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LM4850MT Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4850MT Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 20 page Typical Performance Characteristics Non-LD Specific Characteristics (Continued) Power Supply Rejection Ratio 20001074 Application Information BRIDGED AND SINGLE-ENDED OPERATION As shown in Figure 1, the LM4850 contains three operational amplifiers (A1-A3). These amplifiers can be configured for SE or BTL modes. In the SE mode, the LM4850 operates as a high current output dual op amp. A1 and A3 are independent amplifiers with an externally configured gain of A V =- R F/RI. The outputs of A1 and A3 are used to drive an external set of headphones plugged into the headphone jack. Amplifier A2 is shut down to a high output impedance state in SE mode. This prevents any current flow into the mono bridge-tied load, thereby muting it. In BTL mode, A3 is shut down to a high impedance state. The audio signal from the RIGHT IN pin is directed to the inverting input of A1. As a result, the LEFT IN and RIGHT IN audio signals, V INL and VINR, are summed together at the input of A1. A2 is then activated with a closed-loop gain of A V = -1 fixed by two internal 20k Ω resistors. The outputs of A1 and A2 are then used to drive the mono bridged-tied load. EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION The LM4850’s exposed-DAP (die attach paddle) package (LD) provides 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 traces, ground plane, and surrounding air. The result is a low voltage audio power amplifier that produces 1.7W at ≤ 1% THD+N with a 4Ω load. This high power is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4850’s high power performance and activate un- wanted, though necessary, thermal shutdown protection. The LD package must have its DAP 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, 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 4(2x2) vias. The via diameter should be 0.012in-0.013in with a 1.27mm pitch. Ensure efficient thermal conductivity by plating through the vias. Best thermal performance is achieved with the largest prac- tical heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in 2 area is necessary for 5V operation with a 4 Ω load. Heatsink areas not placed on the same PCB layer as the LM4850 should be 5in 2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚C ambient temperature. In- crease the area to compensate for ambient temperatures above 25˚C. The LM4850’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. An example PCB lay- out for the LD package is shown in the Demonstration Board Layout section. Further detailed and specific infor- mation concerning PCB layout, fabrication, and mounting an LD (LLP) package is available from National Semiconduc- tor’s Package Engineering Group under application note AN1187. BRIDGE CONFIGURATION EXPLANATION When the LM4850 is in BTL mode, the output of amplifier A1 serves as the input to amplifier A2, which results in both amplifiers producing signals identical in magnitude, but out of phase by 180˚. Consequently, the differential gain for the mono channel is: A VD =VOUT /(VINL +VINR) = 2x(RF /RI) (1) Driving a load differentially through the BTL OUT- and BTL OUT+ outputs is an amplifier configuration commonly re- ferred to as ’bridged mode’. Bridged mode operation is dif- ferent from the classical single-ended amplifier configuration where one side of its load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration. It drives a load differentially, which doubles output swing for a specified supply voltage. This produces four times the output power as that produced by a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the ampli- fier is not current limited or clipped. In order to choose an www.national.com 11 |
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