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AN-1192 Datasheet(PDF) 3 Page - National Semiconductor (TI) |
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AN-1192 Datasheet(HTML) 3 Page - National Semiconductor (TI) |
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3 / 21 page  4.0 Thermal Background (Continued) forward. Therefore, the designer must understand the IC’s power dissipation capabilities before using the IC in a booster configuration. 4.1 TYPICAL CHARACTERISTIC DATA The power dissipation capabilities of a power IC are either specified in the datasheet or can be derived from its guar- anteed output power specification. While the power dissipa- tion rating for the LM3886T is 125W, this number can be misleading. Its power dissipation specification is derived from the IC’s junction-to-case thermal resistance, θ JC = 1˚C/W, the maximum junction temperature, T J = 150˚C, and the ambient air, T A = 25˚C. As stated in the datasheet, the device must be derated based on these parameters while operating at elevated temperatures. The heat sinking re- quirements for the application are based on these param- eters so that the IC will not go into Thermal Shutdown (TSD). The real problem for Overture ICs, however, comes from the sensitivity of the output stage’s unique SPiKe™ Protection which dynamically monitors the output transistor’s tempera- ture. While the thermal shutdown circuitry is enabled at T J = 150˚C, SPiKe circuitry is enabled at T J = 250˚C for instanta- neous power spikes in the output stage transistor. As the overall temperature of the IC increases, SPiKe circuitry be- comes even more sensitive causing it to turn on before the 125W limit is reached. TSD circuitry will continue to function globally for the IC in conjunction with the SPiKe circuitry. However, protection circuitry should not be activated under normal operating conditions. The question then becomes, what is the power dissipation limit for the IC such that SPiKe circuitry is not enabled? Knowing the power dissipation limit and keeping the case temperature of the IC as cool as possible will expand the output power capability without activating SPiKe Protection. The other way to determine IC power dissipation capabilities is to analyze the output power specification in the datasheet. In the case of the LM3886T, there are two output power specification guarantees: 60W (min) into a 4 Ω load using ±28V supplies and 50W(typ) into an 8 Ω load from ±35V supplies. Using these two conditions and the theoretical maximum power dissipation equation shown below, results in the following maximum power dissipations: 4.2 SINGLE-ENDED AMPLIFIER Pdmax EQUATION: Pdmax = V CCtot2/2 π2R L Non-Isolated LM3886T: 1. V CC = ±28V, RL =4 Ω Pdmax = V CCtot2/2 π2R L =(±28V) 2/2 π2(4Ω) = 39.7W 2. V CC = ±35V, RL =8 Ω Pdmax = V CCtot2/2 π2R L =(±35V) 2/2 π2(8Ω) = 31.0W These results show that the IC can handle a maximum of ≈ 40W of continuous power dissipation without SPiKe Pro- tection being turned on under continuous sinusoidal input with proper heat sinking. The same theory applies to other Overture ICs as well, like the LM3876T, which is capable of dissipating 31W with proper heat sinking. It should be noted that the results shown above are for the non-isolated power package, where the back of the package is tied to the silicon substrate, or −Vee. The isolated power package has over- molded plastic on the back keeping the package electrically isolated from the silicon substrate. This extra amount of plastic increases the package thermal resistance from 1˚C/W for the non-isolated version to ≈ 2˚C/W for the iso- lated version. The result of increased thermal resistance is higher die temperature under the same conditions even though the heat sink temperature will not change. There are two major points to note: 1. The maximum power dissipation analysis was taken into account using regulated power supplies. The IC for the whole analysis is being tested at the worst case power dissipation point for a constant full-load power supply voltage. When using an unregulated power supply, the no-load voltage will be somewhat higher (15%–35%) causing the overall maximum power dissipation to be higher than expected. 2. In the real “audio” application, the average music power dissipation is much less than the maximum power dissi- pation created by a sinusoidal input. Therefore, the IC will run cooler than expected due to the lower power dissipation. However, when you put these two points together, they mostly cancel out, but only for music stimulus. Product quali- fications may go through worse case power dissipation sce- narios which implies that sinusoids will be used with unregu- lated power supplies. Therefore, when doing the thermal portion of the design, the higher supply voltages will increase the IC power dissipation and must be taken into account. 4.3 BRIDGED-OUTPUT AMPLIFIER Pdmax EQUATION To determine the Pdmax equation for a bridged amplifier solution, the single-ended Pdmax equation is used as a starting point. A bridged amplifier solution requires two am- plifiers and each amplifier will see 1/2 the total impedance. Adding these factors of 2 and 1/2 into the single-ended Pdmax equation results in the total Pdmax equation for a bridged amplifier. Pdmax BTL = 2*[VCCtot2/2 π2(/1⁄2R L)] = 4*(VCCtot2/2 π2R L) The bridged-output Pdmax equation represents the bridged amplifier solution. If a dual amplifier IC is used, then the total Pdmax would need to be dissipated in the single IC package. However, if two individual ICs are used, then the total power dissipation is divided between each IC. Two Non-Isolated LM3886Ts: V CC = ±28V, RL =4 Ω Pdmax = 4V CCtot2/2 π2R L =4(±28V) 2/2 π2(4Ω) = 158.8W Pdmax = 158.8W Pdmax/IC = 79.4W Therefore, using a bridged configuration, V cc would have to be equal to ±20V to keep the IC’s power dissipation within www.national.com 3 |
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