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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LM4781 Fiches technique(PDF) 17 Page - National Semiconductor (TI) |
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LM4781 Fiches technique(HTML) 17 Page - National Semiconductor (TI) |
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17 / 25 page  Application Information (Continued) sible as compared to a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. For each operational amplifier in a bridge con- figuration, the internal power dissipation will increase by a factor of two over the single ended dissipation. Using Equa- tion (2) the load impedance should be divided by a factor of two to find the maximum power dissipation point for each amplifier in a bridge configuration. In the case of an 8 Ω load in a bridge configuration, the value used for R L in Equation (2) would be 4 Ω for each amplifier in the bridge. When using two of the amplifiers of the LM4781 in bridge mode, the third amplifier should have a load impedance equal to or higher than the equivalent impedance seen by each of the bridged amplifiers. In the example above where the bridge load is 8 Ω and each amplifier in the bridge sees a load value of 4 Ω then the third amplifier should also have a 4 Ω load impedance or higher. Using a lower load impedance on the third amplifier will result in higher power dissipation in the third amplifier than the other two amplifiers and may result in unwanted activation of thermal shut down on the third amplifier. Once the impedance seen by each amplifier is known then Equa- tion (2) can be used to calculated the value of P DMAX for each amplifier. The P DMAX of the IC package is found by adding up the power dissipation for each amplifier within the IC package. This value of P DMAX can be used to calculate the correct size heat sink for a bridged amplifier application. Since the inter- nal dissipation for a given power supply and load is in- creased by using bridged-mode, the heatsink’s θ SA will have to decrease accordingly as shown by Equation 4. Refer to the section, Determining the Correct Heat Sink, for a more detailed discussion of proper heat sinking for a given appli- cation. PARALLEL AMPLIFIER APPLICATION Parallel configuration is normally used when higher output current is needed for driving lower impedance loads (i.e. 4 Ω or lower) to obtain higher output power levels. As shown in Figure 3 , the parallel amplifier configuration consist of de- signing the amplifiers in the IC to have identical gain, con- necting the inputs in parallel and then connecting the outputs in parallel through a small external output resistor. Any num- ber of amplifiers can be connected in parallel to obtain the needed output current or to divide the power dissipation across multiple IC packages. Ideally, each amplifier shares the output current equally. Due to slight differences in gain the current sharing will not be equal among all channels. If current is not shared equally among all channels then the power dissipation will also not be equal among all channels. It is recommended that 0.1% tolerance resistors be used to set the gain (R i and Rf) for a minimal amount of difference in current sharing. When operating two or more amplifiers in parallel mode the impedance seen by each amplifier is equal to the total load impedance multiplied by the number of amplifiers driving the load in parallel as shown by Equation (5) below: R L(parallel) =RL(total) * Number of amplifiers (5) Once the impedance seen by each amplifier in the parallel configuration is known then Equation (2) can be used with this calculated impedance to find the amount of power dis- sipation for each amplifier. Total power dissipation (P DMAX) within an IC package is found by adding up the power dissipation for each amplifier in the IC package. Using the calculated P DMAX the correct heat sink size can be deter- mined. Refer to the section, Determining the Correct Heat Sink, for more information and detailed discussion of proper heat sinking. If only two amplifiers of the LM4781 are used in parallel mode then the third amplifier should have a load impedance equal to or higher than the equivalent impedance seen by each of the amplifiers in parallel mode. Having the same load impedance on all amplifiers means that the power dissipation in each amplifier will be equal. Using a lower load impedance on the third amplifier will result in higher power dissipation in the third amplifier than the other two amplifiers and may result in unwanted activation of thermal shut down on the third amplifier. Having a higher impedance on the third amplifier than the equivalent impedance on the two amplifi- ers in parallel will reduce total IC package power dissipation reducing the heat sink size requirement. BI-AMP AND TRI-AMP APPLICATIONS Bi-amping is the practice of using two different amplifiers to power the individual drivers in a speaker enclosure. For example, a two-way speaker enclosure might have a tweeter and a subwoofer. One amplifier would drive the tweeter and another would drive the subwoofer. One advantage is that the gain of each amplifier can be adjusted for the different driver sensitivities. Another advantage is the crossover can be designed before the amplifier stages with low cost op amps instead of large passive components. With the cross- over before the amplifier stages no power is wasted in the passive crossover as each individual amplifier provides the correct frequencies for the driver. Tri-Amping is using three different amplifier stages in the same way bi-amping is done. Bi-amping can also be done on a three-way speaker design by using one amplifier for the subwoofer and another for the midrange and tweeter. The LM4781 is perfectly suited for bi-amp or tri-amp appli- cations with it’s three amplifiers. Two of the amplifiers can be configured for bridge or parallel mode to drive a subwoofer with the third amplifier driving the tweeter or tweeter and midrange. An example would be to use a 4 Ω subwoofer and 8 Ω tweeter/midrange with the LM4781 in parallel and single- ended modes. Each amplifier would see an 8 Ω load but the subwoofer would have twice the output power as the tweeter/midrange. The gain of each amplifier may also be adjusted for the desired response. Using the LM4781 in a tri-amp configuration would allow the gain of each amplifier to be adjusted to achieve the desired speaker response. SINGLE-SUPPLY AMPLIFIER APPLICATION The typical application of the LM4781 is a split supply am- plifier. But as shown in Figure 4, the LM4781 can also be used in a single power supply configuration. This involves using some external components to create a half-supply bias which is used as the reference for the inputs and outputs. Thus, the signal will swing around half-supply much like it swings around ground in a split-supply application. Along with proper circuit biasing, a few other considerations must be accounted for to take advantage of all of the LM4781 functions, like the mute function. CLICKS AND POPS In the typical application of the LM4781 as a split-supply audio power amplifier, the IC exhibits excellent “click” and “pop” performance when utilizing the mute mode. In addition, www.national.com 17 |
Numéro de pièce similaire - LM4781 |
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Description similaire - LM4781 |
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