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LM4881MM Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM4881MM Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 11 page Application Information (Continued) shutdown function should be virtually clickless and popless. While the device will function properly, (no oscillations or mo- torboating), 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 0.1 µF or larger is recommended in all but the most cost sensitive designs. AUDIO POWER AMPLIFIER DESIGN Design a Dual 200mW/8 Ω Audio Amplifier Given: Power Output 200 mWrms Load Impedance 8 Ω Input Level 1 Vrms (max) Input Impedance 20 k Ω Bandwidth 100 Hz–20 kHz ± 0.50 dB A designer must first determine the needed supply rail to ob- tain the specified output power. Calculating the required sup- ply rail involves knowing two parameters, V OPEAK and also the dropout voltage. The latter is typically 530 mV and can be found from the graphs in the Typical Performance Char- acteristics. V OPEAK can be determined from Equation 3. (3) For 200 mW of output power into an 8 Ω load, the required V OPEAK is 1.79 volts. A minimum supply rail of 2.32V results from adding V OPEAK and VOD. Since 5V is a standard supply voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4881 to reproduce peaks in excess of 200 mW without clipping the signal. At this time, the designer must make sure that the power supply choice along with the output imped- ance does not violate the conditions explained in the Power Dissipation section. Remember that the maximum power dissipation point from Equation 1 must be multiplied by two since there are two independent amplifiers inside the pack- age. Once the power dissipation equations have been addressed, the required gain can be determined from Equation 4. (4) A V =Rf/Ri (5) From Equation 4, the minimum gain is: A V = 1.26 Since the desired input impedance was 20 k Ω, and with a gain of 1.26, a value of 27 k Ω is designated for R f, assuming 5% tolerance resistors. This combination results in a nominal gain of 1.35. The final design step is to address the band- width requirements which must be stated as a pair of −3 dB frequency points. Five times away from a −3 dB point is 0.17 dB down from passband response assuming a single pole roll-off. As stated in the External Components section, both R i in conjunction with C i, and Co with RL, create first or- der highpass filters. Thus to obtain the desired frequency low response of 100 Hz within ±0.5 dB, both poles must be taken into consideration. The combination of two single order filters at the same frequency forms a second order response. This results in a signal which is down 0.34 dB at five times away from the single order filter −3 dB point. Thus, a fre- quency of 20 Hz is used in the following equations to ensure that the response is better than 0.5 dB down at 100 Hz. C i ≥ 1/(2π *20kΩ * 20 Hz) = 0.397 µF; use 0.39 µF. C o ≥ 1/(2π *8Ω * 20 Hz) = 995 µF; use 1000 µF. The high frequency pole is determined by the product of the desired high frequency pole, f H, and the closed-loop gain, A V . With a closed-loop gain of 1.35 and f H = 100 kHz, the re- sulting GBWP = 135 kHz which is much smaller than the LM4881 GBWP of 18 MHz. This figure displays that if a de- signer has a need to design an amplifier with a higher gain, the LM4881 can still be used without running into bandwidth limitations. www.national.com 9 |
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