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LM4879SDBD Datasheet(PDF) 11 Page - Texas Instruments |
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LM4879SDBD Datasheet(HTML) 11 Page - Texas Instruments |
11 / 28 page LM4879, LM4879MMBD, LM4879SDBD www.ti.com SNAS142F – MAY 2004 – REVISED OCTOBER 2004 APPLICATION INFORMATION BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4879 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 20 kΩ resistors. Figure 1 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 by 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 the 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. 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, please refer to the AUDIO POWER AMPLIFIER DESIGN section. A bridge configuration, such as the one used in LM4879, 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, the half-supply bias across the load would result in both increased internal IC power dissipation and also possible loudspeaker damage. 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. Since the LM4879 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 2. PDMAX = 4*(VDD) 2/(2π2R L) (2) It is critical that the maximum junction temperature (TJMAX) of 150°C is not exceeded. TJMAX can be determined from the power derating curves by using PDMAX and the PC board foil area. By adding additional copper foil, the thermal resistance of the application can be reduced from a free air value of 150°C/W, resulting in higher PDMAX. Additional copper foil can be added to any of the leads connected to the LM4879. It is especially effective when connected to VDD, GND, and the output pins. Refer to the application information on the LM4879 reference design board for an example of good heat sinking. If TJMAX still exceeds 150°C, then additional changes must be made. These changes can include reduced supply voltage, higher load impedance, or reduced ambient temperature. Internal power dissipation is a function of output power. Refer to the TYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation information for different output powers and output loading. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as possible. Typical applications employ a 5V regulator with 10 µF tantalum or electrolytic capacitor and a ceramic bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4879. The selection of a bypass capacitor, especially CB, is dependent upon PSRR requirements, click and pop performance (as explained in the section, PROPER SELECTION OF EXTERNAL COMPONENTS), system cost, and size constraints. Copyright © 2004, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM4879 LM4879MMBD LM4879SDBD |
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