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