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LM4917 датащи(PDF) 11 Page - National Semiconductor (TI) |
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LM4917 датащи(HTML) 11 Page - National Semiconductor (TI) |
11 / 18 page Application Information ELIMINATING THE OUTPUT COUPLING CAPACITOR The LM4917 features a low noise inverting charge pump that generates an internal negative supply voltage. This allows the outputs of the LM4917 to be biased about GND instead of a nominal DC voltage, like traditional headphone amplifi- ers. Because there is no DC component, the large DC blocking capacitors (typically 220µF) are not necessary. The coupling capacitors are replaced by two, small ceramic charge pump capacitors, saving board space and cost. Eliminating the output coupling capacitors also improves low frequency response. The headphone impedance and the output capacitor form a high pass filter that not only blocks the DC component of the output, but also attenuates low frequencies, impacting the bass response. Because the LM4917 does not require the output coupling capacitors, the low frequency response of the device is not degraded by external components. In addition to eliminating the output coupling capacitors, the ground referenced output nearly doubles the available dy- namic range of the LM4917 when compared to a traditional headphone amplifier operating from the same supply volt- age. OUTPUT TRANSIENT (’CLICK AND POPS’) ELIMINATED The LM4917 contains advanced circuitry that virtually elimi- nates output transients (’clicks and pops’). This circuitry prevents all traces of transients when the supply voltage is first applied or when the part resumes operation after coming out of shutdown mode. To ensure optimal click and pop performance under low gain configurations (less than 0dB), it is critical to minimize the RC combination of the feedback resistor R F and stray input capacitance at the amplifier inputs. A more reliable way to lower gain or reduce power delivered to the load is to place a current limiting resistor in series with the load as explained in the Minimizing Output Noise / Reducing Output Power section. AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 2, the LM4917 has two operational amplifiers internally. The two amplifiers have externally con- figurable gain, and the closed loop gain is set by selecting the ratio of R f to Ri. Consequently, the gain for each channel of the IC is A V = -(Rf /Ri) Since this an output ground-referenced amplifier, by driving the headphone through R OUT (Pin 11) and LOUT (Pin 8), the LM4917 does not require output coupling capacitors. The typical single-ended amplifier configuration where one side of the load is connected to ground requires large, expensive output capacitors. POWER DISSIPATION Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a successful design. Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. P DMAX =(VDD) 2 /(2 π2R L) (1) Since the LM4917 has two operational amplifiers in one package, the maximum internal power dissipation point is twice that of the number which results from Equation 1. Even with the large internal power dissipation, the LM4917 does not require heat sinking over a large range of ambient tem- perature. From Equation 1, assuming a 3V power supply and a16 Ω load, the maximum power dissipation point is 28mW per amplifier. Thus the maximum package dissipation point is 56mW. The maximum power dissipation point obtained must not be greater than the power dissipation that results from Equation 2: P DMAX =(TJMAX -TA)/( θ JA) (2) For package TSSOP, θ JA = 109˚C/W. TJMAX = 150˚C for the LM4917. Depending on the ambient temperature, T A,ofthe system surroundings, Equation 2 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 is greater than that of Equation 2, then either the supply voltage must be de- creased, the load impedance increased or T A reduced. For the typical application of a 3V power supply, with a 16 Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 119.9˚C provided that device operation is around the maximum power dissipation point. Power dissipation is a function of output power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Perfor- mance Characteristics curves for power dissipation infor- mation for lower output powers. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a 3V power supply typi- cally use a 4.7µF in parallel with a 0.1µF ceramic filter capacitors to stabilize the power supply’s output, reduce noise on the supply line, and improve the supply’s transient response. However, their presence does not eliminate the need for a local 0.1µF supply bypass capacitor, C S, con- nected between the LM4917’s supply pins and ground. Keep the length of leads and traces that connect capacitors be- tween the LM4917’s power supply pin and ground as short as possible. MICRO POWER SHUTDOWN The voltage applied to the SD_LC (shutdown left channel) pin and the SD_RC (shutdown right channel) pin controls the LM4917’s shutdown function. When active, the LM4917’s micropower shutdown feature turns off the amplifiers’ bias circuitry, reducing the supply current. The trigger point is 0.3*CPV DD for a logic-low level, and 0.7*CPVDD for logic- high level. The low 0.01µA(typ) shutdown current is achieved by appling a voltage that is as near as ground a possible to the SD_LC/SD_RC pins. A voltage that is higher than ground may increase the shutdown current. There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch, connect an external 100k Ω pull-up resistor between the SD_LC/SD_RC pins and V DD. Connect the switch between the SD_LC/SD_RC pins and ground. Select normal amplifier www.national.com 11 |
Аналогичный номер детали - LM4917 |
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Аналогичное описание - LM4917 |
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