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參數資料
| 型號： | LM4809M |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 0.105 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO8 |
| 封裝： | M08A |
| 文件頁數： | 14/14頁 |
| 文件大小： | 694K |
| 代理商： | LM4809M |
Application Information (Continued)
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4809’s performance requires properly se-
lecting external components. Though the LM4809 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4809 is unity-gain stable, giving a designer maximum
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
CODECs have outputs of 1V
RMS (2.83VP-P). Please refer to
the Audio Power Amplifier Design section for more infor-
mation on selecting the proper gain.
Input and Output Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input and output coupling capacitors (C
I and CO in Figure 1).
A high value capacitor can be expensive and may compro-
mise space efficiency in portable designs. In many cases,
however, the speakers used in portable systems, whether
internal or external, have little ability to reproduce signals
below 150Hz. Applications using speakers with this limited
frequency response reap little improvement by using high
value input and output capacitors.
Besides affecting system cost and size, C
i has an effect on
the LM4809’s click and pop performance. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Thus, pops can be minimized by selecting an input capacitor
value that is no higher than necessary to meet the desired
3dB frequency. Please refer to the Optimizing Click and
Pop Reduction Performance section for a more detailed
discussion on click and pop performance.
As shown in
Figure 1, the input resistor, R
I and the input
capacitor, C
I, produce a 3dB high pass filter cutoff fre-
quency that is found using Equation (3). In addition, the
output load R
L, and the output capacitor CO, produce a -3db
high pass filter cutoff frequency defined by Equation (4).
f
I-3db=1/2πRICI
(3)
f
O-3db=1/2πRLCO
(4)
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
B, the capacitor con-
nected to the BYPASS pin. Since C
B determines how fast
the LM4809 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4809’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
DD), the smaller the turn-on pop. Choosing CB equal to
4.7F along with a small value of C
i (in the range of 0.1F to
0.47F), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
i no larger than neces-
sary for the desired bandwith helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4809 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. During turn-on, the
LM4809’s internal amplifiers are configured as unity gain
buffers. An internal current source charges up the capacitor
on the BYPASS pin in a controlled, linear manner. The gain
of the internal amplifiers remains unity until the voltage on
the BYPASS pin reaches 1/2 V
DD . As soon as the voltage on
the BYPASS pin is stable, the device becomes fully opera-
tional. During device turn-on, a transient (pop) is created
from a voltage difference between the input and output of the
amplifier as the voltage on the BYPASS pin reaches 1/2 V
DD.
For this discussion, the input of the amplifier refers to the
node between R
I and CI. Ideally, the input and output track
the voltage applied to the BYPASS pin. During turn-on, the
buffer-configured amplifier output charges the input capaci-
tor, C
I, through the input resistor, RI. This input resistor
delays the charging time of C
I thereby causing the voltage
difference between the input and output that results in a
transient (pop). Higher value capacitors need more time to
reach a quiescent DC voltage (usually 1/2 V
DD)
when
charged with a fixed current. Decreasing the value of C
I and
R
I will minimize turn-on pops at the expense of the desired
-3dB frequency. Although the BYPASS pin current cannot be
modified, changing the size of C
B alters the device’s turn-on
time and the magnitude of “clicks and pops”. Increasing the
value of C
B reduces the magnitude of turn-on pops. How-
ever, this presents a tradeoff: as the size of C
B increases, the
turn-on time increases. There is a linear relationship be-
tween the size of C
B and the turn-on time. Here are some
typical turn-on times for various values of C
B:
C
B
T
ON
0.1F
80ms
0.22F
170ms
0.33F
270ms
0.47F
370ms
0.68F
490ms
1.0F
920ms
2.2F
1.8sec
3.3F
2.8sec
4.7F
3.4sec
10F
7.7sec
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching V
DD may not
allow the capacitors to fully discharge, which may cause
“clicks and pops”. In a single-ended configuration, the output
is coupled to the load by C
O. This capacitor usually has a
high value. C
O discharges through internal 20k resistors.
Depending on the size of C
O, the discharge time constant
can be relatively large. To reduce transients in single-ended
mode, an external 1k
–5k resistor can be placed in par-
allel with the internal 20k
resistor. The tradeoff for using
this resistor is increased quiescent current.
LM4809
www.national.com
9
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| LM4809MA | 制造商:OC White Company 功能描述:Audio Amp Headphone 2-CH Stereo 0.105W Class-AB 8-Pin SOIC N Rail |
| LM4809MA/NOPB | 功能描述:IC AMP AUDIO PWR .105W AB 8SOIC RoHS:是 類別:集成電路 (IC) >> 線性 - 音頻放大器 系列:Boomer® 產品培訓模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標準包裝:2,500 系列:DirectDrive® 類型:H 類 輸出類型:耳機，2-通道（立體聲） 在某負載時最大輸出功率 x 通道數量:35mW x 2 @ 16 歐姆 電源電壓:1.62 V ~ 1.98 V 特點:I²C，麥克風，靜音，短路保護，音量控制 安裝類型:表面貼裝 供應商設備封裝:25-WLP（2.09x2.09） 封裝/外殼:25-WFBGA，WLCSP 包裝:帶卷 (TR) |
| LM4809MBD | 功能描述:BOARD EVALUATION LM4809M RoHS:否 類別:編程器，開發(fā)系統 >> 評估板 - 音頻放大器 系列:Boomer® 產品培訓模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標準包裝:1 系列:- |
| LM4809MM | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產品: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 |
| LM4809MM/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產品: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 |
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