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LM4923 数据表(PDF) 11 Page - Texas Instruments |
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LM4923 数据表(HTML) 11 Page - Texas Instruments |
11 / 23 page LM4923, LM4923LQBD www.ti.com SNAS211D – JULY 2004 – REVISED FEBRUARY 2009 APPLICATION INFORMATION DIFFERENTIAL AMPLIFIER EXPLANATION The LM4923 is a fully differential audio amplifier that features 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 VDD / 2. When setting the differential gain, the amplifier can be considered to have "halves". Each half uses an input and feedback resistor (Ri1 and RF1) to set its respective closed-loop gain (see Figure 1). With Ri1 = Ri2 and RF1 = RF2, the gain is set at -RF / Ri for each half. This results in a differential gain of AVD = -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 External 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 LM4923 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 LM4923 simply amplifies the difference 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 Vo1 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 assumes that the amplifier is not current limited or clipped. In order to choose an amplifier's closed-loop gain without causing excess clipping, please refer to the Audio Power Amplifier Design section. A bridged configuration, such as the one used in the LM4923, also creates a second advantage over single- ended amplifiers. Since the differential outputs, Vo1 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 configuration, 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 LM4923 include increased power supply rejection ratio, common-mode noise reduction, and click and pop reduction. EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS The LM4923's exposed-DAP (die attach paddle) package (WQFN) 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 LM4923's high power performance and activate unwanted, though necessary, thermal shutdown protection. The WQFN 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 efficient thermal conductivity by plating-through and solder-filling the vias. Copyright © 2004–2009, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM4923 LM4923LQBD |
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