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LM4866MTE Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4866MTE Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 21 page  Application Information EXPOSED-DAP PACKAGE (LLP) PCB MOUNTING CONSIDERATIONS The LM4866’s exposed-DAP (die attach paddle) packages (MTE and LQ) 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 surround- ing PCB copper traces, ground plane and, finally, surround- ing air. The result is a low voltage audio power amplifier that produces 2.2W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful consideration of necessary ther- mal design. Failing to optimize thermal design may compro- mise the LM4866’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The MTE and LQ packages must have their DAPs soldered to a copper pad on the PCB. The DAP’s PCB copper pad is connected to a large plane of continuous unbroken copper. This plane forms a thermal mass and heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper heat sink area with 32(4x8) (MTE) or 6(3x2) (LQ) vias. The via diameter should be 0.012in - 0.013in with a 1.27mm pitch. Ensure efficient ther- mal conductivity by plating-through and solder-filling the vias. Best thermal performance is achieved with the largest prac- tical copper heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in 2 (min) area is necessary for 5V operation with a 4 Ω load. Heatsink areas not placed on the same PCB layer as the LM4866 should be 5in 2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚c ambient temperature. Increase the area to compensate for ambient temperatures above 25˚c. In systems using cooling fans, the LM4866MTE can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a 2.5in 2 exposed copper or 5.0in 2 inner layer copper plane heatsink, the LM4866MTE can continuously drive a 3 Ω load to full power. The LM4866LQ achieves the same output power level without forced air cooling. In all circumstances and conditions, the junction temperature must be held below 150˚C to prevent activating the LM4866’s thermal shutdown protection. The LM4866’s power de-rating curve in the Typi- cal Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts for the exposed-DAP TSSOP and LLP packages are shown in the Demonstration Board Layout section. Further detailed and specific information concerning PCB layout, fabrication, and mounting an LLP package is avail- able from National Semiconductor’s AN-1187. PCB LAYOUT AND SUPPLY REGULATION CONSIDER- ATIONS 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- pendent 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 2.1W to 2.0W. This problem of decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain 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. www.national.com 10 |
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