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LM4871 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM4871 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Application Information (Continued) LD (LLP) package is available from National Semiconduc- tor’s Package Engineering Group under application note AN1187. PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3 Ω AND 4Ω LOADS Power dissipated by a load is a function of the voltage swing across the load and the load’s impedance. As load imped- ance decreases, load dissipation becomes increasingly de- pendant on the interconnect (PCB trace and wire) resistance between the amplifier output pins and the load’s connec- tions. Residual trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1 Ω trace resistance reduces the output power dissipated by a 4 Ω load from 2.0W to 1.95W. This problem of decreased load dissipation is exac- erbated as load impedance decreases. Therefore, to main- tain the highest load dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide as possible. Poor power supply regulation adversely affects maximum output power. A poorly regulated supply’s output voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output signal clipping, and reduced output power. Even with tightly regulated sup- plies, trace resistance creates the same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps maintain full output voltage swing. BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4871 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally config- urable; the second amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first am- plifier is set by selecting the ratio of R f to Ri while the second amplifier’s gain is fixed by the two internal 40k Ω resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two, which results in both amplifiers pro- ducing signals identical in magnitude, but 180˚ out of phase. 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 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. Another advantage of the differential bridge output is no net DC voltage across load. This results from biasing V O1 and V O2 at the same DC voltage, in this case VDD/2 . This eliminates the coupling capacitor that single supply, single- ended amplifiers require. Eliminating an output coupling ca- pacitor in a single-ended configuration forces a single supply amplifier’s half-supply bias voltage across the load. The current flow created by the half-supply bias voltage in- creases internal IC power dissipation and my permanently damage loads such as speakers. 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. Equation 1 states the maximum power dissipation point for a bridge amplifier operating at a given supply voltage and driving a specified output load. P DMAX = 4*(VDD) 2/(2 π2R L) (1) Since the LM4871 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended ampifier. Even with this substantial increase in power dissipation, the LM4871 does not require heatsinking under most operating conditions and output loading. From Equation 1, assuming a 5V power supply and an 8 Ω load, the maximum power dissipation point is 625 mW. The maximum power dissipation point obtained from Equation 1 must not be greater than the power dissi- pation that results from Equation 2: P DMAX =(TJMAX–TA)/ θ JA (2) For the SO package, θ JA = 140˚C/W, for the DIP package, θ JA = 107˚C/W, and for the MSOP package, θ JA = 210˚C/W assuming free air operation. For the LD package soldered to a DAP pad that expands to a copper area of 1.0in 2 on a PCB, the LM4871’s θ JA is 56˚C/W. TJMAX = 150˚C for the LM4871. The θ JA can be decreased by using some form of heat sinking. The resultant θ JA will be the summation of the θ JC, θ CS, and θ SA. θ JC is the junction to case of the package (or to the exposed DAP, as is the case with the LD package), θ CS is the case to heat sink thermal resistance and θ SA is the heat sink to ambient thermal resistance. By adding addi- tional copper area around the LM4871, the θ JA can be reduced from its free air value for the SO and MSOP pack- ages. Increasing the copper area around the LD package from 1.0in 2 to 2.0in2 area results in a θ JA decrease to 46˚C/W. Depending on the ambient temperature, T A, and the θ JA, Equation 2 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 1 is greater than that of Equation 2, then either the supply voltage must be decreased, the load im- pedance increased, the θ JA decreased, or the ambient tem- perature reduced. For the typical application of a 5V power supply, with an 8 Ω load, and no additional heatsinking, the maximum ambient temperature possible without violating the maximum junction temperature is approximately 61˚C pro- vided that device operation is around the maximum power dissipation point and assuming surface mount packaging. For the LD package in a typical application of a 5V power supply, with a 4 Ω load, and 1.0in2 copper area soldered to the exposed DAP pad, the maximum ambient temperature is approximately 77˚C providing device operation is around the maximum power dissipation point. Internal power dissipation is a function of output power. If typical operation is not around the maximum power dissipation point, the ambient temperature can be increased. Refer to the Typical Perfor- mance Characteristics curves for power dissipation infor- mation for different output powers and output loading. www.national.com 9 |
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