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LM4734 Datasheet(Fiches technique) 15 Page  National Semiconductor (TI) 


15 page Application Information (Continued) P DMAX =(VCC) 2 /2 π2R L (1) Thus by knowing the total supply voltage and rated output load, the maximum power dissipation point can be calcu lated. The package dissipation is three times the number which results from Equation (1) since there are three ampli fiers in each LM4734. Refer to the graphs of Power Dissipa tion versus Output Power in the Typical Performance Char acteristics section which show the actual full range of power dissipation not just the maximum theoretical point that re sults from Equation (1). DETERMINING THE CORRECT HEAT SINK The choice of a heat sink for a highpower audio amplifier is made entirely to keep the die temperature at a level such that the thermal protection circuitry is not activated under normal circumstances. The thermal resistance from the die to the outside air, θ JA (junction to ambient), is a combination of three thermal re sistances, θ JC (junction to case), θ CS (case to sink), and θ SA (sink to ambient). The thermal resistance, θ JC (junction to case), of the LM4734TA is 1.0˚C/W. Using Thermalloy Ther macote thermal compound, the thermal resistance, θ CS (case to sink), is about 0.2˚C/W. Since convection heat flow (power dissipation) is analogous to current flow, thermal resistance is analogous to electrical resistance, and tem perature drops are analogous to voltage drops, the power dissipation out of the LM4734 is equal to the following: P DMAX =(TJMAX−TAMB)/ θ JA (2) where T JMAX = 150˚C, TAMB is the system ambient tempera ture and θ JA = θ JC + θ CS + θ SA. 200898B8 Once the maximum package power dissipation has been calculated using Equation 2, the maximum thermal resis tance, θ SA, (heat sink to ambient) in ˚C/W for a heat sink can be calculated. This calculation is made using Equation 4 which is derived by solving for θ SA in Equation 3. θ SA = [(TJMAX−TAMB)−PDMAX( θ JC + θ CS)]/PDMAX (3) Again it must be noted that the value of θ SA is dependent upon the system designer’s amplifier requirements. If the ambient temperature that the audio amplifier is to be working under is higher than 25˚C, then the thermal resistance for the heat sink, given all other things are equal, will need to be smaller. SUPPLY BYPASSING The LM4734 has excellent power supply rejection and does not require a regulated supply. However, to improve system performance as well as eliminate possible oscillations, the LM4734 should have its supply leads bypassed with low inductance capacitors having short leads that are located close to the package terminals. Inadequate power supply bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency instabilities. These instabilities can be eliminated through multiple by passing utilizing a large tantalum or electrolytic capacitor (10µF or larger) which is used to absorb low frequency variations and a small ceramic capacitor (0.1µF) to prevent any high frequency feedback through the power supply lines. If adequate bypassing is not provided, the current in the supply leads which is a rectified component of the load current may be fed back into internal circuitry. This signal causes distortion at high frequencies requiring that the sup plies be bypassed at the package terminals. It is recom mended that a ceramic 0.1µF capacitor and an electrolytic or tantalum 10µF or larger capacitor be placed as close as possible to the IC’s supply pins and then an additional elec trolytic capacitor of 470µF or more on each supply line. BRIDGED AMPLIFIER APPLICATION The LM4734 has three operational amplifiers internally, al lowing for a few different amplifier configurations. One of these configurations is referred to as “bridged mode” and involves driving the load differentially through two of the LM4734’s outputs. This configuration is shown in Figure 2. Bridged mode operation is different from the classical single ended amplifier configuration where one side of its load is connected to ground. A bridge amplifier design has a distinct advantage over the singleended configuration, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Theoretically, four times the output power is pos sible as compared to a singleended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. For each operational amplifier in a bridge con figuration, the internal power dissipation will increase by a factor of two over the single ended dissipation. Using Equa tion (2) the load impedance should be divided by a factor of two to find the maximum power dissipation point for each amplifier in a bridge configuration. In the case of an 8 Ω load in a bridge configuration, the value used for R L in Equation (2) would be 4 Ω for each amplifier in the bridge. When using two of the amplifiers of the LM4734 in bridge mode, the third amplifier should have a load impedance equal to or higher than the equivalent impedance seen by each of the bridged amplifiers. In the example above where the bridge load is 8 Ω and each amplifier in the bridge sees a load value of 4 Ω then the third amplifier should also have a 4 Ω load impedance or higher. Using a lower load impedance on the third amplifier will result in higher power dissipation in the third amplifier than the other two amplifiers and may result in unwanted activation of thermal shut down on the third amplifier. Once the impedance seen by each amplifier is known then Equa tion (2) can be used to calculated the value of P DMAX for each amplifier. The P DMAX of the IC package is found by adding up the power dissipation for each amplifier within the IC package. This value of P DMAX can be used to calculate the correct size heat sink for a bridged amplifier application. Since the inter nal dissipation for a given power supply and load is in creased by using bridgedmode, the heatsink’s θ SA will have to decrease accordingly as shown by Equation 4. Refer to the section, Determining the Correct Heat Sink, for a more detailed discussion of proper heat sinking for a given appli cation. www.national.com 15 
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