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LM4927SD Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM4927SD Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Application Information DIFFERENTIAL AMPLIFIER EXPLANATION The LM4927 is a fully differential audio amplifier that fea- tures differential input and output stages. Internally this is accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the output voltages so that the average value remains V DD / 2. When setting the differential gain, the amplifier can be considered to have "halves". Each half uses an input and feedback resistor (R i1 and RF1) to set its respective closed-loop gain (see Figure 1). With R i1 =Ri2 and RF1 =RF2, the gain is set at -R F /Ri for each half. This results in a differential gain of A VD =-RF/Ri (1) It is extremely important to match the input resistors to each other, as well as the feedback resistors to each other for best amplifier performance. See the Proper Selection of Exter- nal Components section for more information. A differential amplifier works in a manner where the difference between the two input signals is amplified. In most applications, this would require input signals that are 180˚ out of phase with each other. The LM4927 can be used, however, as a single ended input amplifier while still retaining its fully differential benefits. In fact, completely unrelated signals may be placed on the input pins. The LM4927 simply amplifies the differ- ence between them. All of these applications provide what is known as a "bridged mode" output (bridge-tied-load, BTL). This results in output signals at V o1 and Vo2 that are 180˚ out of phase with respect to each other. Bridged mode operation is different from the single-ended amplifier configuration that connects the load between the amplifier output and ground. A bridged amplifier design has distinct advantages over the single- ended configuration: it provides differential drive to the load, thus doubling maximum possible output swing for a specific supply voltage. Four times the output power is possible compared with a single-ended amplifier under the same conditions. This increase in attainable output power as- sumes that the amplifier is not current limited or clipped. In order to choose an amplifier’s closed-loop gain without caus- ing excess clipping, please refer to the Audio Power Am- plifier Design section. A bridged configuration, such as the one used in the LM4927, also creates a second advantage over single- ended amplifiers. Since the differential outputs, V o1 and Vo2, are biased at half-supply, no net DC voltage exists across the load. This assumes that the input resistor pair and the feedback resistor pair are properly matched (see Proper Selection of External Components). BTL configuration eliminates the output coupling capacitor required in single- supply, single-ended amplifier configurations. If an output coupling capacitor is not used in a single-ended output con- figuration, the half-supply bias across the load would result in both increased internal IC power dissipation as well as permanent loudspeaker damage. Further advantages of bridged mode operation specific to fully differential amplifiers like the LM4927 include increased power supply rejection ratio, common-mode noise reduction, and click and pop reduction. EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS The LM4927’s exposed-DAP (die attach paddle) package (LLP) provide a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surrounding PCB copper traces, ground plane and, finally, surrounding air. Failing to optimize thermal design may compromise the LM4927’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The LLP package must have its DAP soldered to a copper pad on the PCB. The DAP’s PCB copper pad is connected to a large plane of continuous unbroken copper. This plane forms a thermal mass and heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper heat sink area with a thermal via. The via diameter should be 0.012in - 0.013in. Ensure effi- cient thermal conductivity by plating-through and solder- filling the vias. Best thermal performance is achieved with the largest prac- tical copper heat sink area. In all circumstances and condi- tions, the junction temperature must be held below 150˚C to prevent activating the LM4927’s thermal shutdown protec- tion. The LM4927’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts are shown in the Demonstration Board Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mounting an LLP package is available from National Semiconductor’s package Engineering Group un- der application note AN1187. PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 4 Ω LOADS Power dissipated by a load is a function of the voltage swing across the load and the load’s impedance. As load imped- ance decreases, load dissipation becomes increasingly de- pendent on the interconnect (PCB trace and wire) resistance between the amplifier output pins and the load’s connec- tions. Residual trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load as desired. This problem of decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load dissipation and widest output volt- age swing, PCB traces that connect the output pins to a load must be as wide as possible. Poor power supply regulation adversely affects maximum output power. A poorly regulated supply’s output voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output signal clipping, and reduced output power. Even with tightly regulated sup- plies, trace resistance creates the same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps maintain full output voltage swing. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifer, whether the amplifier is bridged or single-ended. Equation 2 states the maximum power dissi- pation point for a single-ended amplifier operating at a given supply voltage and driving a specified output load. www.national.com 10 |
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