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LM4892 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4892 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 22 page Typical Performance Characteristics (Continued) Output Power vs Supply Voltage R L =32 Ω Output Power vs Supply Voltage Headphone Output, R L =32 Ω 20012780 20012781 Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4892 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally config- urable, 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 R f to Ri while the second amplifier’s gain is fixed by the two internal 20k Ω 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 A VD= 2 *(Rf/Ri) 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 configura- tion 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 con- ditions. 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 ex- cessive clipping, please refer to the Audio Power Amplifier Design section. A bridge configuration, such as the one used in LM4892, also creates a second advantage over single-ended amplifi- ers. 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 configura- tion. 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 LM4892 has two opera- tional 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 Equa- tion 1. P DMAX = 4*(VDD) 2/(2 π2R L) (1) It is critical that the maximum junction temperature T JMAX of 150˚C is not exceeded. T JMAX can be determined from the power derating curves by using P DMAX 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 P DMAX. Additional copper foil can be added to any of the leads connected to the LM4892. It is especially effective when connected to V DD, GND, and the output pins. Refer to the application information on the LM4892 reference design board for an example of good heat sinking. If T JMAX still exceeds 150˚C, then additional changes must be made. These changes can include re- duced supply voltage, higher load impedance, or reduced ambient temperature. Internal power dissipation is a function of output power. Refer to the Typical Performance Charac- teristics curves for power dissipation information for differ- ent 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 appli- cations employ a 5V regulator with 10µF tantalum or elec- trolytic capacitor and a ceramic bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4892. The selection of a bypass capacitor, especially C B, is dependent upon PSRR www.national.com 10 |
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