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LM3875 Datasheet(PDF) 14 Page - National Semiconductor (TI) |
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LM3875 Datasheet(HTML) 14 Page - National Semiconductor (TI) |
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14 / 20 page  Application Information (Continued) But since we know P DMAX, θ JC, and θ SC for the application and we are looking for θ SA, we have the following: θ SA = [(TJmax −TAmb)−PDMAX ( θ JC + θ CS)]/PDMAX (4) Again it must be noted that the value of θ SA is dependent upon the system designer’s amplifier application and its corresponding parameters as described previously. If the ambient temperature that the audio amplifier is to be working under is higher than the normal 25˚C, then the thermal resistance for the heat sink, given all other things are equal, will need to be smaller. Equations (1), (4) are the only equations needed in the determination of the maximum heat sink thermal resistance. This is, of course, given that the system designer knows the required supply voltages to drive his rated load at a particular power output level and the parameters provided by the semiconductor manufacturer. These parameters are the junction to case thermal resistance, θ JC,TJmax = 150˚C, and the recommended Thermalloy Thermacote thermal com- pound resistance, θ CS. SIGNAL-TO-NOISE RATIO In the measurement of the signal-to-noise ratio, misinterpre- tations of the numbers actually measured are common. One amplifier may sound much quieter than another, but due to improper testing techniques, they appear equal in measure- ments. This is often the case when comparing integrated circuit designs to discrete amplifier designs. Discrete transis- tor amps often “run out of gain” at high frequencies and therefore have small bandwidths to noise as indicated below. 01144911 Integrated circuits have additional open loop gain allowing additional feedback loop gain in order to lower harmonic distortion and improve frequency response. It is this addi- tional bandwidth that can lead to erroneous signal-to-noise measurements if not considered during the measurement process. In the typical example above, the difference in bandwidth appears small on a log scale but the factor of 10 in bandwidth, (200 kHz to 2 MHz) can result in a 10 dB theoretical difference in the signal-to-noise ratio (white noise is proportional to the square root of the bandwidth in a system). In comparing audio amplifiers it is necessary to measure the magnitude of noise in the audible bandwidth by using a “weighting” filter (Note 11). A “weighting” filter alters the frequency response in order to compensate for the average human ear’s sensitivity to the frequency spectra. The weight- ing filters at the same time provide the bandwidth limiting as discussed in the previous paragraph. Note 11: CCIR/ARM: A Practical Noise Measurement Method;byRay Dolby, David Robinson and Kenneth Gundry, AES Preprint No. 1353 (F-3). In addition to noise filtering, differing meter types give differ- ent noise readings. Meter responses include: 1. RMS reading, 2. average responding, 3. peak reading, and 4. quasi peak reading. Although theoretical noise analysis is derived using true RMS based calculations, most actual measurements are taken with ARM (Average Responding Meter) test equip- ment. Typical signal-to-noise figures are listed for an A-weighted filter which is commonly used in the measurement of noise. The shape of all weighting filters is similar, with the peak of the curve usually occurring in the 3 kHz–7 kHz region as shown below. 01144912 SUPPLY BYPASSING The LM3875 has excellent power supply rejection and does not require a regulated supply. However, to eliminate pos- sible oscillations all op amps and power op amps should have their supply leads bypassed with low-inductance ca- pacitors having short leads and 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 bypassing utilizing a large tanta- lum or electrolytic capacitor (10 µF or larger) which is used to absorb low frequency variations and a small ceramic capaci- tor (0.1 µF) 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 low distortion at high frequencies requiring that the supplies be bypassed at the package terminals with an electrolytic capacitor of 470 µF or more. LEAD INDUCTANCE Power op amps are sensitive to inductance in the output lead, particularly with heavy capacitive loading. Feedback to the input should be taken directly from the output terminal, minimizing common inductance with the load. Lead inductance can also cause voltage surges on the sup- plies. With long leads to the power supply, energy is stored in the lead inductance when the output is shorted. This energy can be dumped back into the supply bypass capacitors when the short is removed. The magnitude of this transient is reduced by increasing the size of the bypass capacitor near the IC. With at least a 20 µF local bypass, these voltage surges are important only if the lead length exceeds a couple feet (>1 µH lead inductance). Twisting together the supply and ground leads minimizes the effect. www.national.com 14 |
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