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LM4864MM Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM4864MM Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 10 page Application Information (Continued) 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 LM4864 is tolerant to a variety of external component combinations, consideration to component values must be used to maximize overall sys- tem quality. The LM4864 is unity-gain stable, giving a designer maximum system flexibility. The LM4864 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large in- put 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 complete 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 bandwidth 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 fre- quency 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 attenua- tion. But in many cases the speakers used in portable sys- tems, whether internal or external, have little ability to repro- duce signals below 150 Hz. In this case using a large input capacitor may not increase system performance. 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 ca- pacitor 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 LM4864 turns on. The slower the LM4864’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 Ci (in the range of 0.1 µF to 0.39 µF), should produce a click- less 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 or larger is recommended in all but the most cost sensitive designs. AUDIO POWER AMPLIFIER DESIGN Design a 300 mW/8 Ω Audio Amplifier Given: Power Output 300 mWrms Load Impedance 8 Ω Input Level 1 Vrms Input Impedance 20 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 4 and add the dropout voltage. Using this method, the mini- mum supply voltage would be (V opeak +(2*VOD)), where V OD is extrapolated from the Dropout Voltage vs Supply Volt- age curve in the Typical Performance Characteristics sec- tion. (4) Using the Output Power vs Supply Voltage graph for an 8 Ω load, the minimum supply rail is 3.5V. But since 5V is a stan- dard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that al- lows the LM4864 to reproduce peaks in excess of 500 mW 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 required differential gain can be determined from Equa- tion 5. (5) R F/Ri = AVD/2 (6) From Equation 5, the minimum A VD is 1.55; use AVD = 2. Since the desired input impedance was 20 k Ω, and with a A VD of 2, a ratio of 1:1 of RF to Ri results in an allocation of R i = RF = 20 kΩ. 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 pole gives 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 = 20 kHz x 5 = 100 kHz As stated in the External Components section, R i in con- junction with C i create a highpass filter. C i ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF The high frequency pole is determined by the product of the desired high frequency pole, f H, and the differential gain, A VD. With a AVD = 2 and fH = 100 kHz, the resulting GBWP = 100 kHz which is much smaller than the LM4864 GBWP of 18 MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4864 can still be used without running into bandwidth problems. www.national.com 8 |
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