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LM4861MX bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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LM4861MX bảng dữ liệu(HTML) 9 Page - Texas Instruments |
9 / 20 page LM4861 www.ti.com SNAS095C – MAY 1997 – REVISED MAY 2013 APPLICATION INFORMATION BRIDGE CONFIGURATION EXPLANATION As shown in Figure 2 , the LM4861 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier's gain is externally configurable, while the second amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to Ri while the second amplifier's gain is fixed by the two internal 40kΩ resistors. Figure 2 shows that the output of amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in magnitude, but out of phase 180°. Consequently, the differential gain for the IC is: Avd = 2 * (Rf/ Ri) (1) By driving the load differentially through outputs VO1 and VO2, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier configuration where one side of its load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Consequently, four times the output power is possible 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. In order to choose an amplifier's closed-loop gain without causing excessive clipping which will damage high frequency transducers used in loudspeaker systems, please refer to AUDIO POWER AMPLIFIER DESIGN. A bridge configuration, such as the one used in Boomer Audio Power Amplifiers, 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 eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configuration. Without an output coupling capacitor in a single supply, single-ended amplifier, the half-supply bias across the load would result in both increased internal IC power dissipation and also permanent loudspeaker damage. An output coupling capacitor forms a high pass filter with the load requiring that a large value such as 470 μF be used with an 8Ω load to preserve low frequency response. This combination does not produce a flat response down to 20Hz, but does offer a compromise between printed circuit board size and system cost, versus low frequency response. POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Equation 3 states the maximum power dissipation point for a bridge amplifier operating at a given supply voltage and driving a specified output load. PDMAX = 4*(VDD) 2 / (2π2R L) (2) Since the LM4861 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended amplifier. Even with this substantial increase in power dissipation, the LM4861 does not require heatsinking. From Equation 3, assuming a 5V power supply and an 8 Ω load, the maximum power dissipation point is 625mW.The maximum power dissipation point obtained from Equation 3 must not be greater than the power dissipation that results from Equation 3: PDMAX = (TJMAX − TA) / θJA (3) For the LM4861 surface mount package, θJA = 140°C/W and TJMAX = 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 3 is greater than that of Equation 3, then either the supply voltage must be decreased or the load impedance increased. For the typical application of a 5V power supply, with an 8 Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 62.5°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 can be increased. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output powers. Copyright © 1997–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM4861 |
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