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LM4954 Arkusz danych(PDF) 10 Page - National Semiconductor (TI) |
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LM4954 Arkusz danych(HTML) 10 Page - National Semiconductor (TI) |
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10 / 16 page  Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4954 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 LM4954, 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 LM4954 has two opera- tional amplifiers in one package, the maximum internal power dissipation is four times that of a single-ended ampli- fier. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 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 the free air value, resulting in higher P DMAX. Additional copper foil can be added to any of the leads connected to the LM4954. It is especially effective when connected to V DD, GND, and the output pins. Refer to the application information on the LM4954 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 LM4954. The selection of a bypass capacitor, especially C B, is dependent upon PSRR requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system cost, and size constraints. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4954 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. This shutdown feature turns the amplifier off when a logic low is placed on the shutdown pin. By switching the shutdown pin to ground, the LM4954 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less than 0.4V DC, the idle current may be greater than the typical value of 0.01µA. (Idle current is measured with the shutdown pin tied to ground). The LM4954 has an internal 75k Ω pull- down resistor. If the shutdown pin is left floating the IC will automatically enter shutdown mode. 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 LM4954 is tolerant of external component combinations, consideration to compo- nent values must be used to maximize overall system qual- ity. The LM4954 is unity-gain stable which gives the designer maximum system flexibility. The LM4954 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. Input signals equal to or greater than 1 Vrms are available from sources such as audio codecs. Please refer to the section, Audio Power Amplifier Design, for a more com- plete 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 shown in Figure 1. The input coupling capacitor, C i, forms a first order high pass filter which limits low frequency re- sponse. This value should be chosen based on needed frequency response for a few distinct reasons. Selection Of Input Capacitor Size Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized capacitor is needed to couple in low frequencies without severe attenu- ation. But in many cases the speakers used in portable systems, whether internal or external, have little ability to www.national.com 10 |
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