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LM4872ITPX 参数 Datasheet PDF下载

LM4872ITPX图片预览
型号: LM4872ITPX
PDF下载: 下载PDF文件 查看货源
内容描述: [IC,AUDIO AMPLIFIER,SINGLE,BGA,8PIN,PLASTIC]
分类和应用: 放大器商用集成电路
文件页数/大小: 15 页 / 465 K
品牌: TI [ TEXAS INSTRUMENTS ]
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AUDIO POWER AMPLIFIER DESIGN  
Application Information (Continued)  
A 1W/8AUDIO AMPLIFIER  
width is dictated by the choice of external components  
shown in Figure 1. The input coupling capacitor, Ci, 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.  
Given:  
Power Output  
Load Impedance  
Input Level  
1 Wrms  
8Ω  
1 Vrms  
20 kΩ  
Selection Of Input Capacitor Size  
Input Impedance  
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.  
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 Vopeak using Equation 2  
and add the output voltage. Using this method, the minimum  
In addition to system cost and size, click and pop perfor-  
mance is effected by the size of the input coupling capacitor,  
Ci. A larger input coupling capacitor requires more charge to  
reach its quiescent DC voltage (nominally 1/2 VDD). 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.  
supply voltage would be (Vopeak + (VOD  
+ VODBOT)), where  
VOD  
and VOD  
are extrapolated frToOmP the Dropout Volt-  
TOP  
age BvOsT Supply Voltage curve in the Typical Performance  
Characteristics section.  
(2)  
Besides minimizing the input capacitor size, careful consid-  
eration should be paid to the bypass capacitor value. Bypass  
capacitor, CB, is the most critical component to minimize  
turn-on pops since it determines how fast the LM4872 turns  
on. The slower the LM4872’s outputs ramp to their quiescent  
DC voltage (nominally 1/2 VDD), the smaller the turn-on pop.  
Choosing CB equal to 1.0 µF along with a small value of Ci  
(in the range of 0.1 µF to 0.39 µF), should produce a virtually  
clickless and popless shutdown function. While the device  
will function properly, (no oscillations or motorboating), with  
CB equal to 0.1 µF, the device will be much more susceptible  
to turn-on clicks and pops. Thus, a value of CB equal to  
1.0 µF is recommended in all but the most cost sensitive  
designs.  
Using the Output Power vs Supply Voltage graph for an 8Ω  
load, the minimum supply rail is 4.6V. But since 5V is a  
standard voltage in most applications, it is chosen for the  
supply rail. Extra supply voltage creates headroom that al-  
lows the LM4872 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.  
LOW VOLTAGE APPLICATIONS ( BELOW 3.0 VDD  
)
(3)  
The Lm4872 will function at voltages below 3 volts but this  
mode of operation requires the addition of a 1kresistor  
from each of the differential output pins ( pins 8 and 4 )  
directly to ground. The addition of the pair of 1kresistors (  
R4 & R5 ) assures stable operation below 3 Volt Vdd opera-  
tion. The addition of the two resistors will however increase  
the idle current by as much as 5mA. This is because at 0v  
input both of the outputs of the LM4872’s 2 internal opamps  
go to 1/2 VDD ( 2.5 volts for a 5v power supply ), causing  
current to flow through the 1K resistors from output to  
ground. See fig 4.  
Rf/Ri = AVD/2  
From Equation 3, the minimum AVD is 2.83; use AVD = 3.  
Since the desired input impedance was 20 k, and with a  
AVD impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an  
allocation of Ri = 20 kand Rf = 30 k. 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.  
fL = 100 Hz/5 = 20 Hz  
fH = 20 kHz * 5 = 100 kHz  
Jumper options have been included on the reference design,  
Fig. 4, to accommodate the low voltage application. J2 & J3  
connect R4 and R5 to the outputs. J1 operates the shutdown  
function. J1 must be installed to operate the part. A switch  
may be installed in place of J1 for easier evaluation of the  
shutdown function.  
As stated in the External Components section, Ri in con-  
junction with Ci create a highpass filter.  
Ci 1/(2π*20 k*20 Hz) = 0.397 µF; use 0.39 µF  
The high frequency pole is determined by the product of the  
desired frequency pole, fH, and the differential gain, AVD  
.
With a AVD = 3 and fH = 100 kHz, the resulting GBWP =  
150 kHz which is much smaller than the LM4872 GBWP of  
4 MHz. This figure displays that if a designer has a need to  
design an amplifier with a higher differential gain, the  
LM4872 can still be used without running into bandwidth  
limitations.  
www.national.com  
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