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参数资料
| 型号： | LM3876DWF |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 56 W, 1 CHANNEL, AUDIO AMPLIFIER, UUC |
| 封装： | WAFER |
| 文件页数： | 7/22页 |
| 文件大小： | 626K |
| 代理商： | LM3876DWF |
Application Information (Continued)
ance which would allow the amplifier to provide the desired
output power, keeping in mind the current limiting capabili-
ties of the device. In any case, the output signal swing and
current are found from (where P
O is the average output
power):
(5)
(6)
To determine the maximum supply voltage the following pa-
rameters must be considered. Add the dropout voltage (5V
for LM3876) to the peak output swing, V
opeak, to get the sup-
ply rail value (i.e. ± (V
opeak + Vod) at a current of Iopeak). The
regulation of the supply determines the unloaded voltage,
usually about 15% higher. Supply voltage will also rise 10%
during high line conditions. Therefore, the maximum supply
voltage is obtained from the following equation:
Max. supplies
≈ ± (V
opeak + Vod)(1 + regulation)(1.1) (7)
The input sensitivity and the output power specs determine
the minimum required gain as depicted below:
(8)
Normally the gain is set between 20 and 200; for a 40W, 8
audio amplifier this results in a sensitivity of 894 mV and
89 mV, respectively. Although higher gain amplifiers provide
greater output power and dynamic headroom capabilities,
there are certain shortcomings that go along with the so
called “gain.” The input referred noise floor is increased and
hence the SNR is worse. With the increase in gain, there is
also a reduction of the power bandwidth which results in a
decrease in feedback thus not allowing the amplifier to re-
spond quickly enough to nonlinearities. This decreased abil-
ity to respond to nonlinearities increases the THD + N speci-
fication.
The desired input impedance is set by R
IN. Very high values
can cause board layout problems and DC offsets at the out-
put. The value for the feedback resistance, R
f1, should be
chosen to be a relatively large value (10 k
–100 k), and
the other feedback resistance, Ri, is calculated using stan-
dard op amp configuration gain equations. Most audio ampli-
fiers are designed from the non-inverting amplifier configura-
tion.
DESIGN A 40W/8
AUDIO AMPLIFIER
Given:
Power Output
40W
Load Impedance
8
Input Level
1V(max)
Input Impedance
100 k
Bandwidth
20 Hz–20 kHz ± 0.25 dB
Equations (5), (6) give:
40W/8
V
opeak = 25.3V
I
opeak = 3.16A
Therefore the supply required is: ±30.3V @ 3.16A
With 15% regulation and high line the final supply voltage is
±38.3V using Equation (7). At this point it is a good idea to
check the Power Output vs Supply Voltage to ensure that the
required output power is obtainable from the device while
maintaining low THD + N. It is also good to check the Power
Dissipation vs Supply Voltage to ensure that the device can
handle the internal power dissipation. At the same time de-
signing in a relatively practical sized heat sink with a low
thermal resistance is also important. Refer to Typical Per-
formance Characteristics graphs and the Thermal Con-
siderations section for more information.
The minimum gain from
Equation (8) is:
A
V ≥ 18
We select a gain of 21 (Non-Inverting Amplifier); resulting in
a sensitivity of 894 mV.
Letting R
IN equal 100 k gives the required input imped-
ance, however, this would eliminate the “volume control” un-
less an additional input impedance was placed in series with
the 10 k
potentiometer that is depicted in Figure 1. Adding
the additional 100 k
resistor would ensure the minumum
required input impedance.
For low DC offsets at the output we let R
f1 = 100 k. Solving
for Ri (Non-Inverting Amplifier) gives the following:
Ri = R
f1/(AV 1) = 100k/(21 1) = 5k; use 5.1 k
The bandwidth requirement must be stated as a pole, i.e.,
the 3 dB frequency. Five times away from a pole gives
0.17 dB down, which is better than the required 0.25 dB.
Therefore:
f
L = 20 Hz/5 = 4Hz
f
H = 20 kHz x 5 = 100 kHz
At this point, it is a good idea to ensure that the
Gain-Bandwidth Product for the part will provide the de-
signed gain out to the upper 3 dB point of 100 kHz. This is
why the minimum GBWP of the LM3876 is important.
GBWP
≥ A
V xf3dB = 21 x 100 kHz = 2.1 MHz
GBWP = 2.0 MHz (min) for the LM3876
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci
≥ 1/(2π Ri f
L) = 7.8 F; use 10 F.
Definition of Terms
Input Offset Voltage: The absolute value of the voltage
which must be applied between the input terminals through
two equal resistances to obtain zero output voltage and cur-
rent.
Input Bias Current: The absolute value of the average of
the two input currents with the output voltage and current at
zero.
Input Offset Current: The absolute value of the difference
in the two input currents with the output voltage and current
at zero.
Input Common-Mode Voltage Range (or Input Voltage
Range): The range of voltages on the input terminals for
which the amplifier is operational. Note that the specifica-
tions are not guaranteed over the full common-mode voltage
range unless specifically stated.
Common-Mode
Rejection:
The
ratio
of
the
input
common-mode voltage range to the peak-to-peak change in
input offset voltage over this range.
Power Supply Rejection: The ratio of the change in input
offset voltage to the change in power supply voltages pro-
ducing it.
Quiescent Supply Current: The current required from the
power supply to operate the amplifier with no load and the
output voltage and current at zero.
Slew Rate: The internally limited rate of change in output
voltage with a large amplitude step function applied to the in-
put.
www.national.com
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM3876T | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
| LM3876T | 制造商:National Semiconductor Corporation 功能描述:56W W/Mute, 1 Channel Lm3876T IC Audio Amp |
| LM3876T/NOPB | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
| LM3876TF | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
| LM3876TF NOPB | 制造商:Texas Instruments 功能描述:8MHz Class-AB Bulk |
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