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LM3886 Datasheet(PDF) 19 Page - National Semiconductor (TI) |
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LM3886 Datasheet(HTML) 19 Page - National Semiconductor (TI) |
19 / 24 page Application Information (Continued) 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. 01183314 SUPPLY BYPASSING The LM3886 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. LAYOUT, GROUND LOOPS AND STABILITY The LM3886 is designed to be stable when operated at a closed-loop gain of 10 or greater, but as with any other high-current amplifier, the LM3886 can be made to oscillate under certain conditions. These usually involve printed cir- cuit board layout or output/input coupling. When designing a layout, it is important to return the load ground, the output compensation ground, and the low level (feedback and input) grounds to the circuit board common ground point through separate paths. Otherwise, large cur- rents flowing along a ground conductor will generate volt- ages on the conductor which can effectively act as signals at the input, resulting in high frequency oscillation or excessive distortion. It is advisable to keep the output compensation components and the 0.1 µF supply decoupling capacitors as close as possible to the LM3886 to reduce the effects of PCB trace resistance and inductance. For the same reason, the ground return paths should be as short as possible. In general, with fast, high-current circuitry, all sorts of prob- lems can arise from improper grounding which again can be avoided by returning all grounds separately to a common point. Without isolating the ground signals and returning the grounds to a common point, ground loops may occur. “Ground Loop” is the term used to describe situations occur- ring in ground systems where a difference in potential exists between two ground points. Ideally a ground is a ground, but unfortunately, in order for this to be true, ground conductors with zero resistance are necessary. Since real world ground leads possess finite resistance, currents running through them will cause finite voltage drops to exist. If two ground return lines tie into the same path at different points there will be a voltage drop between them. The first figure below shows a common ground example where the positive input ground and the load ground are returned to the supply ground point via the same wire. The addition of the finite wire resistance, R 2, results in a voltage difference between the two points as shown below. 01183315 The load current I L will be much larger than input bias current I I, thus V1 will follow the output voltage directly, i.e. in phase. Therefore the voltage appearing at the non-inverting input is effectively positive feedback and the circuit may oscillate. If there were only one device to worry about then the values of R 1 and R2 would probably be small enough to be ignored; however, several devices normally comprise a total system. Any ground return of a separate device, whose output is in www.national.com 19 |
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