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LM4923LQ データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM4923LQ データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 12 page Application Information (Continued) vides a quick, smooth transition to shutdown. Another solu- tion is to use a single-throw switch in conjunction with an external pull-up resistor. This scheme guarantees 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 when optimizing device and system performance. Although the LM4923 is tolerant to a variety of external component combinations, consideration of component values must be made when maximizing overall system quality. The LM4923 is unity-gain stable, giving the designer maxi- mum system flexibility. The LM4923 should be used in low closed-loop gain configurations to minimize THD+N values and maximize signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1Vrms are available from sources such as audio codecs. Please refer to the Audio Power Amplifier Design section for a more complete explanation of proper gain selection. When used in its typical application as a fully differential power amplifier the LM4923 does not require input coupling capacitors for input sources with DC common-mode voltages of less than V DD. Exact allowable input common-mode voltage levels are actually a function of V DD,Ri, and Rf and may be determined by Equation 5: V CMi < (VDD-1.2)*((Rf+(Ri)/(Rf)-VDD*(Ri /2Rf) (5) -R F /RI =AVD (6) Special care must be taken to match the values of the feedback resistors (R F1 and RF2) to each other as well as matching the input resistors (R i1 and Ri2) to each other (see Figure 1) more in front. Because of the balanced nature of differential amplifiers, resistor matching differences can re- sult in net DC currents across the load. This DC current can increase power consumption, internal IC power dissipation, reduce PSRR, and possibly damaging the loudspeaker. The chart below demonstrates this problem by showing the ef- fects of differing values between the feedback resistors while assuming that the input resistors are perfectly matched. The results below apply to the application circuit shown in Figure 1, and assumes that V DD =5V, RL =8 Ω, and the system has DC coupled inputs tied to ground. Tolerance R F1 R F2 V 02 -V01 I LOAD 20% 0.8R 1.2R -0.500V 62.5mA 10% 0.9R 1.1R -0.250V 31.25mA 5% 0.95R 1.05R -0.125V 15.63mA 1% 0.99R 1.01R -0.025V 3.125mA 0% RR0 0 Similar results would occur if the input resistors were not carefully matched. Adding input coupling capacitors in be- tween the signal source and the input resistors will eliminate this problem, however, to achieve best performance with minimum component count it is highly recommended that both the feedback and input resistors matched to 1% toler- ance or better. AUDIO POWER AMPLIFIER DESIGN Design a 1W/8 Ω Audio Amplifier Given: Power Output 1Wrms Load Impedance 8 Ω Input Level 1Vrms Input Impedance 20k Ω Bandwidth 100Hz–20kHz ± 0.25dB A designer must first determine the minimum supply rail to obtain the specified output power. The supply rail can easily be found by extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteris- tics section. A second way to determine the minimum supply rail is to calculate the required V OPEAK using Equation 7 and add the dropout voltages. Using this method, the minimum supply voltage is (Vopeak + (V DO TOP +(VDO BOT )), where V DO BOT and V DO TOP are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Perfor- mance Characteristics section. (7) Using the Output Power vs Supply Voltage graph for an 8 Ω load, the minimum supply rail just about 5V. Extra supply voltage creates headroom that allows the LM4923 to repro- duce peaks in excess of 1W without producing audible dis- tortion. 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 Dissipa- tion section. Once the power dissipation equations have been addressed, the required differential gain can be deter- mined from Equation 8. (8) R f /Ri =AVD From Equation 7, the minimum A VD is 2.83. Since the de- sired input impedance was 20k Ω, a ratio of 2.83:1 of R f to Ri results in an allocation of R i = 20k Ω for both input resistors and R f = 60k Ω for both feedback resistors. The final design step is to address the bandwidth requirement which must be stated as a single -3dB frequency point. Five times away from a -3dB point is 0.17dB down from passband response which is better than the required ±0.25dB specified. f H = 20kHz*5= 100kHz The high frequency pole is determined by the product of the desired frequency pole, f H , and the differential gain, AVD . With a A VD = 2.83 and fH = 100kHz, the resulting GBWP = 150kHz which is much smaller than the LM4923 GBWP of 10MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4923 can still be used without running into bandwidth limitations. www.national.com 11 |
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