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LM7372ILDX bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM7372ILDX bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
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10 / 15 page  Simplified Schematic Diagram 20004928 Application Notes The LM7372 is a high speed dual operational amplifier with a very high slew rate and very low distortion, yet like many other op amps, it is used in conventional voltage feedback amplifier applications. Also, again like many op amps, it has a class AB output stage in order to be able to deliver high currents to low impedance loads, yet draw very little quies- cent supply current. For most op-amps in typical applica- tions, this topology means that internal power dissipation is rarely an issue, even with the trend to smaller surface mount packages. However, the LM7372 has been designed for applications where significant levels of power dissipation will be encountered, and an effective means of removing the internal heat generated by this power dissipation is needed to maintain the semiconductor junction temperature at ac- ceptable levels, particularly in environments with elevated ambient temperatures. Several factors contribute to power dissipation and conse- quently higher semiconductor junction temperatures, and these factors need to be well understood if the LM7372 is to perform to the desired specifications in a given application. Since different applications will have different dissipation levels and different compromises can be made between the ways these factors will contribute to the total junction tem- perature, this section will examine the typical application shown on the front page of this data sheet as an example, and offer suggestions for solutions where excessive junction temperatures are encountered. There are two major contributors to the internal power dissi- pation; the product of the supply voltage and the LM7372 quiescent current when no signal is being delivered to the external load, and the additional power dissipated while delivering power to the external load. The first of these components is easy to calculate simply by inspection of the data sheet. The LM7372 quiescent supply current is given as 6.5mA per amplifier, so with a 24Volt supply the power dissipation is PQ=V S x 2Iq (V S =VCC +VEE) = 24 x (6.5 x 10-3) = 312mW This is already a high level of internal power dissipation, and in a small surface mount package with a thermal resistance ( θ JA = 140˚C/Watt (a not unreasonable value for an SO-8 package) would result in a junction temperature 140˚C/W x 0.312W = 43.7˚C above the ambient temperature. A similar calculation using the worst case maximum current limit at an 85˚C ambient will yield a power dissipation of 456mW with a junction temperature of 149˚C, perilously close to the maxi- mum permitted junction temperature of 150˚C! The second contributor to high junction temperature is the additional power dissipated internally when power is being delivered to the external load. This cause of temperature rise can be less amenable to calculation, even when the actual operating conditions are known. For a Class B output stage, one transistor of the output pair will conduct the load current as the output voltage swings positive, with the other transistor drawing no current, and hence dissipating no power. During the other half of the signal swing this situation is reversed, with the lower transis- tor sinking the load current and the upper transistor is cut off. The current in each transistor will be a half wave rectified version of the total load current. Ideally neither transistor will dissipate power when there is no signal swing, but will dissipate increasing power as the output current increases. However, as the signal voltage across the load increases with load current, the voltage across the output transistor (which is the difference voltage between the supply voltage and the instantaneous voltage across the load) will decrease and a point will be reached where the dissipation in the transistor will begin to decrease again. If the signal is driven into a square wave, ideally the transistor dissipation will fall all the way back to zero. www.national.com 10 |
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