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LM4941 Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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LM4941 Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Application Information (Continued) device as possible. Typical applications employ a 5V regu- lator with 10µF and 0.1µF bypass capacitors that increase supply stability. This, however, does not eliminate the need for bypassing the supply nodes of the LM4941. The LM4941 will operate without the bypass capacitor C B, although the PSRR may decrease. A 1µF capacitor is recommended for C B. This value maximizes PSRR performance. Lesser val- ues may be used, but PSRR decreases at frequencies below 1kHz. The issue of C B selection is thus dependant upon desired PSRR and click and pop performance as explained in the section Proper Selection of External Components. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4941 contains shutdown circuitry that is used to turn off the amplifier’s bias circuitry. The device may then be placed into shutdown mode by toggling the Shutdown Select pin to logic low. The trigger point for shutdown is shown as a typical value in the Supply Current vs Shutdown Voltage graphs in the Typical Performance Characteristics section. It is best to switch between ground and supply for maximum perfor- mance. While the device may be disabled with shutdown voltages in between ground and supply, the idle current may be greater than the typical value of 0.1µA. In either case, the shutdown pin should be tied to a definite voltage to avoid unwanted state changes. In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry, which pro- 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 LM4941 is tolerant to a variety of external component combinations, consideration of component values must be made when maximizing overall system quality. The LM4941 is unity-gain stable, giving the designer maxi- mum system flexibility. The LM4941 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 LM4941 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) When using DC coupled inputs, special care must be taken to match the values of the input resistors (R i1 and Ri2)to each other. Because of the balanced nature of differential amplifiers, resistor matching differences can result 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 effects of differing values between the feedback resistors while as- suming 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 i1 R i2 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% R R 0 0 Since the same variations can have a significant effect on PSRR and CMRR performance, it is highly recommended that the input resistors be matched to 1% tolerance or better for best performance. 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 LM4941 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. www.national.com 9 |
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