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LM4929MM датащи(PDF) 11 Page - National Semiconductor (TI) |
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LM4929MM датащи(HTML) 11 Page - National Semiconductor (TI) |
11 / 13 page Application Information (Continued) The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digi- tal output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry elimi- nates the pull-up resistor. Shutdown enable/disable times are controlled by a combina- tion of C B and VDD. Larger values of CB results in longer turn on/off times from Shutdown. Smaller V DD values also in- crease turn on/off time for a given value of C B. Longer shutdown times also improve the LM4929’s resistance to click and pop upon entering or returning from shutdown. For a 2.4V supply and C B = 4.7µF, the LM4929 requires about 2 seconds to enter or return from shutdown. This longer shut- down time enables the LM4929 to have virtually zero pop and click transients upon entering or release from shutdown. Smaller values of C B will decrease turn-on time, but at the cost of increased pop and click and reduced PSRR. Since shutdown enable/disable times increase dramatically as supply voltage gets below 2.2V, this reduced turn-on time may be desirable if extreme low supply voltage levels are used as this would offset increases in turn-on time caused by the lower supply voltage. This technique is not recom- mended for OCL mode since shutdown enable/disable times are very fast (0.5s) independent of supply voltage. 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 LM4929 is tolerant of external component combinations, consideration to compo- nent values must be used to maximize overall system qual- ity. The LM4929 is unity-gain stable which gives the designer maximum system flexibility. The LM4929 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. Input signals equal to or greater than 1V rms are available from sources such as audio codecs. Very large values should not be used for the gain-setting resistors. Values for R i and Rf should be less than 1M Ω. Please refer to the section, Audio Power Amplifier Design, for a more com- plete 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 band- width is dictated by the choice of external components shown in Figure 2. 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 frequency response and turn-on time. SELECTION OF INPUT CAPACITOR SIZE Amplifying the lowest audio frequencies requires a high value input coupling capacitor, C i. A high value capacitor can be expensive and may compromise space efficiency in por- table designs. In many cases, however, the headphones used in portable systems have little ability to reproduce signals below 60Hz. Applications using headphones with this limited frequency response reap little improvement by using a high value input capacitor. In addition to system cost and size, turn on time is affected by the size of the input coupling capacitor C i. A larger input coupling capacitor requires more charge to reach its quies- cent DC voltage. This charge comes from the output via the feedback Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on time can be minimized. A small value of C i (in the range of 0.1µF to 0.39µF), is recommended. AUDIO POWER AMPLIFIER DESIGN A 25mW/32 Ω AUDIO AMPLIFIER Given: Power Output 25mWrms Load Impedance 32 Ω Input Level 1Vrms Input Impedance 20k Ω A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating from the Output Power vs Supply Voltage graphs in the Typical Per- formance Characteristics section, the supply rail can be easily found. 3V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4929 to reproduce peak in excess of 25mW without producing audible distortion. At this time, the de- signer must make sure that the power supply choice along with the output impedance does not violate the conditions explained in the Power Dissipation section. Once the power dissipation equations have been addressed, the required gain can be determined from Equation 2. (4) From Equation 4, the minimum A V is 0.89; use AV = 1. Since the desired input impedance is 20k Ω, and with a A V gain of 1, a ratio of 1:1 results from Equation 1 for R f to Ri. The values are chosen with R i = 20k Ω and R f = 20k Ω. The final design step is to address the bandwidth requirements which must be stated as a pair of -3dB frequency points. Five times away from a -3dB point is 0.17dB down from passband response which is better than the required ± 0.25dB speci- fied. f L = 100Hz/5 = 20Hz f H = 20kHz*5= 100kHz As stated in the External Components section, R i in con- junction with C i creates a C i ≥ 1/(2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF. The high frequency pole is determined by the product of the desired frequency pole, f H, and the differential gain, AV. With an A V = 1 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the LM4929 GBWP of 10MHz. This figure displays that is a designer has a need to design an amplifier with higher differential gain, the LM4929 can still be used without running into bandwidth limitations. Figure 3 shows an optional resistor connected between the amplifier output that drives the headphone jack sleeve and ground. This resistor provides a ground path that supressed power supply hum. Thishum may occur in applications such as notebook computers in a shutdown condition and con- nected to an external powered speaker. The resistor’s 100 Ω value is a suggested starting point. Its final value must be www.national.com 11 |
Аналогичный номер детали - LM4929MM |
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Аналогичное описание - LM4929MM |
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