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LM4884 Datasheet(Fiches technique) 18 Page  National Semiconductor (TI) 


18 page Application Information (Continued) Input Level: 1 V RMS Input Impedance: 20 k Ω Bandwidth: 100 Hz−20 kHz ± 0.25 dB The design begins by specifying the minimum supply voltage necessary to obtain the desired 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 (6), 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 (6). The result is Equation (7). (6) V DD ≥ (V OUTPEAK+(VODTOP +VODBOT)) (7) 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 LM4884 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 of maximum power dissipation as explained above in the Power Dissipation section. After satisfying the LM4884’s power dissipation require ments, the minimum differential gain is found using Equation (8). (8) Thus, a minimum gain of 2.83 allows the LM4884’s to reach full output swing and maintain low noise and THD+N perfor mance. For this example, let A VD = 3. In the example design, the gain will be set to 10dB (A VD = 3.2) by applying a logic low to GAIN 0 and a logic high to GAIN 1. 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 onefifth the lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth limit. This extended bandwith produces a gain variation of 0.17dB at the bandwith’s limits, well within the ±0.25dB desired limit. The results are an f L = 100Hz/5 = 20Hz (9) and an f H = 20kHzx5= 100kHz (10) As mentioned in the External Components section, the inter nal input resistor and C i create a high pass filter that sets the amplifier’s lower bandpass frequency limit. Find the coupling capacitor’s value using Equation (11). f 3dB = 1/2 π(20kΩ)C I (11) The result is (using the minimum R IN resistor value to ensure correct magnitude response at 20Hz) 1/(2 π*20kΩ*20Hz) = 0.398µF (12) 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 up per passband response limit. With A VD = 3.2 and fH = 100kHz, the closedloop gain bandwidth product (GBWP) is 320kHz. This is less than the LM4884’s 3.5MHz GBWP. With this margin, the amplifier can be used in designs that require more differential gain while avoiding performancerestricting bandwidth limitations. Recommended Printed Circuit Board Layout Figures 2 through 6 show the recommended fourlayer PC board layout that is optimized for the 20pin MHpackaged LM4884 and associated external components. This circuit is designed for use with an external 5V supply and 3 Ω (or higher) speakers (or load resistors). This circuit board is easy to use. Apply 5V and ground to the board’s V DD and GND terminals, respectively. Connect speakers (or load resistors) between the board’s OUTA and +OUTA and OUTB and +OUTB pads. Apply balanced dif ferential stereo input signals to the input pins labeled "INA," "+INA," "INB," and "+INB." www.national.com 18 
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