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参数资料
| 型号： | LM4891MM/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 1 W, 1 CHANNEL, AUDIO AMPLIFIER, PDSO8 |
| 封装： | MSOP-8 |
| 文件页数： | 2/20页 |
| 文件大小： | 733K |
| 代理商： | LM4891MM/NOPB |
Application Information (Continued)
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4891 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic high is placed on the shutdown pin.
By switching the shutdown pin to V
DD, the LM4891 supply
current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages more than
1.0V
DC, the idle current may be greater than the typical
value of 0.1A. (Idle current is measured with the shutdown
pin tied to V
DD).
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch in conjunction with an
external pull-up resistor. When the switch is closed, the
shutdown pin is connected to ground which enables the
amplifier. If the switch is open, then the external pull-up
resistor to V
DD will disable the LM4891. This scheme guar-
antees that the shutdown pin will not float thus preventing
unwanted state changes.
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 LM4891 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4891 is unity-gain stable which gives the designer
maximum system flexibility. The LM4891 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 1 Vrms are available
from sources such as audio codecs. 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 Figure 1. The input coupling capacitor, C
i, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100 Hz to 150 Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
i. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD). This
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B, is the most critical component to minimize
turn-on pops since it determines how fast the LM4891 turns
on. The slower the LM4891’s outputs ramp to their quiescent
DC voltage (nominally 1/2 V
DD), the smaller the turn-on pop.
Choosing C
B equal to 1.0 F along with a small value of Ci
(in the range of 0.1 F to 0.39 F), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
C
B equal to 0.1 F, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of C
B equal to
1.0 F is recommended in all but the most cost sensitive
designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8
AUDIO AMPLIFIER
Given:
Power Output
1 Wrms
Load Impedance
8
Input Level
1 Vrms
Input Impedance
20 k
Bandwidth
100 Hz–20 kHz ± 0.25 dB
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. A second way to determine the minimum sup-
ply rail is to calculate the required V
opeak using Equation 2
and add the output voltage. Using this method, the minimum
supply voltage would be (V
opeak +(VODTOP +VODBOT)), where
V
ODBOT and VODTOP are extrapolated from the Dropout Volt-
age vs Supply Voltage curve in the Typical Performance
Characteristics section.
(2)
5V is a standard voltage, in most applications, chosen for the
supply rail. Extra supply voltage creates headroom that al-
lows the LM4891 to reproduce peaks in excess of 1W with-
out producing audible distortion. At this time, the designer
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 differential gain can be determined from Equa-
tion 3.
(3)
A
VD =(Rf/Ri)2
From Equation 3, the minimum A
VD is 2.83; use AVD =3.
Since the desired input impedance was 20 k
, and with a
A
VD of 3, a ratio of 1.5:1 of Rf to Ri results in an allocation of
R
i =20k
and R
f =30k
. 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 better than the required ±0.25 dB specified.
f
L = 100 Hz/5 = 20 Hz
f
H =20kHz*5=100 kHz
LM4891
www.national.com
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相关代理商/技术参数 |
参数描述 |
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| LM4891MX/NOPB | 功能描述:IC AMP AUDIO PWR 1W MONO 8SOIC RoHS:是 类别:集成电路 (IC) >> 线性 - 音頻放大器 系列:Boomer® 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:2,500 系列:DirectDrive® 类型:H 类 输出类型:耳机，2-通道（立体声） 在某负载时最大输出功率 x 通道数量:35mW x 2 @ 16 欧姆 电源电压:1.62 V ~ 1.98 V 特点:I²C，麦克风，静音，短路保护，音量控制 安装类型:表面贴装 供应商设备封装:25-WLP（2.09x2.09） 封装/外壳:25-WFBGA，WLCSP 包装:带卷 (TR) |
| LM4892 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:1 Watt Audio Power Amplifier with Headphone Sense |
| LM48920B WAF | 制造商:Texas Instruments 功能描述: |
| LM4892B E WAF | 制造商:Texas Instruments 功能描述: |
| LM4892BUPN E WAF | 制造商:Texas Instruments 功能描述: |
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