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## LM4947 Datasheet(PDF) 28 Page - Texas Instruments

 Part No. LM4947 Description Mono Class D and Stereo Audio Sub-System Download 37 Pages Scroll/Zoom 100% Maker TI1 [Texas Instruments] Homepage http://www.ti.com Logo

## LM4947 Datasheet(HTML) 28 Page - Texas Instruments

 28 / 37 page LM4947, LM4947TLEVALSNAS349D – JUNE 2006 – REVISED MAY 2013www.ti.comLM4947 and in the transducer load. The amount of power dissipation in the LM4947 is very low. This is becausethe ON resistance of the switches used to form the output waveforms is typically less than 0.25Ω. This leavesonly the transducer load as a potential "sink" for the small excess of input power over audio band output power.The LM4947 dissipates only a fraction of the excess power requiring no additional PCB area or copper plane toact as a heat sink.The LM4947 also has a pair of single-ended amplifiers driving stereo headphones, RHP and LHP. The maximuminternal power dissipation for RHP and LHP is given by Equation 7 and Equation 8. From Equation 7 andEquation 8, assuming a 5V power supply and a 32Ω load, the maximum power dissipation for LHP and RHP is40mW, or 80mW total.PDMAX-LHP = (VDD)2 / (2π2 RL): Single-ended Mode(7)PDMAX-RHP = (VDD)2 / (2π2 RL): Single-ended Mode(8)The maximum internal power dissipation of the LM4947 occurs when all 3 amplifiers pairs are simultaneously on;and is given by Equation 9.PDMAX-TOTAL = PDMAX-SPKROUT + PDMAX-LHP + PDMAX-RHP(9)The maximum power dissipation point given by Equation 9 must not exceed the power dissipation given byEquation 10:PDMAX = (TJMAX - TA) / θJA(10)The LM4947's TJMAX = 150°C. In the ITL package, the LM4947's θJA is 65°C/W. At any given ambienttemperature TA, use Equation 10 to find the maximum internal power dissipation supported by the IC packaging.Rearranging Equation 10 and substituting PDMAX-TOTAL for PDMAX' results in Equation 11. This equation gives themaximum ambient temperature that still allows maximum stereo power dissipation without violating the LM4947'smaximum junction temperature.TA = TJMAX - PDMAX-TOTAL θJA(11)For a typical application with a 5V power supply and an 8Ω load, the maximum ambient temperature that allowsmaximum stereo power dissipation without exceeding the maximum junction temperature is approximately 104°Cfor the ITL package.TJMAX = PDMAX-TOTAL θJA + TA(12)Equation 12 gives the maximum junction temperature TJMAX. If the result violates the LM4947's 150°C, reducethe maximum junction temperature by reducing the power supply voltage or increasing the load resistance.Further allowance should be made for increased ambient temperatures.The above examples assume that a device is a surface mount part operating around the maximum powerdissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures areallowed as output power or duty cycle decreases. If the result of Equation 9 is greater than that of Equation 10,then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. If thesemeasures are insufficient, a heat sink can be added to reduceθJA. The heat sink can be created using additionalcopper 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.When adding a heat sink, theθJA is the sum of θJC, θCS, and θSA. (θJC is the junction-to-case thermal impedance,θCS is the case-to-sink thermal impedance, and θSA is the sink-to-ambient thermal impedance). Refer to theTYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation information at lower output powerlevels.POWER SUPPLY BYPASSINGAs with any power amplifier, proper supply bypassing is critical for low noise performance and high power supplyrejection. Applications that employ a 5V regulator typically use a 1µF in parallel with a 0.1µF filter capacitors tostabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient response.However, their presence does not eliminate the need for a local 1.1µF tantalum bypass capacitance connectedbetween the LM4947's supply pins and ground. Keep the length of leads and traces that connect capacitorsbetween the LM4947's power supply pin and ground as short as possible. Connecting a 2.2µF capacitor, CB,between the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier'sPSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however,increases turn-on time and can compromise the amplifier's click and pop performance. The selection of bypasscapacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (asexplained in the section, SELECTING EXTERNAL COMPONENTS), system cost, and size constraints.28Submit Documentation FeedbackCopyright © 2006–2013, Texas Instruments IncorporatedProduct Folder Links: LM4947 LM4947TLEVAL