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LM4918 数据表(PDF) 10 Page - National Semiconductor (TI) |
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LM4918 数据表(HTML) 10 Page - National Semiconductor (TI) |
10 / 17 page Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4918 consists of two pairs of operational amplifiers, forming a two-channel (channel L and channel R) stereo amplifier. (Though the following discusses channel L, it applies equally to channel R.) External resistors Rf and Ri (set as R0 and R1, respectively for the L channel, and R7 and R6, respectively for the R channel on the demo board circuit) set the closed-loop gain of the first op-amp, whereas two internal 20k Ω resistors set the second op-amps gain at -1. The LM4918 drives a load, such as a speaker, connected between the two amplifier outputs, VoL- and VoL+ (VoR-, and VoR+ for the R channel). Figure 1shows that the first op-amp’s output serves as the second op-amp’s input. This results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Taking advantage of this phase difference, a load is placed between VoL- and VoL+ and driven differentially (commonly referred to as “bridge mode”). This results in a differential gain of A VD=2*(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. Another advantage of the differ- ential 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 (such as when using the LM4918 to drive single-ended headphone loads). Eliminating an output coupling 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 π2R 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 LM4918 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- fier. From Equation (2), assuming a 5V power supply and a 8 Ω load, the maximum single channel power dissipation is 0.158W or 0.317W for stereo operation. P DMAX =4*(VDD) 2 /(2 π2R L) Bridge-Mode (3) The LM4918’s power dissipation is twice that given by Equa- tion (2) or Equation (3) when operating in the single-ended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation (3) must not ex- ceed the power dissipation given by Equation (4): P DMAX’ =(TJMAX -TA)/ θ JA (4) The LM4918’s T JMAX = 150˚C. In the LQ package soldered to a DAP pad that expands to a copper area of 1in 2 on a PCB, the LM4918’s θ JA is 51˚C/W. At any given ambient temperature T A, use Equation (4) to find the maximum inter- nal power dissipation supported by the IC packaging. Rear- ranging Equation (4) and substituting P DMAX for PDMAX’ re- sults in Equation (5). This equation gives the maximum ambient temperature that still allows maximum stereo power dissipation without violating the LM4918’s maximum junction temperature. T A =TJMAX - 2*PDMAX θ JA (5) For a typical application with a 5V power supply and an 8 Ω load, the maximum ambient temperature that allows maxi- mum stereo power dissipation without exceeding the maxi- mum junction temperature is approximately 85˚C for the LQ package. T JMAX =PDMAX θ JA +TA (6) Equation (6) gives the maximum junction temperature T J- MAX . If the result violates the LM4918’s 150˚C, reduce the maximum junction temperature by reducing the power sup- ply voltage or increasing the load resistance. Further allow- ance should be made for increased ambient temperatures. The above examples assume that a device is a surface mount part operating around the maximum power dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are allowed as output power or duty cycle decreases. If the result of Equation (2) is greater than that of Equation (3), then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. If these measures are insufficient, a heat sink can be added to reduce θ JA. The heat sink can be created using additional copper area around the package, with connections to the ground pin(s), supply pin and amplifier output pins. External, solder attached SMT heatsinks such as the Thermalloy 7106D can also improve power dissipation. Refer to the Typical Performance Characteristics curves for power dissi- pation information at lower output power levels. EXPOSED-DAP MOUNTING CONSIDERATIONS The LM4918’s exposed-DAP (die attach paddle) packages (LD) provide a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surrounding PCB copper area heatsink, copper traces, ground plane, and finally, surrounding air. The result is a low voltage audio power amplifier that produces 1.1W dissipation in an 8 Ω load at ≤ 1% THD+N. This power is achieved through careful consideration of necessary thermal design. Failing to opti- www.national.com 10 |
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