20 Hz-20 kHz ±0.25 dB
A designer must first determine the needed supply rail to ob-
tain the specified output power. Calculating the required sup-
ply rail involves knowing two parameters, V
opeak and also the
dropout voltage. The latter is typically 0.7V. V
opeak can be
determined from equation 3.
For 500 mW of output power into an 8
load, the required
V
opeak is 2.83V. A minimum supply rail of 3.53V results from
adding V
opeak and Vod. But 3.53V is not a standard voltage
that exists in many applications and for this reason, a supply
rail of 5V is designated. Extra supply voltage creates dy-
namic headroom that allows the LM4860 to reproduce peaks
in excess of 500 mW without clipping the signal. At this time,
the designer must make sure that the power supply choice
along with the output impedance does not violate the condi-
tions explained in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 4.
From equation 4, the minimum A
vd is:
A
vd =2
Since the desired input impedance was 20 k
, and with an
A
vd of 2, a ratio of 1:1 of Rf to Riresults in an allocation of
R
i =Rf =20 k. Since the Avd was less than 10, a feedback
capacitor is not needed. The final design step is to address
the bandwidth requirements which must be stated as a pair
of 3 dB frequency points. Five times away from a 3 dB
point is 0.17 dB down from passband response which is bet-
ter than the required ±0.25 dB specified. This fact results in
a low and high frequency pole of 4 Hz and 100 kHz respec-
tively. As stated in the External Components section, R
i in
conjunction with C
i create a highpass filter.
C
i ≥ 1/(2π * 20 k * 4 Hz) = 1.98 F;
use 2.2 F.
The high frequency pole is determined by the product of the
desired high frequency pole, f
H, and the differential gain, Avd.
With a A
vd = 2 and fH = 100 kHz, the resulting GBWP =
100 kHz which is much smaller than the LM4860 GBWP of
7 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4860 can still be used without running into bandwidth
problems.
LM4860
www.national.com
8
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4860MX | 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 |
| LM4860MX/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 |
| LM4861 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:1.1W Audio Power Amplifier with Shutdown Mode |
| LM4861_03 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:1.1W Audio Power Amplifier with Shutdown Mode |
| LM4861M | 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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