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LM4863M bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM4863M bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Application Information (Continued) PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications us- ing integrated power amplifiers is critical to optimize device and system performance. While the LM4863 is tolerant to a variety of external component combinations, consideration to component values must be used to maximize overall sys- tem quality. The LM4863 is unity-gain stable, giving the designer maxi- mum system performance. The LM4863 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. In- put signals equal to or greater than 1 Vrms are available from sources such as audio codecs. Please refer to the sec- tion, Audio Power Amplifier Design, for a more complete explanation of proper gain selection. Besides gain, one of the major considerations is the closed-loop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, C i, forms a first order high pass filter which limits low frequency re- sponse. This value should be chosen based on needed fre- quency response for a few distinct reasons. CLICK AND POP CIRCUITRY The LM4863 contains circuitry to minimize turn-on transients or “clicks and pops”. In this case, turn-on refers to either power supply turn-on or the device coming out of shutdown mode. When the device is turning on, the amplifiers are inter- nally configured as unity gain buffers. An internal current source ramps up the voltage of the bypass pin. Both the in- puts and outputs ideally track the voltage at the bypass pin. The device will remain in buffer mode until the bypass pin has reached its half supply voltage, 1/2 V DD. As soon as the bypass node is stable, the device will become fully opera- tional, where the gain is set by the external resistors. Although the bypass pin current source cannot be modified, the size of C B can be changed to alter the device turn-on time and the amount of “clicks and pops”. By increasing amount of turn-on pop can be reduced. However, the tradeoff for using a larger bypass capacitor is an increase in turn-on time for this device. There is a linear relationship be- tween the size of C B and the turn-on time. Here are some typical turn-on times for a given C B: C B T ON 0.01 µF 20 ms 0.1 µF 200 ms 0.22 µF 420 ms 0.47 µF 840 ms 1.0 µF 2 Sec In order eliminate “clicks and pops”, all capacitors must be discharged before turn-on. Rapid on/off switching of the de- vice or the shutdown function may cause the “click and pop” circuitry to not operate fully, resulting in increased “click and pop” noise. In a single-ended configuration, the output cou- pling capacitor, C O, is of particular concern. This capacitor discharges through the internal 20 k Ω resistors. Depending on the size of C O, the time constant can be relatively large. To reduce transients in single-ended mode, an external 1k Ω–5 kΩ resistor can be placed in parallel with the internal 20 k Ω resistor. The tradeoff for using this resistor is an in- crease in quiescent current. The value of C I will also reflect turn-on pops. Clearly, a cer- tain size for C I is needed to couple in low frequencies without excessive attenuation. But in many cases, the speakers used in portable systems, whether integral or external, have little ability to reproduce signals below 100 Hz to 150 Hz. In this case, using a large input and output capacitor may not increase system performance. In most cases, choosing a small value of C I in the range of 0.1 µF to 0.33 µF), along with C B equal to 1.0 µF should produce a virtually clickless and popless turn-on. In cases where C I is larger than 0.33 µF, it may be advantageous to increase the value of C B. Again, it should be understood that increasing the value of C B will reduce the “clicks and pops” at the expense of a longer device turn-on time. DS012881-24 FIGURE 2. Headphone Circuit www.national.com 10 |
Số phần tương tự - LM4863M |
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Mô tả tương tự - LM4863M |
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