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LM4954TLX bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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LM4954TLX bảng dữ liệu(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Application Information (Continued) reproduce signals below 100Hz to 150Hz. 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 affected 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 1/2V 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. Choos- ing C B equal to 2.2µF along with a small value of Ci (in the range of 0.1µF to 0.39µF), should produce a virtually click- less and popless shutdown function. While the device will function properly, (no oscillations or motorboating), with C B equal to 0.1µF. AUDIO POWER AMPLIFIER DESIGN 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. 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 2. (2) A VD =(Rf/Ri)2 The LM4954 is unity-gain stable and requires no external components besides gain-setting resistors, an input coupling capacitor, and proper supply bypassing in the typical appli- cation. However, if a closed-loop differential gain of greater than 10 is required, a feedback capacitor (C F) may be needed as shown in Figure 2 to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that elimi- nates possible high frequency oscillations. Care should be taken when calculating the -3dB frequency in that an incor- rect combination of R F and CF will cause rolloff before 20kHz. A typical combination of feedback resistor and ca- pacitor that will not produce audio band high frequency rolloff is R F = 20k Ω and C F = 25pf. These components result in a -3dB point of approximately 320 kHz. To calculate the value of the capacitor for a given -3dB point, use Equation 3 below: C F = 1/(2 πf 3dBRF) (F) (3) 20129108 FIGURE 2. HIGHER GAIN AUDIO AMPLIFIER www.national.com 11 |
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