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LM4820-6 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4820-6 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 18 page  Application Information (Continued) 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 LM4820-6 turns on. The slower the LM4820-6’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 µF along with a small value of C i (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 C B equal to 0.1 µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of C B equal to 1.0 µF is recommended in all but the most cost sensitive designs. AUDIO POWER AMPLIFIER DESIGN A 1W/8 Ω AUDIO AMPLIFIER Given: Power Output 1 Wrms Load Impedance 8 Ω Input Level 1 Vrms Input Impedance 25 k Ω 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 3. Using this method, the minimum supply voltage would be (V opeak +(VODTOP +VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance Characteristics section. (3) 2.7V DD to 5VDD is a standard supply voltage range for most applications. Extra supply voltage creates headroom that allows the LM4820-6 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the de- signer 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 differential gain is determined from Equations 4 or 5. (4) or A VD =2(Rf/Ri ) (5) R f =Ri = 25kΩ A VD =2(25kΩ/25kΩ ) A VD =2 The last step in this design example is setting the amplifier’s -3dB frequency bandwidth. To achieve the desired ±0.25dB pass band magnitude variation limit, the low frequency re- sponse must extend to at least one-fifth the lower bandwidth limit. The high frequency response must extend to at least five times the upper bandwidth limit. The gain variation for both response limits is 0.17dB, well within the ±0.25dB desired limit. The results are f L = 100Hz/5 = 20Hz and f H = 20kHzx5= 100kHz As mentioned in the Selecting Proper External Compo- nents section, R i and Ci create a highpass filter that sets the amplifier’s lower bandpass frequency limit. To find the cou- pling capacitor’s value, use Equation 6 C i ≥ 1/(2πRifL) (6) The result is 1/(2 π*25kΩ*20kHz) = .318µf Use a 0.33µf capacitor, the closest standard value. The product of the desired high frequency cutoff (100kHz in this example ) and the differential gain A VD, determines the upper passband response limit. With A VD = 2 and fH = 100kHz, the closed-loop gain bandwidth product (GBWP) is 200kHz. This is less than the LM4820-6’s 25MHz GBWP. With this margin, the amplifier can be used in designs that require more differential gain while avoiding performance, restricting bandwidth limitations. www.national.com 10 |
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