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LM4835MTE Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4835MTE Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 16 page  Application Information (Continued) put signals to obtain a given output power. Input signals equal to or greater than 1Vrms are available from sources such as audio codecs. 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 3. Both the input coupling capacitor, C I, and the output coupling capacitor form first order high pass filters which limit low frequency response given in Equations 4 and 5. f IC = 1/(2πRiCi) (4) f OC = 1/(2πRLCO) (5) These values should be chosen based on required fre- quency response. Selection of Input and Output Capacitor Size Large input and output capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized capacitor is needed to couple in low frequencies without se- vere attenuation. In many cases the speakers used in por- table systems, whether internal or external, have little ability to reproduce signals below 100 Hz–150 Hz. In this case, us- inga large input or output capacitor may not increase system performance. In addition to system cost and size, click and pop perfor- mance is effected by the size of the input coupling capacitor, C i. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally 1/2 V DD.) This charge comes from the output through the feedback and is apt to create pops once the device is enabled. By minimizing the capacitor size based on necessary low frequency re- sponse, turn-on pops can be minimized. CLICK AND POP CIRCUITRY The LM4835 contains circuitry to minimize turn-on transients or “click 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 muted. An internal current source ramps up the voltage of the bypass pin. Both the inputs and outputs ideally track the voltage at the bypass pin. The device will remain in mute mode until the bypass pin has reached its half supply volt- age, 1/2 V DD. As soon as the bypass node is stable, the de- vice will become fully operational. Although the bypass pin current source cannot be modified, the size of the bypass capacitor, C B, can be changed to alter the device turn-on time and the amount of “click and pop”. By increasing C B, the amount of turn-on pop can be reduced. However, the trade-off for using a larger bypass capacitor is an increase in the turn-on time for the device. Reducing C B will decrease turn-on time and increase “click and pop”. There is a linear relationship between the size of C B and the turn-on time. Here are some typical turn-on times for differ- ent values of C B: C B T ON 0.01 µF 2 ms 0.1 µF 20 ms 0.22 µF 42 ms 0.47 µF 84 ms 1.0 µF 200 ms C B T ON 4.7 µF 1sec In order to eliminate “click and pop”, all capacitors must be discharged before turn-on. Rapid on/off switching of the de- vice or shutdown function may cause the “click and pop” cir- cuitry to not operate fully, resulting in increased “click and pop” noise. In systems where the line out and headphone jack are the same, the output coupling cap, C O, is of particular concern. C O is chosen for a desired cutoff frequency with a head- phone load. This desired cutoff frequency will change when the headphone load is replaced by a high impedance line out load(powered speakers). The input impedance of head- phones are typically between 32 Ω and 64Ω. Whereas, the input impedance of powered speakers can vary from 1k Ω to 100k Ω. As the RC time constant of the load and the output coupling capacitor increases, the turn off transients are in- creased. To improve click and pop performance in this situation, exter- nal resistor R7 should be added as shown in Figure 4. The recommended value for R7 is between 150 Ω to 1kΩ.To achieve virtually clickless and popless performance R7 = 150 Ω,C O = 220µF, and CB = 1.0µF should be used. Lower values of R6 will result in better click and pop performance. However, it should be understood that lower resistance val- ues of R7 will increase current consumption. DOCKING STATION In an application such as a notebook computer, docking sta- tion or line level outputs may be required. Pin 9 and Pin 13 can drive loads greater than 1k Ω rail to rail. These pins are tied to the output of the input op-amp to drive powered speakers and other high impedance loads. Output coupling capacitors need to be placed in series with the load. The rec- ommended values of the capacitors are between 0.33µF to 1.0µF with the positive side of the capacitors toward the IC. The outputs of the docking station pins cannot be attenuated with the DC volume control. However the gain of the outputs can be configured by adjusting the feedback and input resis- tors for the input op-amp. The input op-amp is in an inverting configuration where the gain is: R F /Ri =- Av Note that by adjusting the gain of the input op-amp the over- all gain of the output amplifiers are also affected. Although the single ended outputs of the output amplifiers can be used to drive line level outputs, it is recommended to use Pins 9 and 13 to achieve better performance. DS100139-5 FIGURE 3. Resistor for Varying Output Loads www.national.com 11 |
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