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LM4872IBP bảng dữ liệu(PDF) 8 Page - National Semiconductor (TI) |
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LM4872IBP bảng dữ liệu(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4872 has two operational am- plifiers internally, allowing for a few different amplifier con- figurations. 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 10 k Ω resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two which results in both am- plifiers 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 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. Four times the output power is possible as compared to a single-ended amplifier under the same condi- tions. 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 LM4872, 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 suc- cessful amplifier, whether the amplifier is bridged or single- ended. A direct consequence of the increased power deliv- ered to the load by a bridge amplifier is an increase in internal power dissipation. Since the LM4872 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 LM4872. It is especially effective when connected to V DD,GND, and the output pins. Refer to the application information on the LM4872 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. The National Reference Design board using a 5V supply and an 8 ohm load will run in a 110˚C am- bient environement without exceeding T JMAX. 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 applica- tions employ a 5V regulator with 10 µF Tantalum or electro- lytic capacitor and a 0.1 µF bypass capacitor which aid in supply stability. This does not eliminate the need for bypass- ing the supply nodes of the LM4872. The selection of bypass capacitor, especially C B, is dependent upon PSRR require- ments, click and pop performance as explained in the sec- tion, Proper Selection of External Components, system cost, and size constraints. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4872 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. This shutdown feature turns the am- plifier off when a logic high is placed on the shutdown pin. The trigger point between a logic low and logic high level is typically half- supply. It is best to switch between ground and supply to provide maximum device performance. By switch- ing the shutdown pin to V DD, the LM4872 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less than V DD, the idle current may be greater than the typical value of 0.01 µA. In either case, the shutdown pin should be tied to a stable volt- age to avoid unwanted state changes. In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which pro- vides a quick, smooth transition into shutdown. Another solu- tion is to use a single-pole, single-throw switch in conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground and enables the amplifier. If the switch is open, then the external pull-up re- sistor will disable the LM4872. This scheme guarantees that the shutdown pin will not float thus preventing unwanted state changes. PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications us- ing integrated power amplifiers is critical to optimize device and system performance. While the LM4872 is tolerant of external component combinations, consideration to compo- nent values must be used to maximize overall system qual- ity. The LM4872 is unity-gain stable which gives a designer maximum system flexibility. The LM4872 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. In- put signals equal to or greater than 1 Vrms are available from sources such as audio codecs. Please refer to the sec- tion, Audio Power Amplifier Design, for a more complete explanation of proper gain selection. Besides gain, one of the major considerations is the closed- loop bandwidth of the amplifier. To a large extent, the band- width is dictated by the choice of external components www.national.com 8 |
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