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APPLICATION INFORMATION (continued)<br /><br />Inductor<br /><br />Proper inductor selection is key to the performance-switching power-supply designs. One important factor to<br />consider is whether the regulator is used in continuous mode (inductor current flows continuously and never<br />drops to zero) or in discontinuous mode (inductor current goes to zero during the normal switching cycle). Each<br />mode has distinctively different operating characteristics and, therefore, can affect the regulator performance<br />and requirements. In many applications, the continuous mode is the preferred mode of operation, since it offers<br />greater output power with lower peak currents, and also can result in lower output ripple voltage. The<br />advantages of continuous mode of operation come at the expense of a larger inductor required to keep inductor<br />current continuous, especially at low output currents and/or high input voltages.<br />The TL2575 and TL2575HV can operate in either continuous or discontinuous mode. With heavy load currents,<br />the inductor current flows continuously and the regulator operates in continuous mode. Under light load, the<br />inductor fully discharges and the regulator is forced into the discontinuous mode of operation. For light loads<br />(approximately 200 mA or less), this discontinuous mode of operation is perfectly acceptable and may be<br />desirable solely to keep the inductor value and size small. Any buck regulator eventually operates in<br />discontinuous mode when the load current is light enough.<br />The type of inductor chosen can have advantages and disadvantages. If high performance/quality is a concern,<br />then more-expensive toroid core inductors are the best choice, as the magnetic flux is contained completely<br />within the core, resulting in less EMI and noise in nearby sensitive circuits. Inexpensive bobbin core inductors,<br />however, generate more EMI as the open core does not confine the flux within the core. Multiple switching<br />regulators located in proximity to each other are particularly susceptible to mutual coupling of magnetic fluxes<br />from each other ’ s open cores. In these situations, closed magnetic structures (such as a toroid, pot core, or<br />E-core) are more appropriate.<br />Regardless of the type and value of inductor used, the inductor never should carry more than its rated current.<br />Doing so may cause the inductor to saturate, in which case the inductance quickly drops, and the inductor looks<br />like a low-value resistor (from the dc resistance of the windings). As a result, switching current rises dramatically<br />(until limited by the current-by-current limiting feature of the TL2575 and TL2575HV) and can result in<br />overheating of the inductor and the IC itself. Note that different types of inductors have different saturation<br />characteristics.
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