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LM4830N bảng dữ liệu(PDF) 9 Page - National Semiconductor (TI) |
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LM4830N bảng dữ liệu(HTML) 9 Page - National Semiconductor (TI) |
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9 / 14 page  Application Information POWER AMPLIFIER HANDSFREE MODE As shown in Figure 1, amplifier A1 can be used in one of two modes, bridged output or single-ended output. This IC was intended to be used in systems requiring both internal speaker drive and external mono-headphone drive capabil- ity. Headphones generally have a much higher impedance than that of speakers since headphones don’t require as much output power. This also allows headphones to be driven single-endedly. Shown in Figure 1, the output can be automatically switched from bridged speaker drive to single-ended headphone drive using a control pin in the headphone jack that is tied to the Headset (HS) pin, pin 3. When the voltage at the HS pin input changes from 0V to 5V, V O2 of the bridged amplifier output is put into high imped- ance. This allows the permanently connected internal speaker of the system to be disabled when a headphone is plugged into the headphone jack. Output V O1 then drives the headphone single-endedly through the output coupling cap, C C.CC should be chosen to allow the full audio bandwidth to be amplified. Since C C and R L create a high-pass filter, CC must be big enough to allow frequencies down to 20 Hz to be amplified. The following equation should be used for proper component selection. C C = 1/(2π(20 Hz)(R L)) where 16Ω≤ RL ≤ 600Ω (1) As usual, the output drive limitations are the maximum sup- ply voltage swing, current drive capability, and power dissi- pation. In bridged-output drive mode, the power amplifier will drive 4 Ω or 8Ω with normal music signals over time. How- ever, trying to put a sinewave through the amplifier at the worst case power dissipation point could cause the amplifier to go into thermal shutdown. In single-ended drive mode, the amplifier is intended to drive 32 Ω headphones. It will drive lower impedances with the limitations of voltage swing and current drive capability. The result of driving lower impedance loads single-endedly is lower achievable output power. Headset and Shutdown Pin Table HS Pin SD Pin IC Operation Microphone Low Low All Outputs On MIC1 On High Low 1/2 A1 On MIC2 On (V O1 On Only) X High Whole IC Off NA X — “Don’t Care” NA — Not Applicable POWER DISSIPATION Power dissipation is a major concern when using any power amplifier and must be thoroughly understood to ensure a successful design. Equation 2 states the maximum power dissipation point for a bridged amplifier operating at a given supply voltage and driving a specified output load. P DMAX = 4(VDD) 2/(2 π2 R L) (2) Although the LM4830 has three amplifiers in the package, the bridged amplifier produces the majority of the power dis- sipation because it supplies the largest amount of output power. If each of the amplifiers in the LM4830 were of the same power level, each of their power dissipations would need to be taken into account. However, this is not the case and the bridged power amplifier is the only major power dis- sipation contributor. Even with the large internal power dissipation created by the bridged amplifier, the LM4830 does not require heatsinking over a large range of ambient temperatures. Using Equation 2, assuming a 5V power supply and a 8 Ω load, the maximum power dissipation point is 633 mW. P DMAX = (TJMAX −TA)/θJA (3) For the LM4830 surface mount package, θ JA = 79˚C/W and T JMAX = 150˚C. Depending on the ambient temperature, TA, of the system surroundings, Equation 3 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 2 is greater than that of Equation 3, then either the supply voltage must be de- creased, the load impedance increased, or the ambient tem- perature reduced. For the typical application of a 5V power supply, with a bridged 8 Ω load, the maximum ambient tem- perature possible without violating the maximum junction temperature is approximately 100˚C provided that device op- eration is around the maximum power dissipation point. The average power dissipation caused by typical music material played at a reasonable level is generally lower than the maximum power dissipation point. Refer to the Typical Per- formance Characteristics curves for power dissipation in- formation for lower output powers. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is criti- cal for low noise performance and high power supply rejec- tion. The capacitor location on both the half-supply bypass and power supply pins should be as close to the device as possible. The effect of a larger half-supply bypass capacitor is improved low frequency PSRR due to increased half-supply stability. Typical applications employ a 5V regula- tor with 10 µF and a 0.1 µF bypass capacitors which aid in supply stability, but do not eliminate the need for bypassing the supply nodes of the LM4830. The selection of bypass ca- pacitors, especially C b, is thus dependent upon desired low frequency PSRR, system cost, and size constraints. GROUNDING In order to achieve the best possible performance, there are certain grounding techniques that should be followed. All in- put reference grounds should be tied with their respective source grounds and brought back to the power supply ground separately from the output load ground returns. Those input grounds should also be tied in with the half-supply bypass ground, pin 16. As an example, the AC in- put ground reference for the power amplifier, A1, is V IN+, pin 7. This ground should be tied as close as possible to the By- pass ground (pin 16), as shown in Figure 1. In order to tie in the signal source ground, the audio jack ground on V IN− should also be tied to the Bypass ground. As stated above, the ground returns for the output loads should be brought back to the supply ground individually. This will keep large signal currents on those ground lines from interfering with the stable AC input ground references. In addition, the signal ground reference for the preamp, A2, (the ground end of capacitor C I) should be tied together with the mic inputs’ signal ground reference from the microphone. LAYOUT ISSUES As stated in the Grounding section, placement of ground re- turn lines is imperative in maintaining the highest level of system performance. It is not only important to route the cor- rect ground return lines together, but also equally important to be aware of where those ground return lines are routed in conjunction with each other. As an example, the output load www.national.com 9 |
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