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LM4818M bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM4818M bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Application Information (Continued) operation by closing the switch. Opening the switch con- nects the shutdown pin to V DD through the pull-up resistor, activating micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This pre- vents unwanted state changes. In a system with a micropro- cessor or a microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUT- DOWN pin with active circuitry eliminates the pull-up resistor PROPER SELECTION OF EXTERNAL COMPONENTS Optimizing the LM4818’s performance requires properly se- lecting external components. Though the LM4818 operates well when using external components with wide tolerances, best performance is achieved by optimizing component val- ues. The LM4818 is unity gain stable, giving the designer maxi- mum design flexibility. The gain should be set to no more than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-noise ra- tio. These parameters are compromised as the closed-loop gain increases. However, low gain demands input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources such as audio CO- DECs have outputs of 1V RMS (2.83VP-P). Please refer to the Audio Power Amplifier Design section for more informa- tion on selecting the proper gain. Another important consideration is the amplifier’s close-loop bandwidth. 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 that limits low frequency response. This value should be chosen based on needed frequency response for a few distinct reasons discussed below Input Capacitor Value Selection Amplifying the lowest audio frequencies requires a high value input coupling capacitor (C i in Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. Applications using speakers with limited frequency response reap little improvement by using a large input capacitor. Besides affecting system cost and size, C i has an effect on the LM4818’s click and pop performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero to a quies- cent state. The magnitude of the pop is directly proportional to the input capacitor’s value. Higher value capacitors need more time to reach a quiescent DC voltage (usually 1/2 V DD) when charged with a fixed current. The amplifier’s output charges the input capacitor through the feedback resistor, R F. Thus, selecting an input capacitor value that is no higher than necessary to meet the desired -3dB frequency can minimize pops. As shown in Figure 1, the input resistor (R i) and the input capacitor, C i produce a -3dB high pass filter cutoff frequency that is found using Equation (5). f -3dB = 1/(2 πRiCi) (Hz) (5) As an example when using a speaker with a low frequency limit of 150Hz, C i, using Equation (5) is 0.063µF. The 0.39µF C i shown in Figure 1 allows the LM4818 to drive a high efficiency, full range speaker whose response extends down to 20Hz. Besides optimizing the input capacitor value, the bypass capacitor value, C B requires careful consideration. The by- pass capacitor’s value is the most critical to minimizing turn-on pops because it determines how fast the LM4818 turns on. The slower the LM4818’s outputs ramp to their quiescent DC voltage (nominally 1/2V DD), the smaller the turn-on pop. While the device will function properly (no os- cillations or motorboating), with C B less than 1.0µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of C B equal to or greater than 1.0µF is recommended in all but the most cost sensitive designs. Bypass Capacitor Value Selection Besides minimizing the input capacitor size, careful consid- eration should be paid to the value of C B, the capacitor connected to the BYPASS pin. Since C B determines how fast the LM4818 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4818’s outputs ramp to their quiescent DC voltage (nomi- nally 1/2V DD), the smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of C i (in the range of 0.1µF to 0.39µF) produces a click-less and pop-less shut- down function. As discussed above, choosing C i no larger than necessary for the desired bandwidth helps minimize clicks and pops. If using the optional capacitor, C B2, the total capacitance see at the BYPASS pin is C B +CB2. When using the values shown in Figure 1, Typical Audio Amplifier Application Circuit, for C B and CB2 the change in the capacitance seen by the BYPASS pin is not significant rela- tive to capacitor value tolerances. Optimizing Click and Pop Reduction Performance The LM4818 contains circuitry that minimizes turn-on and shutdown transients or ’clicks and pops’. For this discussion, turn on refers to either applying the power or supply voltage or when the shutdown mode is deactivated. While the power supply is ramping to it’s final value, the LM4818’s internal amplifiers are configured as unity gain buffers. An internal current source charges the voltage of the bypass capacitor, C B, connected to the BYPASS pin in a controlled, linear manner. Ideally, the input and outputs track the voltage charging on the bypass capacitor. The gain of the internal amplifiers remains unity until the bypass capacitor is fully charged to 1/2V DD. As soon as the voltage on the bypass capacitor is stable, the device becomes fully operational. Although the BYPASS pin current cannot be modified, changing the size of the bypass capacitor, C B, alters the device’s turn-on time and magnitude of ’clicks and pops’. Increasing the value of C B reduces the magnitude of turn-on pops. However, this presents a tradeoff: as the size of C B increases, the turn-on time (Ton) increases. There is a linear relationship between the size of C B and the turn on time. If using the optional capacitor, C B2, the total capacitance see at the BYPASS pin is C B and CB2. The total capacitance see at the BYPASS pin must be considered for the table below and when optimizing click and pop performance. Below are some typical turn-on times for various values of C B: C B T ON 0.01µF 20ms 0.1µF 200ms 0.22µF 440ms www.national.com 10 |
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