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LM4816 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4816 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 13 page Application Information (Continued) the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches 1/2 V DD. As soon as the voltage on the BYPASS pin is stable, the device becomes fully operational. Although the bypass pin current cannot be modified, chang- ing the size of C B alters the device’s turn-on time and the magnitude of "clicks and pops". Increasing the value of C B reduces the magnitude of turn-on pops. However, this pre- sents a tradeoff: as the size of C B increases, the turn-on time increases. There is a linear relationship between the size of C B and the turn-on time. Here are some typical turn-on times for various values of C B: C B T ON 0.01µF 20 ms 0.1µF 200 ms 0.22µF 440 ms 0.47µF 940 ms 1.0µF 2 Sec In order eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching V DD may not allow the capacitors to fully discharge, which may cause "clicks and pops". NO LOAD STABILITY The LM4816 may exhibit low level oscillation when the load resistance is greater than 10k Ω. This oscillation only occurs as the output signal swings near the supply voltages. Pre- vent this oscillation by connecting a 5k Ω between the output pins and ground. AUDIO POWER AMPLIFIER DESIGN Audio Amplifier Design: Driving 1W into an 8 Ω Load The following are the desired operational parameters: Power Output: 1W RMS Load Impedance: 8 Ω Input Level: 1V RMS Input Impedance: 20k Ω Bandwidth: 100Hz−20 kHz ± 0.25 dB The design begins by specifying the minimum supply voltage necessary to obtain the specified output power. One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical Performance Char- acteristics section. Another way, using Equation (4), is to calculate the peak output voltage necessary to achieve the desired output power for a given load impedance. To ac- count for the amplifier’s dropout voltage, two additional volt- ages, based on the Dropout Voltage vs Supply Voltage in the Typical Performance Characteristics curves, must be added to the result obtained by Equation (8). The result in Equation (9). (8) V DD ≥ (V OUTPEAK +(VODTOP +VODBOT)) (9) The Output Power vs Supply Voltage graph for an 8 Ω load indicates a minimum supply voltage of 4.6V. This is easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom, allowing the LM4816 to produce peak output power in excess of 1W without clipping or other audible distortion. The choice of supply voltage must also not create a situation that violates maximum power dissipation as explained above in the Power Dissipation section. After satisfying the LM4816’s power dissipation require- ments, the minimum differential gain is found using Equation (10). (10) Thus, a minimum gain of 2.83 allows the LM4816’s to reach full output swing and maintain low noise and THD+N perfor- mance. For this example, let A VD =3. The amplifier’s overall gain is set using the input (R i) and feedback (R f) resistors. With the desired input impedance set at 20k Ω, the feedback resistor is found using Equation (11). R f/Ri =AVD/2 (11) The value of R f is 30k Ω. 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 and 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 an f L = 100Hz/5 = 20Hz (12) and an F H = 20kHzx5 = 100kHz (13) As mentioned in the External Components section, R i and C i create a highpass filter that sets the amplifier’s lower bandpass frequency limit. Find the coupling capacitor’s value using Equation (14). (14) the result is 1/(2 π*20kΩ*20Hz) = 0.398µF (15) Use a 0.39µF 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 = 3 and fH = 100kHz, the closed-loop gain bandwidth product (GBWP) is 300kHz. This is less than the LM4816’s 3.5MHz GBWP. With www.national.com 11 |
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