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LM4866LQ Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4866LQ Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 21 page Application Information (Continued) BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4866 consists of two pairs of operational amplifiers, forming a two-channel (channel A and channel B) stereo amplifier. (Though the following discusses channel A, it applies equally to channel B.) External resistors R f and Ri set the closed-loop gain of Amp1A, whereas two internal 20k Ω resistors set Amp2A’s gain at -1. The LM4866 drives a load, such as a speaker, connected between the two amplifier outputs, -OUTA and +OUTA. Figure 1 shows that Amp1A’s output serves as Amp2A’s input. This results in both amplifiers producing signals iden- tical in magnitude, but 180˚ out of phase. Taking advantage of this phase difference, a load is placed between -OUTA and +OUTA and driven differentially (commonly referred to as ’bridge mode’). This results in a differential gain of A VD = 2x(Rf /Ri) (1) Bridge mode amplifiers are different from single-ended am- plifiers that drive loads connected between a single amplifi- er’s output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-ended con- figuration: its differential output doubles the voltage swing across the load. This produces four times the output power when compared to a single-ended amplifier under the same conditions. This increase in attainable output power as- sumes that the amplifier is not current limited or that the output signal is not clipped. To ensure minimum output sig- nal clipping when choosing an amplifier’s closed-loop gain, refer to the Audio Power Amplifier Design section. Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by biasing channel A’s and channel B’s outputs at half-supply. This eliminates the coupling capacitor that single supply, single-ended amplifiers require. Eliminating an output cou- pling capacitor in a single-ended configuration forces a single-supply amplifier’s half-supply bias voltage across the load. This increases internal IC power dissipation and may permanently damage loads such as speakers. POWER DISSIPATION Power dissipation is a major concern when designing a successful single-ended or bridged amplifier. Equation (2) states the maximum power dissipation point for a single- ended amplifier operating at a given supply voltage and driving a specified output load P DMAX =(VDD) 2 /(2 π2 R L) Single-Ended (2) However, a direct consequence of the increased power de- livered to the load by a bridge amplifier is higher internal power dissipation for the same conditions. The LM4866 has two operational amplifiers per channel. The maximum internal power dissipation per channel operating in the bridge mode is four times that of a single-ended ampli- 20018601 * Refer to the section Proper Selection of External Components, for a detailed discussion of CB size. FIGURE 1. Typical Audio Amplifier Application Circuit Pin out shown for the LLP package. Refer to the Connection Diagrams for the pinout of the TSSOP package. www.national.com 11 |
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