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參數(shù)資料
| 型號(hào)： | LM4818M/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 0.35 W, 1 CHANNEL, AUDIO AMPLIFIER, PDSO8 |
| 封裝： | SOP-8 |
| 文件頁(yè)數(shù)： | 2/16頁(yè) |
| 文件大?。?/td> | 483K |
| 代理商： | LM4818M/NOPB |
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Application Information (Continued)
operation by closing the switch. Opening the switch con-
nects the shutdown pin to V
DD through the pull-up resistor,
activating micro-power shutdown. The switch and resistor
guarantee that the SHUTDOWN pin will not float. This pre-
vents unwanted state changes. In a system with a micropro-
cessor or a microcontroller, use a digital output to apply the
control voltage to the SHUTDOWN pin. Driving the SHUT-
DOWN pin with active circuitry eliminates the pull-up resistor
PROPER SELECTION OF EXTERNAL COMPONENTS
Optimizing the LM4818’s performance requires properly se-
lecting external components. Though the LM4818 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4818 is unity gain stable, giving the designer maxi-
mum design flexibility. The gain should be set to no more
than a given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V
RMS (2.83VP-P). Please refer to the
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Another important consideration is the amplifier’s close-loop
bandwidth. To a large extent, the bandwidth is dictated by
the choice of external components shown in Figure 1. The
input coupling capacitor, C
i, forms a first order high pass filter
that limits low frequency response. This value should be
chosen based on needed frequency response for a few
distinct reasons discussed below
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (C
i in Figure 1). A high value
capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with limited frequency response reap little
improvement by using a large input capacitor.
Besides affecting system cost and size, C
i has an effect on
the LM4818’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s value. Higher value capacitors need
more time to reach a quiescent DC voltage (usually 1/2 V
DD)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
F. Thus, selecting an input capacitor value that is no higher
than necessary to meet the desired -3dB frequency can
minimize pops.
As shown in Figure 1, the input resistor (R
i) and the input
capacitor, C
i produce a -3dB high pass filter cutoff frequency
that is found using Equation (5).
f
-3dB = 1/(2
πR
iCi) (Hz)
(5)
As an example when using a speaker with a low frequency
limit of 150Hz, C
i, using Equation (5) is 0.063F. The 0.39F
C
i shown in Figure 1 allows the LM4818 to drive a high
efficiency, full range speaker whose response extends down
to 20Hz.
Besides optimizing the input capacitor value, the bypass
capacitor value, C
B requires careful consideration. The by-
pass capacitor’s value is the most critical to minimizing
turn-on pops because it determines how fast the LM4818
turns on. The slower the LM4818’s outputs ramp to their
quiescent DC voltage (nominally 1/2V
DD), the smaller the
turn-on pop. While the device will function properly (no os-
cillations or motorboating), with C
B less than 1.0F, the
device will be much more susceptible to turn-on clicks and
pops. Thus, a value of C
B equal to or greater than 1.0F is
recommended in all but the most cost sensitive designs.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of C
B, the capacitor
connected to the BYPASS pin. Since C
B determines how
fast the LM4818 settles to quiescent operation, its value is
critical when minimizing turn-on pops. The slower the
LM4818’s outputs ramp to their quiescent DC voltage (nomi-
nally 1/2V
DD), the smaller the turn-on pop. Choosing CB
equal to 1.0F along with a small value of C
i (in the range of
0.1F to 0.39F) produces a click-less and pop-less shut-
down function. As discussed above, choosing C
i no larger
than necessary for the desired bandwidth helps minimize
clicks and pops. If using the optional capacitor, C
B2, the total
capacitance see at the BYPASS pin is C
B +CB2. When using
the values shown in Figure 1, Typical Audio Amplifier
Application Circuit, for C
B and CB2 the change in the
capacitance seen by the BYPASS pin is not significant rela-
tive to capacitor value tolerances.
Optimizing Click and Pop Reduction Performance
The LM4818 contains circuitry that minimizes turn-on and
shutdown transients or "clicks and pops". For this discus-
sion, turn on refers to either applying the power or supply
voltage or when the shutdown mode is deactivated. While
the power supply is ramping to it’s final value, the LM4818’s
internal amplifiers are configured as unity gain buffers. An
internal current source charges the voltage of the bypass
capacitor, C
B, connected to the BYPASS pin in a controlled,
linear manner. Ideally, the input and outputs track the voltage
charging on the bypass capacitor. The gain of the internal
amplifiers remains unity until the bypass capacitor is fully
charged to 1/2V
DD. As soon as the voltage on the bypass
capacitor is stable, the device becomes fully operational.
Although the BYPASS pin current cannot be modified,
changing the size of the bypass capacitor, C
B, alters the
device’s turn-on time and magnitude of "clicks and pops".
Increasing the value of C
B reduces the magnitude of turn-on
pops. However, this presents a tradeoff: as the size of C
B
increases, the turn-on time (Ton) increases. There is a linear
relationship between the size of C
B and the turn on time. If
using the optional capacitor, C
B2, the total capacitance see
at the BYPASS pin is C
B and CB2. The total capacitance see
at the BYPASS pin must be considered for the table below
and when optimizing click and pop performance. Below are
some typical turn-on times for various values of C
B:
LM4818
www.national.com
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4818MX | 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 |
| LM4818MX/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 |
| LM4819 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:350mW Audio Power Amplifier with Shutdown Mode |
| LM4819LD | 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 |
| LM4819LD/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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